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L_0811 | bohrs atomic model | T_4171 | As a young man, Bohr worked in Rutherfords lab in England. Because Rutherfords model was weak on the position of the electrons, Bohr focused on them. He hypothesized that electrons can move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. He called these fixed distances energy levels, or electron shells. He thought of them as concentric spheres, with the nucleus at the center of each sphere. In other words, the shells consisted of sphere within sphere within sphere. Furthermore, electrons with less energy would be found at lower energy levels, closer to the nucleus. Those with more energy would be found at higher energy levels, farther from the nucleus. Bohr also hypothesized that if an electron absorbed just the right amount of energy, it would jump to the next higher energy level. Conversely, if it lost the same amount of energy, it would jump back to its original energy level. However, an electron could never exist in between two energy levels. These ideas are illustrated in the Figure 1.2. Q: How is an atom like a ladder? A: Energy levels in an atom are like the rungs of a ladder. Just as you can stand only on the rungs and not in between them, electrons can orbit the nucleus only at fixed distances from the nucleus and not in between them. | text | null |
L_0811 | bohrs atomic model | T_4172 | Bohrs model of the atom is actually a combination of two different ideas: Rutherfords atomic model of electrons orbiting the nucleus and German scientist Max Plancks idea of a quantum, which Planck published in 1901. A quantum (plural, quanta) is the minimum amount of energy that can be absorbed or released by matter. It is a discrete, or distinct, amount of energy. If energy were water and you wanted to add it to matter in the form of a drinking glass, you couldnt simply pour the water continuously into the glass. Instead, you could add it only in small fixed quantities, for example, by the teaspoonful. Bohr reasoned that if electrons can absorb or lose only fixed quantities of energy, then they must vary in their energy by these fixed amounts. Thus, they can occupy only fixed energy levels around the nucleus that correspond to quantum increases in energy. This is a two-dimensional model of a three-dimensional atom. The concen- tric circles actually represent concentric spheres. Q: The idea that energy is transferred only in discrete units, or quanta, was revolutionary when Max Planck first proposed it in 1901. However, what scientists already knew about matter may have made it easier for them to accept the idea of energy quanta. Can you explain? A: Scientists already knew that matter exists in discrete units called atoms. This idea had been demonstrated by John Dalton around 1800. Knowing this may have made it easier for scientists to accept the idea that energy exists in discrete units as well. | text | null |
L_0813 | bond polarity | T_4176 | Covalent bonds are chemical bonds between atoms of nonmetals that share valence electrons. In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar covalent bonds. The Figure than the hydrogen atoms do because the nucleus of the oxygen atom has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge, and the hydrogen atoms become slightly positive in charge. Click image to the left or use the URL below. URL: In other covalent bonds, electrons are shared equally. These bonds are called nonpolar covalent bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral in charge. The oxygen (O2 ) molecule in the Figure 1.2 has two nonpolar bonds. The two oxygen nuclei have an equal force of attraction for their four shared electrons. | text | null |
L_0813 | bond polarity | T_4176 | Covalent bonds are chemical bonds between atoms of nonmetals that share valence electrons. In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar covalent bonds. The Figure than the hydrogen atoms do because the nucleus of the oxygen atom has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge, and the hydrogen atoms become slightly positive in charge. Click image to the left or use the URL below. URL: In other covalent bonds, electrons are shared equally. These bonds are called nonpolar covalent bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral in charge. The oxygen (O2 ) molecule in the Figure 1.2 has two nonpolar bonds. The two oxygen nuclei have an equal force of attraction for their four shared electrons. | text | null |
L_0813 | bond polarity | T_4177 | A covalent compound is a compound in which atoms are held together by covalent bonds. If the covalent bonds are polar, then the covalent compound as a whole may be polar. A polar covalent compound is one in which there is a slight difference in electric charge between opposite sides of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in the Figure 1.3. In both molecules, the oxygen atoms attract electrons more strongly than the carbon or hydrogen atoms do, so both molecules have polar bonds. However, only formaldehyde is a polar compound. Carbon dioxide is nonpolar. Q: Why is carbon dioxide nonpolar? A: The symmetrical arrangement of atoms in carbon dioxide results in opposites sides of the molecule having the same charge. | text | null |
L_0815 | buoyancy | T_4182 | Buoyant force is an upward force that fluids exert on any object that is placed in them. The ability of fluids to exert this force is called buoyancy. What explains buoyant force? A fluid exerts pressure in all directions, but the pressure is greater at greater depth. Therefore, the fluid below an object, where the fluid is deeper, exerts greater pressure on the object than the fluid above it. You can see in the Figure 1.1 how this works. Buoyant force explains why the girl pictured above can float in water. Q: Youve probably noticed that some things dont float in water. For example, if you drop a stone in water, it will sink to the bottom rather than floating. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? A: The answer has to do with their weight. | text | null |
L_0815 | buoyancy | T_4183 | Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure 1.2. On the left, the objects weight is the same as the buoyant force acting on it, so the object floats. On the right, the objects weight is greater than the buoyant force acting on it, so the object sinks. | text | null |
L_0815 | buoyancy | T_4183 | Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure 1.2. On the left, the objects weight is the same as the buoyant force acting on it, so the object floats. On the right, the objects weight is greater than the buoyant force acting on it, so the object sinks. | text | null |
L_0815 | buoyancy | T_4184 | Density, or the amount of mass in a given volume, is also related to the ability of an object to float. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why the giant ice berg in the Figure 1.3 is floating in the ocean. Q: Can you think of more examples of substances that float in a fluid because they are low in density? A: Oil is less dense than water, so oil from a spill floats on ocean water. Helium is less dense than air, so balloons filled with helium float in air. | text | null |
L_0816 | calculating acceleration from force and mass | T_4185 | A change in an objects motionsuch as Xander speeding up on his scooteris called acceleration. Acceleration occurs whenever an object is acted upon by an unbalanced force. The greater the net force acting on the object, the greater its acceleration will be, but the mass of the object also affects its acceleration. The smaller its mass is, the greater its acceleration for a given amount of force. Newtons second law of motion summarizes these relationships. According to this law, the acceleration of an object equals the net force acting on it divided by its mass. This can be represented by the equation: Acceleration = Net force Mass or a = F m | text | null |
L_0816 | calculating acceleration from force and mass | T_4186 | This equation for acceleration can be used to calculate the acceleration of an object that is acted on by a net force. For example, Xander and his scooter have a total mass of 50 kilograms. Assume that the net force acting on Xander and the scooter is 25 Newtons. What is his acceleration? Substitute the relevant values into the equation for acceleration: F = 25 N = 0.5 N a= m 50 kg kg The Newton is the SI unit for force. It is defined as the force needed to cause a 1-kilogram mass to accelerate at 1 m/s2 . Therefore, force can also be expressed in the unit kg m/s2 . This way of expressing force can be substituted for Newtons in Xanders acceleration so the answer is expressed in the SI unit for acceleration, which is m/s2 : 2 0.5 kgm/s a = 0.5kgN = = 0.5 m/s2 kg Q: Why are there no kilograms in the final answer to this problem? A: The kilogram units in the numerator and denominator of the fraction cancel out. As a result, the answer is expressed in the correct SI units for acceleration. | text | null |
L_0816 | calculating acceleration from force and mass | T_4187 | Its often easier to measure the mass and acceleration of an object than the net force acting on it. Mass can be measured with a balance, and average acceleration can be calculated from velocity and time. However, net force may be a combination of many unseen forces, such as gravity, friction with surfaces, and air resistance. Therefore, it may be more useful to know how to calculate the net force acting on an object from its mass and acceleration. The equation for acceleration above can be rewritten to solve for net force as: Net Force = Mass Acceleration, or F=ma Look at Xander in the Figure 1.1. Hes riding his scooter down a ramp. Assume that his acceleration is 0.8 m/s2 . How much force does it take for him to accelerate at this rate? Substitute the relevant values into the equation for force to find the answer: F = m a = 50 kg 0.8 m/s2 = 40 kg m/s2 , or 40 N Q: If Xander and his scooter actually had a mass of 40 kg instead of 50 kg, how much force would it take for him to accelerate at 0.8 m/s2 ? | text | null |
L_0817 | calculating acceleration from velocity and time | T_4188 | Calculating acceleration is complicated if both speed and direction are changing or if you want to know acceleration at any given instant in time. However, its relatively easy to calculate average acceleration over a period of time when only speed is changing. Then acceleration is the change in velocity (represented by v) divided by the change in time (represented by t): acceleration = v t | text | null |
L_0817 | calculating acceleration from velocity and time | T_4189 | Look at the cyclist in the Figure 1.1. With the help of gravity, he speeds up as he goes downhill on a straight part of the trail. His velocity changes from 1 meter per second at the top of the hill to 6 meters per second by the time he reaches the bottom. If it takes him 5 seconds to reach the bottom, what is his average acceleration as he races down the hill? v t 6 m/s 1 m/s = 5s 5 m/s = 5s 1 m/s = 1s = 1 m/s2 acceleration = In words, this means that for each second the cyclist travels downhill, his velocity (in this case, his speed) increases by 1 meter per second on average. Note that the answer to this problem is expressed in m/s2 , which is the SI unit for acceleration. Q: The cyclist slows down at the end of the race. His velocity changes from 6 m/s to 2 m/s during a period of 4 seconds without any change in direction. What was his average acceleration during these 4 seconds? A: Use the equation given above for acceleration: v t 6 m/s 2 m/s = 4s 4 m/s = 4s 1 m/s = 1s = 1 m/s2 acceleration = | text | null |
L_0819 | calculating work | T_4195 | Work is the use of force to move an object. It is directly related to both the force applied to the object and the distance the object moves. Work can be calculated with this equation: Work = Force x Distance. | text | null |
L_0819 | calculating work | T_4196 | The equation for work can be used to calculate work if force and distance are known. To use the equation, force is expressed in Newtons (N), and distance is expressed in meters (m). For example, assume that Clarissa uses 100 Newtons of force to push the mower and that she pushes it for a total of 200 meters as she cuts the grass in her grandmothers yard. Then, the amount of work Clarissa does is: Work = 100 N 200 m = 20,000 N m Notice that the unit for work in the answer is the Newton meter (N m). This is the SI unit for work, also called the joule (J). One joule equals the amount of work that is done when 1 N of force moves an object over a distance of 1 m. Q: After Clarissa mows her grandmothers lawn, she volunteers to mow a neighbors lawn as well. If she pushes the mower with the same force as before and moves it over a total of 234 meters, how much work does she do mowing the neighbors lawn? A: The work Clarissa does can be calculated as: Work = 100 N 234 m = 23,400 N m, or 23,400 J | text | null |
L_0819 | calculating work | T_4197 | The work equation given above can be rearranged to find force or distance if the other variables are known: Force = Work Distance Distance = Work Force After Clarissa finishes mowing both lawns, she pushes the lawn mower down the sidewalk to her own house. If she pushes the mower over a distance of 30 meters and does 2700 joules of work, how much force does she use? Substitute the known values into the equation for force: J Force = 2700 30 m = 90 N Q: When Clarissa gets back to her house, she hangs the 200-Newton lawn mower on some hooks in the garage (see the Figure 1.1). To lift the mower, she does 400 joules of work. How far does she lift the mower to hang it? A: Substitute the known values into the equation for distance: | text | null |
L_0820 | carbohydrate classification | T_4198 | Carbohydrates are one of four classes of biochemical compounds. The other three classes are proteins, lipids, and nucleic acids. In addition to cellulose, carbohydrates include sugars and starches. Carbohydrate molecules contain atoms of carbon, hydrogen, and oxygen. Living things use carbohydrates mainly for energy. Q: Which carbohydrates do you use for energy? A: You may eat a wide variety of carbohydratesfrom sugars in fruits to starches in potatoes. However, body cells use only sugars for energy. | text | null |
L_0820 | carbohydrate classification | T_4199 | Sugars are simple carbohydrates. Molecules of sugars have relatively few carbon atoms. Glucose (C6 H12 O6 ) is one of the smallest sugar molecules. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose obtain it by consuming plants or these other organisms. In the Figure 1.1, you can see structural formulas for glucose and two other sugars, named fructose and sucrose. Fructose is a sugar that is found in fruits. It is an isomer of glucose. Isomers are compounds that have the same atoms but different arrangements of atoms. Do you see how the atoms are arranged differently in fructose than in glucose? Youre probably most familiar with the sugar sucrose, because sucrose is table sugar. Its the sugar that you spoon onto your cereal or into your iced tea. Q: Compare the structure of sucrose with the structures of glucose and fructose. How is sucrose related to the other two sugars? A: Sucrose consists of one molecule of glucose and one molecule of fructose bonded together. | text | null |
L_0820 | carbohydrate classification | T_4200 | Starches are complex carbohydrates. They are polymers of glucose. A polymer is a large molecule that consists of many smaller, repeating molecules, called monomers. The monomers are joined together by covalent bonds. Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers get starches by eating plants. Common sources of starches in the human diet are pictured in the Figure 1.2. Our digestive system breaks down starches to sugar, which our cells use for energy. | text | null |
L_0820 | carbohydrate classification | T_4201 | Cellulose is another complex carbohydrate that is a polymer of glucose. However, glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers, as you can see in the Figure 1.3. Have you ever eaten raw celery? If you have, then you probably noticed that Foods that are good sources of starches. the stalks contain long, stringy fibers. The fibers are mostly cellulose. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. Cellulose also provides needed fiber in the human diet. We cant digest cellulose, but it helps keep food wastes moving through the digestive tract. | text | null |
L_0820 | carbohydrate classification | T_4201 | Cellulose is another complex carbohydrate that is a polymer of glucose. However, glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers, as you can see in the Figure 1.3. Have you ever eaten raw celery? If you have, then you probably noticed that Foods that are good sources of starches. the stalks contain long, stringy fibers. The fibers are mostly cellulose. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. Cellulose also provides needed fiber in the human diet. We cant digest cellulose, but it helps keep food wastes moving through the digestive tract. | text | null |
L_0821 | carbon bonding | T_4202 | Carbon is a very common ingredient of matter because it can combine with itself and with many other elements. It can form a great diversity of compounds, ranging in size from just a few atoms to thousands of atoms. There are millions of known carbon compounds, and carbon is the only element that can form so many different compounds. | text | null |
L_0821 | carbon bonding | T_4203 | Carbon is a nonmetal in group 14 of the periodic table. Like other group 14 elements, carbon has four valence electrons. Valence electrons are the electrons in the outer energy level of an atom that are involved in chemical bonds. The valence electrons of carbon are shown in the electron dot diagram in the Figure 1.1. Q: How many more electrons does carbon need to have a full outer energy level? A: Carbon needs four more valence electrons, or a total of eight valence electrons, to fill its outer energy level. A full outer energy level is the most stable arrangement of electrons. Q: How can carbon achieve a full outer energy level? A: Carbon can form four covalent bonds. Covalent bonds are chemical bonds that form between nonmetals. In a covalent bond, two atoms share a pair of electrons. By forming four covalent bonds, carbon shares four pairs of electrons, thus filling its outer energy level and achieving stability. | text | null |
L_0821 | carbon bonding | T_4204 | A carbon atom can form covalent bonds with other carbon atoms or with the atoms of other elements. Carbon often forms bonds with hydrogen. Compounds that contain only carbon and hydrogen are called hydrocarbons. Methane (CH4 ), which is modeled in the Figure 1.2, is an example of a hydrocarbon. In methane, a single carbon atom forms covalent bonds with four hydrogen atoms. The diagram on the left in the Figure 1.2 shows all the shared valence electrons. The diagram on the right in the Figure 1.2, called a structural formula, represents each pair of shared electrons with a dash (-). Methane (CH4 ) | text | null |
L_0821 | carbon bonding | T_4205 | Carbon can form single, double, or even triple bonds with other carbon atoms. In a single bond, two carbon atoms share one pair of electrons. In a double bond, they share two pairs of electrons, and in a triple bond they share three pairs of electrons. Examples of compounds with these types of bonds are represented by the structural formulas in the Figure 1.3. Q: How many bonds do the carbon atoms share in each of these compounds? A: In ethane, the two carbon atoms share a single bond. In ethene they share a double bond, and in ethyne they share a triple bond. | text | null |
L_0822 | carbon monomers and polymers | T_4206 | Carbon has a unique ability to form covalent bonds with many other atoms. It can bond with other carbon atoms as well as with atoms of other elements. Because of this ability, carbon often forms polymers. A polymer is a large molecule that is made out of many smaller molecules that are joined together by covalent bonds. The smaller, repeating molecules are called monomers. (The prefix mono- means one and the prefix poly- means many.) Polymers may consist of just one type of monomer or of more than one type. Polymers are similar to the strings of beads pictured in the Figure 1.1. Like beads on a string, monomers in a polymer may be all the same or different from one another. | text | null |
L_0822 | carbon monomers and polymers | T_4207 | Many polymers of carbon occur naturally. Two examples are rubber and cellulose. Rubber is a natural polymer of the monomer named isoprene (C5 H8 ). This polymer comes from rubber trees, which grow in tropical areas. Structural formulas for rubber and isoprene are shown in the Figure 1.2. Note that just a small section of the rubber polymer is represented by the structural formula. Cellulose is a natural polymer of the monomer named glucose (C6 H12 O6 ). This polymer makes up the cell walls of plants and is the most common compound in living things. Structural formulas for cellulose and glucose are also shown in the Figure 1.2). As you can see from the structural formula for cellulose, when two glucose monomers bond together, a molecule of water (H2 O) is released. Q: How are the glucose molecules arranged in the cellulose polymer? A: The glucose molecules alternate between right-side up and upside down. | text | null |
L_0822 | carbon monomers and polymers | T_4207 | Many polymers of carbon occur naturally. Two examples are rubber and cellulose. Rubber is a natural polymer of the monomer named isoprene (C5 H8 ). This polymer comes from rubber trees, which grow in tropical areas. Structural formulas for rubber and isoprene are shown in the Figure 1.2. Note that just a small section of the rubber polymer is represented by the structural formula. Cellulose is a natural polymer of the monomer named glucose (C6 H12 O6 ). This polymer makes up the cell walls of plants and is the most common compound in living things. Structural formulas for cellulose and glucose are also shown in the Figure 1.2). As you can see from the structural formula for cellulose, when two glucose monomers bond together, a molecule of water (H2 O) is released. Q: How are the glucose molecules arranged in the cellulose polymer? A: The glucose molecules alternate between right-side up and upside down. | text | null |
L_0822 | carbon monomers and polymers | T_4208 | Synthetic carbon polymers are produced in labs or factories. Plastics are common examples of synthetic carbon polymers. You are probably familiar with the plastic called polyethylene. All of the plastic items pictured in the Figure 1.3 are made of polyethylene. It consists of repeating monomers of ethylene (C2 H4 ). Structural formulas for ethylene and polyethylene are also shown in the Figure 1.4. Click image to the left or use the URL below. URL: | text | null |
L_0822 | carbon monomers and polymers | T_4208 | Synthetic carbon polymers are produced in labs or factories. Plastics are common examples of synthetic carbon polymers. You are probably familiar with the plastic called polyethylene. All of the plastic items pictured in the Figure 1.3 are made of polyethylene. It consists of repeating monomers of ethylene (C2 H4 ). Structural formulas for ethylene and polyethylene are also shown in the Figure 1.4. Click image to the left or use the URL below. URL: | text | null |
L_0823 | catalysts | T_4209 | A catalyst is a substance that increases the rate of a chemical reaction. The presence of a catalyst is one of several factors that influence the rate of chemical reactions. (Other factors include the temperature, concentration, and surface area of reactants.) A catalyst isnt a reactant in the chemical reaction it speeds up. As a result, it isnt changed or used up in the reaction, so it can go on to catalyze many more reactions. Q: How is a catalyst like a tunnel through a mountain? A: Like a tunnel through a mountain, a catalyst provides a faster pathway for a chemical reaction to occur. | text | null |
L_0823 | catalysts | T_4210 | Catalysts interact with reactants so the reaction can occur by an alternate pathway that has a lower activation energy. Activation energy is the energy needed to start a reaction. When activation energy is lower, more reactant particles have enough energy to react so the reaction goes faster. Many catalysts work like the one in the Figure 1.1. The catalyst brings the reactants together by temporarily bonding with them. This makes it easier and quicker for the reactants to react together. Q: In the Figure 1.1, look at the energy needed in the catalytic and non-catalytic pathways of the reaction. How does the amount of energy compare? How does this affect the reaction rate along each pathway? A: The catalytic pathway of the reaction requires far less energy. Therefore, the reaction will occur faster by this pathway because more reactants will have enough energy to react. | text | null |
L_0823 | catalysts | T_4211 | Chemical reactions constantly occur inside living things. Many of these reactions require catalysts so they will occur quickly enough to support life. Catalysts in living things are called enzymes. Enzymes may be extremely effective. A reaction that takes a split second to occur with an enzyme might take many years without it! More than 1000 different enzymes are necessary for human life. Many enzymes are needed for the digestion of food. An example is amylase, which is found in the mouth and small intestine. Amylase catalyzes the breakdown of starch to sugar. You can see how it affects the rate of starch digestion in the Figure 1.2. A: The starches in the cracker start to break down to sugars with the help of the enzyme amylase. Try this yourself and see if you can taste the reaction. | text | null |
L_0824 | cellular respiration reactions | T_4212 | Cellular Respiration is the process in which the cells of living things break down the organic compound glucose with oxygen to produce carbon dioxide and water. The overall chemical equation for cellular respiration is: C6 H12 O6 + 6O2 6CO2 + 6H2 O As the Figure 1.1 shows, cellular respiration occurs in the cells of all kinds of organisms, including those that make their own food (autotrophs) as well as those that get their food by consuming other organisms (heterotrophs). Q: How is cellular respiration related to breathing? A: Breathing consists of inhaling and exhaling, and its purpose is to move gases into and out of the body. Oxygen needed for cellular respiration is brought into the body with each inhalation. Carbon dioxide and water vapor produced by cellular respiration are released from the body with each exhalation. | text | null |
L_0824 | cellular respiration reactions | T_4213 | The reactions of cellular respiration are catabolic reactions. In catabolic reactions, bonds are broken in larger molecules and energy is released. In cellular respiration, bonds are broken in glucose, and this releases the chemical energy that was stored in the glucose bonds. Some of this energy is converted to heat. The rest of the energy is used to form many small molecules of a compound called adenosine triphosphate, or ATP. ATP molecules contain just the right amount of stored chemical energy to power biochemical reactions inside cells. Click image to the left or use the URL below. URL: | text | null |
L_0828 | chemical bond | T_4220 | A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds. Q: Why do you think that chemical bonds form? A: Chemical bonds form because they give atoms a more stable arrangement of electrons. | text | null |
L_0828 | chemical bond | T_4221 | To understand why chemical bonds form, consider the common compound known as water, or H2 O. It consists of two hydrogen (H) atoms and one oxygen (O) atom. As you can see in the on the left side of the Figure 1.1, each hydrogen atom has just one electron, which is also its sole valence electron. The oxygen atom has six valence electrons. These are the electrons in the outer energy level of the oxygen atom. In the water molecule on the right in the Figure 1.1, each hydrogen atom shares a pair of electrons with the oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. The hydrogen atoms each have a pair of shared electrons, so their first and only energy level is full. The oxygen atom has a total of eight valence electrons, so its outer energy level is full. A full outer energy level is the most stable possible arrangement of electrons. It explains why elements form chemical bonds with each other. | text | null |
L_0828 | chemical bond | T_4222 | Not all chemical bonds form in the same way as the bonds in water. There are actually three different types of chemical bonds, called covalent, ionic, and metallic bonds. Each type of bond is described below. Click image to the left or use the URL below. URL: A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented in the Figure 1.1 contains covalent bonds. An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals instead of molecules. Table salt contains ionic bonds. A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround itboth its own valence electrons and those of other ions of the same metal. The ions and electrons form a lattice-like structure. Only metals, such as the copper pictured in the Figure 1.2, form metallic bonds. | text | null |
L_0830 | chemical equations | T_4226 | A chemical equation is a shorthand way to sum up what occurs in a chemical reaction. The general form of a chemical equation is: Reactants Products The reactants in a chemical equation are the substances that begin the reaction, and the products are the substances that are produced in the reaction. The reactants are always written on the left side of the equation and the products on the right. The arrow pointing from left to right shows that the reactants change into the products during the reaction. This happens when chemical bonds break in the reactants and new bonds form in the products. As a result, the products are different chemical substances than the reactants that started the reaction. Q: What is the general equation for the reaction in which iron rusts? A: Iron combines with oxygen to produce rust, which is the compound named iron oxide. This reaction could be represented by the general chemical equation below. Note that when there is more than one reactant, they are separated by plus signs (+). If more than one product were produced, plus signs would be used between them as well. Iron + Oxygen Iron Oxide | text | null |
L_0830 | chemical equations | T_4227 | When scientists write chemical equations, they use chemical symbols and chemical formulas instead of names to represent reactants and products. Look at the chemical reaction illustrated in the Figure 1.1. In this reaction, carbon reacts with oxygen to produce carbon dioxide. Carbon is represented by the chemical symbol C. The chemical symbol for oxygen is O, but pure oxygen exists as diatomic (two-atom) molecules, represented by the chemical formula O2 . A molecule of the compound carbon dioxide consists of one atom of carbon and two atoms of oxygen, so carbon dioxide is represented by the chemical formula CO2 . Q: What is the chemical equation for this reaction? A: The chemical equation is: C + O2 CO2 Q: How have the atoms of the reactants been rearranged in the products of the reaction? What bonds have been broken, and what new bonds have formed? A: Bonds between the oxygen atoms in the oxygen molecule have been broken, and new bonds have formed between the carbon atom and the two oxygen atoms. | text | null |
L_0830 | chemical equations | T_4228 | All chemical equations, like equations in math, must balance. This means that there must be the same number of each type of atom on both sides of the arrow. Thats because matter is always conserved in a chemical reaction. This is the law of conservation of mass. Look at the equation above for the reaction between carbon and oxygen in the formation of carbon dioxide. Count the number of atoms of each type. Are the numbers the same on both sides of the arrow? The answer is yes, so the equation is balanced. | text | null |
L_0830 | chemical equations | T_4229 | Lets return to the chemical reaction in which iron (Fe) combines with oxygen (O2 ) to form rust, or iron oxide (Fe2 O3 ). The equation for this reaction is: 4Fe+ 3O2 2Fe2 O3 This equation illustrates the use of coefficients to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula that shows how many atoms or molecules of the substance are involved in the reaction. From the equation for rusting, you can see that four atoms of iron combine with three molecules of oxygen to form two molecules of iron oxide. Q: Is the equation for the rusting reaction balanced? How can you tell? A: Yes, the equation is balanced. You can tell because there is the same number of each type of atom on both sides of the arrow. First count the iron atoms. There are four iron atoms in the reactants. There are also four iron atoms in the products (two in each of the two iron oxide molecules). Now count the oxygen atoms. There are six on each side of the arrow, confirming that the equation is balanced in terms of oxygen as well as iron. | text | null |
L_0831 | chemical formula | T_4230 | In a chemical formula, the elements in a compound are represented by their chemical symbols, and the ratio of different elements is represented by subscripts. Consider the compound water as an example. Each water molecule contains two hydrogen atoms and one oxygen atom. Therefore, the chemical formula for water is: H2 O The subscript 2 after the H shows that there are two atoms of hydrogen in the molecule. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used in the chemical formula. | text | null |
L_0831 | chemical formula | T_4231 | The Table 1.1 shows four examples of compounds and their chemical formulas. The first two compounds are ionic compounds, and the second two are covalent compounds. Each formula shows the ratio of ions or atoms that make up the compound. Name of Compound Type of Compound Sodium chloride ionic Calcium iodide ionic Hydrogen peroxide covalent Carbon dioxide covalent Ratio of Ions or Atoms of Each Element 1 sodium ion (Na+ ) 1 chloride ion (Cl ) 1 calcium ion (Ca2+ ) 2 io- dide ions (I ) 2 hydrogen atoms (H) 2 oxygen atoms (O) 1 carbon atom (C) 2 oxy- gen atoms (O) Chemical Formulas NaCl CaI2 H2 O2 CO2 There is a different rule for writing the chemical formula for each type of compound. Ionic compounds are compounds in which positive metal ions and negative nonmetal ions are joined by ionic bonds. In these compounds, the chemical symbol for the positive metal ion is written first, followed by the symbol for the negative nonmetal ion. Click image to the left or use the URL below. URL: Q: The ionic compound lithium fluoride consists of a ratio of one lithium ion (Li+ ) to one fluoride ion (F ). What is the chemical formula for this compound? A: The chemical formula is LiF. Covalent compounds are compounds in which nonmetals are joined by covalent bonds. In these compounds, the element that is farther to the left in the periodic table is written first, followed by the element that is farther to the right. If both elements are in the same group of the periodic table, the one with the higher period number is written first. Click image to the left or use the URL below. URL: Q: A molecule of the covalent compound nitrogen dioxide consists of one nitrogen atom (N) and two oxygen atoms (O). What is the chemical formula for this compound? A: The chemical formula is NO2 . | text | null |
L_0833 | chemical reaction overview | T_4235 | A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. Chemical reactions are represented by chemical equations, like the one below, in which reactants (on the left) are connected by an arrow to products (on the right). Reactants Products Chemical reactions may occur quickly or slowly. Look at the two pictures in the Figure 1.1. Both represent chemical reactions. In the picture on the left, a reaction inside a fire extinguisher causes foam to shoot out of the extinguisher. This reaction occurs almost instantly. In the picture on the right, a reaction causes the iron tool to turn to rust. This reaction occurs very slowly. In fact, it might take many years for all of the iron in the tool to turn to rust. Q: What happens during a chemical reaction? Where do the reactants go, and where do the products come from? A: During a chemical reaction, chemical changes take place. Some chemical bonds break and new chemical bonds form. | text | null |
L_0833 | chemical reaction overview | T_4236 | The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. This happens because chemical bonds break in the reactants and new chemical bonds form in the products. Consider the chemical reaction in which water forms from oxygen and hydrogen gases. The Figure 1.2 represents this reaction. Bonds break in molecules of hydrogen and oxygen, and then new bonds form in molecules of water. In both reactants and products there are four hydrogen atoms and two oxygen atoms, but the atoms are combined differently in water. | text | null |
L_0833 | chemical reaction overview | T_4237 | The chemical reaction in the Figure 1.2, in which water forms from hydrogen and oxygen, is an example of a synthesis reaction. In this type of reaction, two or more reactants combine to synthesize a single product. There are several other types of chemical reactions, including decomposition, replacement, and combustion reactions. The Table 1.1 compares these four types of chemical reactions. Type of Reaction Synthesis Decomposition General Equation A+B C AB A + B Example 2Na + Cl2 2NaCl 2H2 O 2H2 + O2 Type of Reaction Single Replacement Double Replacement Combustion General Equation A+BC B+ AC AB+ CD AD + CB fuel + oxygen carbon dioxide + water Example 2K + 2H2 O 2KOH + H2 NaCl+ AgF NaF + AgCl CH4 + 2O2 CO2 + 2H2 O Q: The burning of wood is a chemical reaction. Which type of reaction is it? A: The burning of woodor of anything elseis a combustion reaction. In the combustion example in the table, the fuel is methane gas (CH4 ). Click image to the left or use the URL below. URL: | text | null |
L_0833 | chemical reaction overview | T_4238 | All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In terms of energy, there are two types of chemical reactions: endothermic reactions and exothermic reactions. In exothermic reactions, more energy is released when bonds form in products than is used to break bonds in reactants. These reactions release energy to the environment, often in the form of heat or light. In endothermic reactions, more energy is used to break bonds in reactants than is released when bonds form in products. These reactions absorb energy from the environment. Q: When it comes to energy, which type of reaction is the burning of wood? Is it an endothermic reaction or an exothermic reaction? How can you tell? A: The burning of wood is an exothermic reaction. You can tell by the heat and light energy given off by a wood fire. | text | null |
L_0834 | chemical reaction rate | T_4239 | How fast a chemical reaction occurs is called the reaction rate. Several factors affect the rate of a given chemical reaction. They include the: temperature of reactants. concentration of reactants. surface area of reactants. presence of a catalyst. | text | null |
L_0834 | chemical reaction rate | T_4240 | When the temperature of reactants is higher, the rate of the reaction is faster. At higher temperatures, particles of reactants have more energy, so they move faster. As a result, they are more likely to bump into one another and to collide with greater force. For example, food spoils because of chemical reactions, and these reactions occur faster at higher temperatures (see the bread on the left in the Figure 1.1). This is why we store foods in the refrigerator or freezer (like the bread on the right in the Figure 1.1). The lower temperature slows the rate of spoilage. Left image: Bread after 1 month on a warm countertop. Right image: Bread after 1 month in a cold refrigerator. | text | null |
L_0834 | chemical reaction rate | T_4241 | Concentration is the number of particles of a substance in a given volume. When the concentration of reactants is higher, the reaction rate is faster. At higher concentrations, particles of reactants are crowded closer together, so they are more likely to collide and react. Did you ever see a sign like the one in the Figure 1.2? You might see it where someone is using a tank of pure oxygen for a breathing problem. Combustion, or burning, is a chemical reaction in which oxygen is a reactant. A greater concentration of oxygen in the air makes combustion more rapid if a fire starts burning. Q: It is dangerous to smoke or use open flames when oxygen is in use. Can you explain why? A: Because of the higher-than-normal concentration of oxygen, the flame of a match, lighter, or cigarette could spread quickly to other materials or even cause an explosion. | text | null |
L_0834 | chemical reaction rate | T_4242 | When a solid substance is involved in a chemical reaction, only the matter at the surface of the solid is exposed to other reactants. If a solid has more surface area, more of it is exposed and able to react. Therefore, increasing the surface area of solid reactants increases the reaction rate. Look at the hammer and nails pictured in the Figure 1.3. Both are made of iron and will rust when the iron combines with oxygen in the air. However, the nails have a greater surface area, so they will rust faster. | text | null |
L_0834 | chemical reaction rate | T_4243 | Some reactions need extra help to occur quickly. They need another substance called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction. A catalyst isnt a reactant, so it isnt changed or used up in the reaction. Therefore, it can catalyze many other reactions. | text | null |
L_0835 | chemistry of compounds | T_4244 | A compound is a unique substance that forms when two or more elements combine chemically. Compounds form as a result of chemical reactions. The elements in compounds are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions that share or transfer valence electrons. Click image to the left or use the URL below. URL: Water is an example of a common chemical compound. As you can see in the Figure 1.1, each water molecule consists of two atoms of hydrogen and one atom of oxygen. Water always has this 2:1 ratio of hydrogen to oxygen. Like water, all compounds consist of a fixed ratio of elements. It doesnt matter how much or how little of a compound there is. It always has the same composition. Q: Sometimes the same elements combine in different ratios. How can this happen if a compound always consists of the same elements in the same ratio? A: If the same elements combine in different ratios, they form different compounds. | text | null |
L_0835 | chemistry of compounds | T_4245 | Look at the Figure 1.2 of water (H2 O) and hydrogen peroxide (H2 O2 ), and read about these two compounds. Both compounds consist of hydrogen and oxygen, but they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Q: Read the Figure 1.3 about carbon dioxide (CO2 ) and carbon monoxide (CO). Both compounds consist of carbon and oxygen, but in different ratios. How can you tell that carbon dioxide and carbon monoxide are different compounds? Carbon Dioxide: Every time you exhale, you release carbon dioxide into the air. Its an odorless, colorless gas. Car- bon dioxide contributes to global climate change, but it isnt directly harmful to hu- man health. Carbon Monoxide: Carbon monoxide is produced when matter burns. Its a colorless, odorless gas that is very harmful to human health. In fact, it can kill people in minutes. Because you cant see or smell carbon monoxide, it must be detected with an alarm. | text | null |
L_0835 | chemistry of compounds | T_4245 | Look at the Figure 1.2 of water (H2 O) and hydrogen peroxide (H2 O2 ), and read about these two compounds. Both compounds consist of hydrogen and oxygen, but they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Q: Read the Figure 1.3 about carbon dioxide (CO2 ) and carbon monoxide (CO). Both compounds consist of carbon and oxygen, but in different ratios. How can you tell that carbon dioxide and carbon monoxide are different compounds? Carbon Dioxide: Every time you exhale, you release carbon dioxide into the air. Its an odorless, colorless gas. Car- bon dioxide contributes to global climate change, but it isnt directly harmful to hu- man health. Carbon Monoxide: Carbon monoxide is produced when matter burns. Its a colorless, odorless gas that is very harmful to human health. In fact, it can kill people in minutes. Because you cant see or smell carbon monoxide, it must be detected with an alarm. | text | null |
L_0835 | chemistry of compounds | T_4246 | There are two basic types of compounds that differ in the nature of the bonds that hold their atoms or ions together. They are covalent and ionic compounds. Both types are described below. Click image to the left or use the URL below. URL: Covalent compounds consist of atoms that are held together by covalent bonds. These bonds form between nonmetals that share valence electrons. Covalent compounds exist as individual molecules. Water is an example of a covalent compound. Ionic compounds consist of ions that are held together by ionic bonds. These bonds form when metals transfer electrons to nonmetals. Ionic compounds exist as a matrix of many ions, called a crystal. Sodium chloride (table salt) is an example of an ionic compound. | text | null |
L_0836 | color | T_4247 | Visible light is light that has wavelengths that can be detected by the human eye. The wavelength of visible light determines the color that the light appears. As you can see in the Figure 1.1, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between are all the other colors of light that we can see. Only seven main colors of light are actually represented in the diagram. | text | null |
L_0836 | color | T_4248 | A prism, like the one in the Figure 1.2, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass or plastic. Matter that is transparent allows light to pass through it. A prism transmits light but slows it down. When light passes from air to the glass of the prism, the change in speed causes the light to change direction and bend. Different wavelengths of light bend at different angles. This makes the beam of light separate into light of different wavelengths. What we see is a rainbow of colors. Q: Look back at the rainbow that opened this article. Do you see all the different colors of light, from red at the top to violet at the bottom? What causes a rainbow to form? A: Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow of colors. | text | null |
L_0836 | color | T_4248 | A prism, like the one in the Figure 1.2, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass or plastic. Matter that is transparent allows light to pass through it. A prism transmits light but slows it down. When light passes from air to the glass of the prism, the change in speed causes the light to change direction and bend. Different wavelengths of light bend at different angles. This makes the beam of light separate into light of different wavelengths. What we see is a rainbow of colors. Q: Look back at the rainbow that opened this article. Do you see all the different colors of light, from red at the top to violet at the bottom? What causes a rainbow to form? A: Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow of colors. | text | null |
L_0836 | color | T_4249 | An opaque object is one that doesnt let light pass through it. Instead, it reflects or absorbs the light that strikes it. Many objects, such as the leaves pictured in the Figure 1.3, reflect just one or a few wavelengths of visible light and absorb the rest. The wavelengths that are reflected determine the color that an object appears to the human eye. For example, the leaves appear green because they reflect green light and absorb light of other wavelengths. A transparent or translucent material, such as window glass, transmits some or all of the light that strikes it. This means that the light passes through the material rather than being reflected by it. In this case, we see the material because of the transmitted light. Therefore, the wavelength of the transmitted light determines the color that the object appears. Look at the beautiful stained glass windows in the Figure 1.4. The different colors of glass transmit The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes green leaves, the blue light is absorbed and no light is reflected. Q: What color do you see if an object absorbs all of the light that strikes it? A: When all of the light is absorbed, none is reflected, so the object looks black. But black isnt a color of light. Black is the absence of light. | text | null |
L_0836 | color | T_4249 | An opaque object is one that doesnt let light pass through it. Instead, it reflects or absorbs the light that strikes it. Many objects, such as the leaves pictured in the Figure 1.3, reflect just one or a few wavelengths of visible light and absorb the rest. The wavelengths that are reflected determine the color that an object appears to the human eye. For example, the leaves appear green because they reflect green light and absorb light of other wavelengths. A transparent or translucent material, such as window glass, transmits some or all of the light that strikes it. This means that the light passes through the material rather than being reflected by it. In this case, we see the material because of the transmitted light. Therefore, the wavelength of the transmitted light determines the color that the object appears. Look at the beautiful stained glass windows in the Figure 1.4. The different colors of glass transmit The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes green leaves, the blue light is absorbed and no light is reflected. Q: What color do you see if an object absorbs all of the light that strikes it? A: When all of the light is absorbed, none is reflected, so the object looks black. But black isnt a color of light. Black is the absence of light. | text | null |
L_0836 | color | T_4250 | The human eye can distinguish only red, green, and blue light. These three colors are called the primary colors of light. All other colors of light can be created by combining the primary colors. Look at the Venn diagram 1.5. Red and green light combine to form yellow light. Red and blue light combine to form magenta light, and blue and green light combine to form cyan light. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram, where all three primary colors of light combine. The result is white light. | text | null |
L_0836 | color | T_4251 | Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is the dark green pigment called chlorophyll, which is found in plants. Chlorophyll absorbs all but green wavelengths of visible light. Pigments are also found in many manufactured products. They are used to color paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary colors of pigments are the same as the secondary colors of light: cyan, magenta, and yellow. Q: A color printer needs just three colors of ink to print all of the colors that we can see. Which colors are they? A: The three colors of ink in a color printer are the three primary pigment colors: cyan, magenta, and yellow. These three colors can be combined in different ratios to produce all other colors, so they are the only colors needed for full-color printing. | text | null |
L_0837 | combining forces | T_4252 | You have probably heard of the famous equation E = mc2 . The "E" represent the amount of energy. The "m" represents mass. The "c" represent the speed of light. Writing a "c" is much easier than writing the actual speed of light. The speed of light is a really large number. The speed of light is about 300 million meters per second. Thats really, really fast. Light always travels at the same speed through space. In outer space, there is not any matter to get in its way. Think about riding your bicycle. When you ride on a hard surface, it is easy to pedal. You can go really fast. Imagine how your speed would change if you were riding through deep sand. You would find it hard to pedal. You would not be able to go as fast. The same is true for light. When there is no matter around, like in outer space, it can go fast. When matter gets in its way, it slows down. Light travels through some matter faster than through others. Table 1.1 gives the speed of light in six common materials. Material Air Water Glass Vegetable oil Alcohol Diamond Speed of Light (m/s) 299 million meters per second 231 million meters per second 200 million meters per second 150 million meters per second 140 million meters per second 125 million meters per second No matter how slow light travels, it still goes really, really fast. The important thing to remember is that it does travel. It is hard for us to imagine light taking time to cover a distance. Think about when you enter your science classroom. You step through the door. You tell your teacher, "Hello." You walk to your desk and sit down. It may take around 10 to 20 seconds to walk this distance. Imagine now your teacher turns the light off. She carries a small lamp over to the door you just entered. She asks you to watch carefully as she switches on the light. She flips the switch and you immediately see the light. The light just covered the same distance you just walked. Thats how fast light is. For us, it is hard to imagine that it moves. Now lets think about light traveling between the Sun and Earth. The Sun is 93 million miles away. What if we were able to turn off the Sun for just a second? How long would it take us to notice? Would we notice instantly like in the classroom? Remember, the Sun is a long way away. We wouldnt notice the change for a little over 8 minutes. That is because the Sun is a long way away. Even when moving as fast as light, it takes time to travel from the Sun to Earth. What do you think happens when it hits the air in our atmosphere? Air is made up of matter. When light travels through matter it slows down. How do scientists know it slows down? What evidence do scientists have? When sunlight hits Earths atmosphere it bends just a little. If sunlight goes through water droplets it bends even more. The bending of light through droplets of water is why we can see rainbows. It also explains why the straw in a glass of water appears to be broken. | text | null |
L_0837 | combining forces | T_4253 | When light passes from one medium (or type of matter) to another, it changes speed. You can actually see this happen. If light strikes a new substance at an angle, the light appears to bend. This is what explains the straw looking broken in the picture above. So, does light always bend as it travels into a new medium? If light travels straight into a new substance it is not bent. You may know this angle as perpendicular. The light still slows down, just does not appear to bend. Any angle other than perpendicular the light will bend as it slows down. The bending of light is called refraction. Figure 1.1 shows how refraction occurs. Notice that the angle of light changes again as it passes from the glass back to the air. In this case, the speed increases, and the ray of light resumes its initial direction. For a more detailed explanation of refraction, watch this video: Click image to the left or use the URL below. URL: | text | null |
L_0838 | combustion reactions | T_4254 | A combustion reaction occurs when a substance reacts quickly with oxygen (O2 ). For example, in the Figure usually referred to as fuel. The products of a complete combustion reaction include carbon dioxide (CO2 ) and water vapor (H2 O). The reaction typically gives off heat and light as well. The general equation for a complete combustion reaction is: Fuel + O2 CO2 + H2 O The burning of charcoal is a combustion reaction. | text | null |
L_0838 | combustion reactions | T_4255 | The fuel that burns in a combustion reaction contains compounds called hydrocarbons. Hydrocarbons are compounds that contain only carbon (C) and hydrogen (H). The charcoal pictured in the Figure 1.1 consists of hydrocarbons. So do fossil fuels such as natural gas. Natural gas is a fuel that is commonly used in home furnaces and gas stoves. The main component of natural gas is the hydrocarbon called methane (CH4 ). You can see a methane flame in the Figure 1.2. The combustion of methane is represented by the equation: CH4 + 2O2 CO2 + 2H2 O The combustion of methane gas heats a pot on a stove. Q: Sometimes the flame on a gas stove isnt just blue but has some yellow or orange in it. Why might this occur? A: If the flame isnt just blue, the methane isnt getting enough oxygen to burn completely, leaving some of the carbon unburned. The flame will also not be as hot as a completely blue flame for the same reason. | text | null |
L_0840 | compound machine | T_4258 | A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. You can read below about two examplesthe wheelbarrow and corkscrew. Other compound machines, such as bicycles, consist of many simple machines. Big compound machines such as cars may consist of hundreds or even thousands of simple machines. | text | null |
L_0840 | compound machine | T_4259 | Look at the wheelbarrow in the Figure 1.1. It is used to carry heavy objects. It consists of two simple machines: a lever and a wheel and axle. Effort is applied to the lever by picking up the handles of the wheelbarrow. The lever, in turn, applies upward force to the load. The force is increased by the lever, making the load easier to lift. Effort is applied to the wheel of the wheelbarrow by pushing it over the ground. The rolling wheel turns the axle and increases the force, making it easier to push the load. | text | null |
L_0840 | compound machine | T_4260 | The corkscrew in the Figure 1.2 is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction. | text | null |
L_0840 | compound machine | T_4260 | The corkscrew in the Figure 1.2 is also a compound machine. It is used to pierce a cork and pull it out of the neck of a bottle. It consists of a screw and two levers. Turning the handle on top twists the screw down into the center of the cork. Then, pushing down on the two levers causes the screw to pull upward, bringing the cork with it. The levers increase the force and change its direction. | text | null |
L_0840 | compound machine | T_4261 | Friction is a force that opposes motion between any surfaces that are touching. All machines have moving parts and friction, so they have to use some of the work that is applied to them to overcome friction. This makes all machines less than 100 percent efficient. Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of many simple machines, friction can become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts of a machine so they slide over each other more easily. This is how friction is reduced in a car engine. | text | null |
L_0840 | compound machine | T_4262 | The mechanical advantage of a machine is the factor by which it changes the force applied to the machine. Many machines increase the force applied to them, and this is how they make work easier. Compound machines tend to have a greater mechanical advantage than simple machines. Thats because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater its mechanical advantage tends to be. Q: Assume that the lever and the wheel and axle of a wheelbarrow each have a mechanical advantage of 2. What is the mechanical advantage of the wheelbarrow? A: The mechanical advantage of the wheelbarrow is the product of the mechanical advantage of the lever (2) and the mechanical advantage of the wheel and axle (2). Therefore, the wheelbarrow has a mechanical advantage of 4. | text | null |
L_0841 | compounds | T_4263 | A compound is a unique substance that forms when two or more elements combine chemically. For example, the compound carbon dioxide forms when one atom of carbon (grey in the model above) combines with two atoms of oxygen (red in the model). Another example of a compound is water. It forms when two hydrogen atoms combine with one oxygen atom. Click image to the left or use the URL below. URL: Q: How could a water molecule be represented? A: It could be represented by a model like the one for carbon dioxide in the opening image. You can see a sample Figure 1.1. A model of water. Two things are true of all compounds: A compound always has the same elements in the same proportions. For example, carbon dioxide always has two atoms of oxygen for each atom of carbon, and water always has two atoms of hydrogen for each atom of oxygen. A compound always has the same composition throughout. For example, all the carbon dioxide in the atmosphere and all the water in the ocean have these same proportions of elements. Q: How do you think the properties of compounds compare with the properties of the elements that form them? A: You might expect the properties of a compound to be similar to the properties of the elements that make up the compound. But you would be wrong. | text | null |
L_0841 | compounds | T_4264 | The properties of compounds are different from the properties of the elements that form themsometimes very different. Thats because elements in a compound combine and become an entirely different substance with its own unique properties. Do you put salt on your food? Table salt is the compound sodium chloride. It contains sodium and chlorine. As shown in the Figure 1.2, sodium is a solid that reacts explosively with water, and chlorine is a poisonous gas. But together in table salt, sodium and chlorine form a harmless unreactive compound that you can safely eat. Q: The compound sodium chloride is very different from the elements sodium and chlorine that combine to form it. What are some properties of sodium chloride? A: Sodium chloride is an odorless white solid that is harmless unless consumed in large quantities. In fact, it is a necessary component of the human diet. | text | null |
L_0841 | compounds | T_4265 | Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure 1.3. It is made up of many sodium and chloride ions. Sodium and chlorine combine to form sodium chloride, or table salt. A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework. Click image to the left or use the URL below. URL: Compounds such as carbon dioxide and water form molecules instead of crystals. A molecule is the smallest particle of a compound that still has the compounds properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. | text | null |
L_0841 | compounds | T_4265 | Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure 1.3. It is made up of many sodium and chloride ions. Sodium and chlorine combine to form sodium chloride, or table salt. A sodium chloride crystal consists of many sodium ions (blue) and chloride ions (green) arranged in a rigid framework. Click image to the left or use the URL below. URL: Compounds such as carbon dioxide and water form molecules instead of crystals. A molecule is the smallest particle of a compound that still has the compounds properties. It consists of two or more atoms bonded together. You saw models of carbon dioxide and water molecules above. | text | null |
L_0843 | conservation of energy in chemical reactions | T_4269 | All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Like the combustion reaction in a furnace, some chemical reactions require less energy to break bonds in reactants than is released when bonds form in products. These reactions, called exothermic reactions, release energy. In other chemical reactions, it takes more energy to break bonds in reactants than is released when bonds form in products. These reactions, called endothermic reactions, absorb energy. | text | null |
L_0843 | conservation of energy in chemical reactions | T_4270 | Whether a chemical reaction absorbs or releases energy, there is no overall change in the amount of energy during the reaction. Thats because energy cannot be created or destroyed. This is the law of conservation of energy. Energy may change form during a chemical reactionfor example, from chemical energy to heat energy when gas burns in a furnacebut the same amount of energy remains after the reaction as before. This is true of all chemical reactions. Q: If energy cant be destroyed during a chemical reaction, what happens to the energy that is absorbed in an endothermic reaction? A: The energy is stored in the bonds of the products as chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. This is represented by the graph on the left in the Figure 1.1. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. You can see this in the graph on the right in the Figure 1.1. Note: H represents the change in en- ergy. Q: What happens to the excess energy in the reactants of an exothermic reaction? A: The excess energy is generally released to the surroundings when the reaction occurs. In a home heating system, for example, the energy that is released during combustion in the furnace is used to heat the home. | text | null |
L_0845 | conservation of mass and energy in nuclear reactions | T_4273 | Einsteins equation is possibly the best-known equation of all time. Theres reason for that. The equation is incredibly important. It changed how scientists view energy and matter, which are two of the most basic concepts in all of science. The equation shows that energy and matter are two forms of the same thing. This new idea turned science upside down when Einstein introduced it in the early 1900s. Amazingly, the idea has withstood the test of time as more and more evidence has been gathered to support it. You can listen to an explanation of Einsteins equation at URL: https://youtu.be/hW7DW9NIO9M Q: What do the letters in Einsteins equation stand for? A: E stands for energy, m stands for mass, and c stands for the speed of light. The speed of light is 300,000 kilometers (186,000 miles) per second, so c2 is a very big number. Therefore, the amount of energy in even a small mass of matter is tremendous. Suppose, for example, that you have 1 gram of matter. Thats about the mass of a paperclip. Multiplying this mass by c2 would yield enough energy to power 3,600 homes for a year! | text | null |
L_0845 | conservation of mass and energy in nuclear reactions | T_4274 | Einsteins equation helps scientists understand what happens in nuclear reactions and why they produce so much energy. When the nucleus of a radioisotope undergoes fission or fusion in a nuclear reaction, it loses a tiny amount of mass. What happens to the lost mass? It isnt really lost at all. It is converted to energy. How much energy? E = mc2 . The change in mass is tiny, but it results in a great deal of energy. Q: In a nuclear reaction, mass decreases and energy increases. What about the laws of conservation of mass and conservation of energy? Are mass and energy not conserved in nuclear reactions? Do we need to throw out these laws when it comes to nuclear reactions? A: No, the laws still apply. However, its more correct to say that the sum of mass and energy is always conserved in a nuclear reaction. Mass changes to energy, but the total amount of mass and energy combined remains the same. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4275 | A chemical reaction occurs when some substances change chemically to other substances. Chemical reactions are represented by chemical equations. Consider a simple chemical reaction, the burning of methane. In this reaction, methane (CH4 ) combines with oxygen (O2 ) in the air and produces carbon dioxide (CO2 ) and water vapor (H2 O). The reaction is represented by the following chemical equation: CH4 + 2O2 CO2 + 2H2 O This equation shows that one molecule of methane combines with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water vapor. All chemical equations must be balanced. This means that the same number of each type of atom must appear on both sides of the arrow. Q: Is the chemical equation for the burning of methane balanced? Count the atoms of each type on both sides of the arrow to find out. A: Yes, the equation is balanced. There is one carbon atom on both sides of the arrow. There are also four hydrogen atoms and four oxygen atoms on both sides of the arrow. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4276 | Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. | text | null |
L_0846 | conservation of mass in chemical reactions | T_4277 | How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in the Figure 1.1. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was the discovery of oxygen. Click image to the left or use the URL below. URL: Q: Lavoisier carried out his experiments inside a sealed glass jar. Why was sealing the jar important for his results? What might his results have been if he hadnt sealed the jar? A: Sealing the jar was important so that any gases produced in the reactions were captured and could be measured. If he hadnt sealed the jar, gases might have escaped detection. Then his results would have shown that there was less mass after the reactions than before. In other words, he would not have been able to conclude that mass is conserved in chemical reactions. | text | null |
L_0847 | convection | T_4278 | Convection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid). Thermal energy is the total kinetic energy of moving particles of matter, and the transfer of thermal energy is called heat. Convection is one of three ways that thermal energy can be transferred (the other ways are conduction and thermal radiation). Thermal energy is always transferred from matter with a higher temperature to matter with a lower temperature. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0847 | convection | T_4279 | The Figure 1.1 shows how convection occurs, using hot water in a pot as an example. When particles in one area of a fluid (in this case, the water at the bottom of the pot) gain thermal energy, they move more quickly, have more collisions, and spread farther apart. This decreases the density of the particles, so they rise up through the fluid. As they rise, they transfer their thermal energy to other particles of the fluid and cool off in the process. With less energy, the particles move more slowly, have fewer collisions, and move closer together. This increases their density, so they sink back down through the fluid. When they reach the bottom of the fluid, the cycle repeats. The result is a loop of moving particles called a convection current. | text | null |
L_0847 | convection | T_4280 | Convection currents transfer thermal energy through many fluids, not just hot water in a pot. For example, convection currents transfer thermal energy through molten rock below Earths surface, through water in the oceans, and through air in the atmosphere. Convection currents in the atmosphere create winds. You can see one way this happens in the Figure 1.2. The land heats up and cools off faster than the water because it has lower specific heat. Therefore, the land gets warmer during the day and cooler at night than the water does. During the day, warm air rises above the land and cool air from the water moves in to take its place. During the night, the opposite happens. Warm air rises above the water and cool air from the land moves out to take its place. Q: During the day, in which direction is thermal energy of the air transferred? In which direction is it transferred during the night? A: During the day, thermal energy is transferred from the air over the land to the air over the water. During the night, thermal energy is transferred in the opposite direction. | text | null |
L_0848 | cooling systems | T_4281 | A refrigerator is an example of a cooling system. Another example is an air conditioner. The purpose of any cooling system is to transfer thermal energy in order to keep things cool. A refrigerator, for example, transfers thermal energy from the cool air inside the refrigerator to the warm air in the kitchen. If youve ever noticed how warm the back of a running refrigerator gets, then you know that it releases a lot of thermal energy into the room. Q: Thermal energy always moves from a warmer area to a cooler area. How can thermal energy move from the cooler air inside a refrigerator to the warmer air in a room? A: The answer is work. | text | null |
L_0848 | cooling systems | T_4282 | A refrigerator must do work to reverse the normal direction of thermal energy flow. Work involves the use of force to move something, and doing work takes energy. In a refrigerator, the energy is usually provided by electricity. You can read in detail in the Figure 1.1 how a refrigerator does its work. | text | null |
L_0848 | cooling systems | T_4283 | The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance such as FreonTM that has a low boiling point and changes between liquid and gaseous states as it passes through the refrigerator. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it transfers thermal energy to the warm air outside the refrigerator and changes back to a liquid. Work is done by a refrigerator to move the refrigerant through the different components of the refrigerator. | text | null |
L_0849 | covalent bonding | T_4284 | A covalent bond is the force of attraction that holds together two atoms that share a pair of valence electrons. The shared electrons are attracted to the nuclei of both atoms. This forms a molecule consisting of two or more atoms. Covalent bonds form only between atoms of nonmetals. | text | null |
L_0849 | covalent bonding | T_4285 | The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the Figure 1.1. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the Figure 1.2 shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so its not a new compound. Oxygen normally occurs in diatomic (two-atom) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. | text | null |
L_0849 | covalent bonding | T_4285 | The two atoms that are held together by a covalent bond may be atoms of the same element or different elements. When atoms of different elements form covalent bonds, a new substance, called a covalent compound, results. Water is an example of a covalent compound. A water molecule is modeled in the Figure 1.1. A molecule is the smallest particle of a covalent compound that still has the properties of the compound. Q: How many valence electrons does the oxygen atom (O) share with each hydrogen atom (H)? How many covalent bonds hold the water molecule together? A: The oxygen atom shares one pair of valence electrons with each hydrogen atom. Each pair of shared electrons represents one covalent bond, so two covalent bonds hold the water molecule together. The diagram in the Figure 1.2 shows an example of covalent bonds between two atoms of the same element, in this case two atoms of oxygen. The diagram represents an oxygen molecule, so its not a new compound. Oxygen normally occurs in diatomic (two-atom) molecules. Several other elements also occur as diatomic molecules: hydrogen, nitrogen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). Q: How many electrons do these two oxygen atoms share? How many covalent bonds hold the oxygen molecule together? A: The two oxygen atoms share two pairs of electrons, so two covalent bonds hold the oxygen molecule together. | text | null |
L_0849 | covalent bonding | T_4286 | Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the oxygen atoms in the Figure 1.2. Alone, each oxygen atom has six valence electrons. By sharing two pairs of valence electrons, each oxygen atom has a total of eight valence electrons. This fills its outer energy level, giving it the most stable arrangement of electrons. The shared electrons are attracted to both oxygen nuclei, and this force of attraction holds the two atoms together in the oxygen molecule. | text | null |
L_0850 | crystalline carbon | T_4287 | Graphite is one of three forms of crystalline, or crystal-forming, carbon. Carbon also exists in an amorphous, or shapeless, form in substances such as coal and charcoal. Different forms of the same element are called allotropes. Besides graphite, the other allotropes of crystalline carbon are diamond and fullerenes. All three forms exist as crystals rather than molecules. In a crystal, many atoms are bonded together in a repeating pattern that may contains thousands of atoms. The arrangement of atoms in the crystal differs for each form of carbon and explains why the different forms have different properties. Click image to the left or use the URL below. URL: Q: How do you think the properties of diamond might differ from the properties of graphite? A: Diamond is clear whereas graphite is black. Diamond is also very hard, so it doesnt break easily. Graphite, in contrast, is soft and breaks very easily. | text | null |
L_0850 | crystalline carbon | T_4288 | Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure 1.1). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure 1.2). | text | null |
L_0850 | crystalline carbon | T_4288 | Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure 1.1). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure 1.2). | text | null |
L_0850 | crystalline carbon | T_4289 | Graphite is a form of crystalline carbon in which each carbon atom is covalently bonded to three other carbon atoms. The carbon atoms are arranged in layers, with strong bonds within each layer but only weak bonds between layers (see Figure 1.3). The weak bonds between layers allow the layers to slide over one another, so graphite is relatively soft and slippery. This makes it useful as a lubricant. Q: Why do graphites properties make it useful for pencil leads? A: Being slippery, graphite slides easily over paper when you write. Being soft, it rubs off on the paper, allowing you to leave marks. Graphites softness also allows you to sharpen it easily. (Imagine trying to sharpen a diamond!) | text | null |
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