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L_0785 | chemical reactions and energy | T_4063 | Any factor that helps reactants come together so they can react lowers the amount of activation energy needed to start the reaction. If the activation energy is lowered, more reactant particles can react, and the reaction occurs more quickly. How fast a reaction occurs is called the reaction rate. Factors that affect the reaction rate include: temperature of reactants concentration of reactants surface area of reactants presence of catalysts | text | null |
L_0785 | chemical reactions and energy | T_4064 | 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. They are more likely to bump into one another and to collide with greater force. For example, when you fry an egg, turning up the heat causes the egg to cook faster. The same principle explains why storing food in a cold refrigerator reduces the rate at which food spoils (see Figure 8.16). Both food frying and food spoiling are chemical reactions that happen faster at higher temperatures. | text | null |
L_0785 | chemical reactions and energy | T_4065 | 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 Figure 8.17? You might see it where someone is using a tank of pure oxygen for a breathing problem. The greater concentration of oxygen in the air makes combustion rapid if a fire starts burning. | text | null |
L_0785 | chemical reactions and energy | T_4066 | 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. For example, crushing a solid into a powder exposes more of the substance to other reactants. This may greatly speed up the reaction. You can see another example in Figure 8.18. Iron rusts when it combines with oxygen in the air. The iron hammer head and iron nails will both rust eventually. Which will rust faster? | text | null |
L_0785 | chemical reactions and energy | T_4067 | 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 but is not changed or used up in the reaction. The catalyst can go on to catalyze many more reactions. Catalysts are not reactants, but they help reactants come together so they can react. You can see one way this happens in the animation at the URL below. By helping reactants come together, a catalyst decreases the activation energy needed to start a chemical reaction. This speeds up the reaction. Living things depend on catalysts to speed up many chemical reactions inside their cells. 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 billions of years without it! | text | null |
L_0786 | properties of carbon | T_4068 | Carbon is a nonmetal in group 14 of the periodic table. Like other group 14 compounds, 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 Figure 9.1. | text | null |
L_0786 | properties of carbon | T_4069 | Because it has four valence electrons, carbon needs four more electrons to fill its outer energy level. It can achieve this by forming 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. A carbon atom can form bonds with other carbon atoms or with the atoms of other elements. Carbon often forms bonds with hydrogen. You can see an example in Figure 9.2. The compound represented in the figure is methane (CH4 ). The carbon atom in a methane molecule forms bonds with four hydrogen atoms. The diagram on the left shows all the shared electrons. The diagram on the right represents each pair of shared electrons with a dash (). This type of diagram is called a structural formula. | text | null |
L_0786 | properties of carbon | T_4070 | 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 shown in Figure 9.3. | text | null |
L_0786 | properties of carbon | T_4071 | Because of carbons ability to form so many covalent bonds, it often forms polymers. A polymer is a large molecule that consists of many smaller molecules joined together by covalent bonds. The smaller 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 a little like the strings of beads in Figure 9.4. What do the individual beads represent? Many polymers occur naturally. You will read about natural polymers in this chapters "Hydrocarbons" and "Carbon and Living Things" lessons. Other polymers are synthetic. This means that they are produced in labs or factories. Synthetic polymers are created in synthesis reactions in which monomers bond together to form much larger compounds. Plastics are examples of synthetic polymers. The plastic items in Figure 9.5 are all made of polythene (also called polyethylene). It consists of repeating monomers of ethene (C2 H4 ). To learn more about polymers and how they form, go to this URL: (2:13). | text | null |
L_0786 | properties of carbon | T_4071 | Because of carbons ability to form so many covalent bonds, it often forms polymers. A polymer is a large molecule that consists of many smaller molecules joined together by covalent bonds. The smaller 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 a little like the strings of beads in Figure 9.4. What do the individual beads represent? Many polymers occur naturally. You will read about natural polymers in this chapters "Hydrocarbons" and "Carbon and Living Things" lessons. Other polymers are synthetic. This means that they are produced in labs or factories. Synthetic polymers are created in synthesis reactions in which monomers bond together to form much larger compounds. Plastics are examples of synthetic polymers. The plastic items in Figure 9.5 are all made of polythene (also called polyethylene). It consists of repeating monomers of ethene (C2 H4 ). To learn more about polymers and how they form, go to this URL: (2:13). | text | null |
L_0786 | properties of carbon | T_4072 | Exploratorium Staff Scientist Julie Yu changes and manipulates the physical and chemical properties of plastic bottles by exposing them to heat. This is how plastic bags and bottles can be recycled and used over and over again. For more information on properties of plastic, see http://science.kqed.org/quest/video/quest-lab-properties-of-plas MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0786 | properties of carbon | T_4073 | Pure carbon can exist in different forms, depending on how its atoms are arranged. The forms include diamond, graphite, and fullerenes. All three forms exist as crystals, but they have different structures. Their different structures, in turn, give them different properties. You can learn more about them in Table 9.1. atoms affect the properties of the substances formed? Structure Diamond crystal Description Diamond Diamond is a form of carbon in which each carbon atom is bonded to four other carbon atoms. This forms a strong, rigid, three- dimensional structure. Diamond is the hardest natural substance. It is used for cutting and grinding tools as well as for rings and other pieces of jewelry. Graphite Graphite is a form of carbon in which carbon atoms are arranged in layers. Bonds are strong between carbon atoms within each layer but relatively weak between atoms in different layers. The weak bonds between layers allow the layers to slide over one another. This makes graphite relatively soft and slippery. It is used as a lubricant. It also makes up the "lead" in pencils. Fullerene A fullerene (also called a bucky- ball) is a form of carbon in which carbon atoms are arranged in hol- low spheres. Each carbon atom is bonded to three others by sin- gle covalent bonds. The pattern of atoms resembles the pattern on the surface of a soccer ball. Fullerenes were first discovered in 1985. They have been found in soot and me- teorites. Possible commercial uses of fullerenes are under investiga- tion. To learn how this form of carbon got its funny names, go to this URL: This metal cutter has a diamond blade. | text | null |
L_0787 | hydrocarbons | T_4074 | Hydrocarbons are compounds that contain only carbon and hydrogen. Hydrocarbons are the simplest type of carbon-based compounds. Nonetheless, they can vary greatly in size. The smallest hydrocarbons have just one or two carbon atoms, but large hydrocarbons may have hundreds. The size of hydrocarbon molecules influences their properties. For example, it influences their boiling and melting points. As a result, some hydrocarbons are gases at room temperature, while others are liquids or solids. Hydrocarbons are generally nonpolar and do not dissolve in water. In fact, they tend to repel water. Thats why they are used in floor wax and similar products. Hydrocarbons can be classified in two basic classes. The classes are saturated hydrocarbons and unsaturated hydrocarbons. This classification is based on the number of bonds between carbon atoms. You can learn more about both types of hydrocarbons at this URL: (6:41). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0787 | hydrocarbons | T_4075 | Saturated hydrocarbons contain only single bonds between carbon atoms. They are the simplest hydrocarbons. They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible. In other words, the carbon atoms are saturated with hydrogen. You can see an example of a saturated hydrocarbon in Figure Saturated hydrocarbons are given the general name of alkanes. The name of specific alkanes always ends in -ane. The first part of the name indicates how many carbon atoms each molecule of the alkane has. The smallest alkane is methane. It has just one carbon atom. The next largest is ethane, with two carbon atoms. The chemical formulas and properties of methane, ethane, and several other alkanes are listed in Table 9.2. The boiling and melting points of alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally have higher boiling and melting points. This table shows only alkanes with relatively few carbon atoms. Some alkanes have many more carbon atoms. What properties might larger alkanes have? For example, do you think that any of them might be solids? Alkane Methane Ethane Propane Butane Pentane Hexane Heptane Octane Chemical Formula CH4 C2 H6 C3 H8 C4 H10 C5 H12 C6 H14 C7 H16 C8 H18 Boiling Point (C) -162 -89 -42 0 36 69 98 126 Melting Point (C) -183 -172 -188 -138 -130 -95 -91 -57 State (at 20C) gas gas gas gas liquid liquid liquid liquid | text | null |
L_0787 | hydrocarbons | T_4076 | Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different shapes, or structures. Hydrocarbons may form straight chains, branched chains, or rings. Figure 9.8 shows an example of an alkane with each shape. In straight-chain molecules, all the carbon atoms are lined up in a row like cars of a train. They form what is called the backbone of the molecule. In branched-chain molecules, at least one of the carbon atoms branches off to the side from the backbone. In cyclic molecules, the chain of carbon atoms is joined at the two ends to form a ring. | text | null |
L_0787 | hydrocarbons | T_4077 | Even compounds with the same number of carbon and hydrogen atoms can have different shapes. These compounds are called isomers. Look at the examples in Figure 9.9. The figure shows the structural formulas of butane and its isomer iso-butane. Both molecules have four carbon atoms and ten hydrogen atoms (C4 H10 ), but the atoms are arranged differently. Butane is a straight-chain molecule. Iso-butane is branched. You can see three-dimensional models of these two isomers at the URLs below. You can rotate the molecule models to get a better idea of their shapes. | text | null |
L_0787 | hydrocarbons | T_4078 | Ring-shaped alkanes are called cycloalkanes. They usually contain just five or six carbon atoms because larger rings are not very stable. However, rings can join together to create larger molecules consisting of two or more rings. Compared with the straight- and branched-chain alkanes, cycloalkanes have higher boiling and melting points. | text | null |
L_0787 | hydrocarbons | T_4079 | Unsaturated hydrocarbons contain at least one double or triple bond between carbon atoms. As a result, the carbon atoms are unable to bond with as many hydrogen atoms as they would if they were joined only by single bonds. This makes them unsaturated with hydrogen. Unsaturated hydrocarbons are classified on the basis of their bonds as alkenes, alkynes, or aromatic hydrocarbons. | text | null |
L_0787 | hydrocarbons | T_4080 | Unsaturated hydrocarbons that contain at least one double bond are called alkenes. The name of a specific alkene always ends in ene, with a prefix indicating the number of carbon atoms. Figure 9.10 shows the structural formula for the smallest alkene. It has just two carbon atoms and is named ethene. Ethene is produced by most fruits and vegetables. It speeds up ripening and also rotting. Figure 9.11 shows the effects of ethene on bananas. Like alkanes, alkenes can have different shapes. They can form straight chains, branched chains, or rings. Alkenes can also form isomers, or compounds with the same atoms but different shapes. Generally, the physical properties of alkenes are similar to those of alkanes. Smaller alkenes, such as ethene, have relatively high boiling and melting points. They are gases at room temperature. Larger alkenes have lower boiling and melting points. They are liquids or waxy solids at room temperature. | text | null |
L_0787 | hydrocarbons | T_4081 | Unsaturated hydrocarbons that contain at least one triple bond are called alkynes. The name of specific alkynes always end in yne, with a prefix for the number of carbon atoms. Figure 9.12 shows the smallest alkyne, called ethyne, which has just two carbon atoms. Ethyne is also called acetylene. It is burned in acetylene torches, like the one in Figure 9.13. Acetylene produces so much heat when it burns that it can melt metal. Breaking all those bonds between carbon atoms releases a lot of energy. Alkynes may form straight or branched chains. They rarely occur as cycloalkynes. In fact, alkynes of all shapes are relatively rare, at least in nature. | text | null |
L_0787 | hydrocarbons | T_4081 | Unsaturated hydrocarbons that contain at least one triple bond are called alkynes. The name of specific alkynes always end in yne, with a prefix for the number of carbon atoms. Figure 9.12 shows the smallest alkyne, called ethyne, which has just two carbon atoms. Ethyne is also called acetylene. It is burned in acetylene torches, like the one in Figure 9.13. Acetylene produces so much heat when it burns that it can melt metal. Breaking all those bonds between carbon atoms releases a lot of energy. Alkynes may form straight or branched chains. They rarely occur as cycloalkynes. In fact, alkynes of all shapes are relatively rare, at least in nature. | text | null |
L_0787 | hydrocarbons | T_4081 | Unsaturated hydrocarbons that contain at least one triple bond are called alkynes. The name of specific alkynes always end in yne, with a prefix for the number of carbon atoms. Figure 9.12 shows the smallest alkyne, called ethyne, which has just two carbon atoms. Ethyne is also called acetylene. It is burned in acetylene torches, like the one in Figure 9.13. Acetylene produces so much heat when it burns that it can melt metal. Breaking all those bonds between carbon atoms releases a lot of energy. Alkynes may form straight or branched chains. They rarely occur as cycloalkynes. In fact, alkynes of all shapes are relatively rare, at least in nature. | text | null |
L_0787 | hydrocarbons | T_4082 | Unsaturated cyclic hydrocarbons are called aromatic hydrocarbons. Thats because they have a strong aroma, or scent. Their molecules consist of six carbon atoms in a ring shape, connected by alternating single and double bonds. Aromatic hydrocarbons may have a single ring or multiple rings joined together by bonds between their carbon atoms. Benzene is the smallest aromatic hydrocarbon. It has just one ring. You can see its structural formula in Figure 9.14. Benzene has many uses. For example, it is used in air fresheners and mothballs because of its strong scent. You can learn more about benzene and other aromatic hydrocarbons at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0787 | hydrocarbons | T_4083 | It is hard to overstate the importance of hydrocarbons to modern life. Hydrocarbons have even been called the driving force of western civilization. You saw some ways they are used in Figure 9.6. Several other ways are illustrated in Figure 9.15. Their most important use is as fuels. Gasoline, natural gas, fuel oil, diesel fuel, jet fuel, coal, kerosene, and propane are just some of the hydrocarbon compounds that are burned for fuel. Hydrocarbons are also used to manufacture many products, including plastics and synthetic fabrics such as polyester. The main source of hydrocarbons is fossil fuels coal, petroleum, and natural gas. Fossil fuels form over hundreds of millions of years when dead organisms are covered with sediments and put under great pressure. Giant ferns in ancient swamps turned into coal deposits. Dead organisms in ancient seas gradually formed deposits of petroleum and natural gas. You can read more about these sources of hydrocarbons in the chapter Introduction to Energy and at the URL below. | text | null |
L_0788 | carbon and living things | T_4084 | A biochemical compound is any carbon-based compound found in living things. Like hydrocarbons, all biochemi- cal compounds contain hydrogen as well as carbon. However, biochemical compounds also contain other elements, such as oxygen and nitrogen. Almost all biochemical compounds are polymers. They consist of many, smaller monomer molecules. Biochemical polymers are referred to as macromolecules. The prefix macro means "large," and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. Biochemical compounds make up the cells and tissues of organisms. They are also involved in life processes, such as making and using food for energy. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. However, they can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in Table 9.3 and described in the rest of this lesson. Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starches cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids Functions provide energy to cells store energy in plants makes up the cell walls of plants speed up biochemical re- actions regulate life processes fats oils store energy in animals store energy in plants DNA RNA stores genetic information in cells helps cells make proteins | text | null |
L_0788 | carbon and living things | T_4085 | Carbohydrates are biochemical compounds that include sugars, starches, and cellulose. They contain oxygen in addition to carbon and hydrogen. Organisms use carbohydrates mainly for energy. | text | null |
L_0788 | carbon and living things | T_4086 | Sugars are simple carbohydrates. Molecules of sugar have just a few carbon atoms. The simplest sugar is glucose (C6 H12 O6 ). Glucose is the sugar that the cells of living things use for energy. 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. You can see the structural formula of glucose and two other sugars in Figure 9.16. The other sugars in the figure are fructose and sucrose. Fructose is an isomer of glucose. It is found in fruits. It has the same atoms as glucose, but they are arranged differently. Sucrose is table sugar. It consists of one molecule of glucose and one molecule of fructose. | text | null |
L_0788 | carbon and living things | T_4087 | Starches are complex carbohydrates. They are polymers of glucose. They consist of hundreds of glucose monomers bonded together. Plants make starch to store extra sugars. Consumers get starch from plants. Common sources of starch in the human diet are pictured in Figure 9.17. Our digestive system breaks down starch to simple sugars, which our cells use for energy. | text | null |
L_0788 | carbon and living things | T_4087 | Starches are complex carbohydrates. They are polymers of glucose. They consist of hundreds of glucose monomers bonded together. Plants make starch to store extra sugars. Consumers get starch from plants. Common sources of starch in the human diet are pictured in Figure 9.17. Our digestive system breaks down starch to simple sugars, which our cells use for energy. | text | null |
L_0788 | carbon and living things | T_4088 | Cellulose is another complex carbohydrate that is a polymer of glucose. However, the glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers (see Figure 9.18). Have you ever eaten raw celery? If you have, then you probably noticed that 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 trunks and stems. 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_0788 | carbon and living things | T_4089 | Proteins are biochemical compounds that contain oxygen, nitrogen, and sulfur in addition to carbon and hydrogen. Protein molecules consist of one or more chains of small molecules called amino acids. | text | null |
L_0788 | carbon and living things | T_4090 | Amino acids are the "building blocks" of proteins. There are 20 different common amino acids. The structural formula of the simplest amino acid, called glycine, is shown in Figure 9.19. Other amino acids have a similar structure. The sequence of amino acids and the number of amino acid chains in a protein determine the proteins shape. The shape of a protein, in turn, determines its function. Shapes may be very complex. You can learn more about the structure of proteins at the URL below. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0788 | carbon and living things | T_4091 | Proteins are the most common biochemicals. They have many different functions, including: making up tissues as components of muscle. speeding up biochemical reactions as enzymes. regulating life processes as hormones. helping defend against infections as antibodies. transporting materials as components of the blood (see the example in Figure 9.20). | text | null |
L_0788 | carbon and living things | T_4092 | Lipids are biochemical compounds such as fats and oils. Organisms use lipids to store energy. In addition to carbon and hydrogen, lipids contain oxygen. | text | null |
L_0788 | carbon and living things | T_4093 | Lipids are made up of long carbon chains called fatty acids. Like hydrocarbons, fatty acids may be saturated or unsaturated. Figure 9.21 shows structural formulas for two small fatty acids. One is saturated and one is unsaturated. In saturated fatty acids, there are only single bonds between carbon atoms. As a result, the carbons are saturated with hydrogen atoms. Saturated fatty acids are found in fats. Fats are solid lipids that animals use to store energy. In unsaturated fatty acids, there is at least one double bond between carbon atoms. As a result, some carbons are not bonded to as many hydrogen atoms as possible. Unsaturated fatty acids are found in oils. Oils are liquid lipids that plants use to store energy. | text | null |
L_0788 | carbon and living things | T_4094 | Some lipids contain the element phosphorus as well as oxygen, carbon, and hydrogen. These lipids are called phospholipids. Two layers of phospholipid molecules make up most of the cell membrane in the cells of living things. Figure 9.22 shows how phospholipid molecules are arranged in a cell membrane. One end (the head) of each phospholipid molecule is polar and attracts water. This end is called hydrophilic ("water loving"). The other end (the tail) is nonpolar and repels water. This end is called hydrophobic ("water hating"). The nonpolar tails are on the inside of the membrane. The polar heads are on the outside of the membrane. These differences in polarity allow some molecules to pass through the membrane while keeping others out. You can see how this works in the video at the URL below. | text | null |
L_0788 | carbon and living things | T_4095 | Nucleic acids are biochemical molecules that contain oxygen, nitrogen, and phosphorus in addition to carbon and hydrogen. There are two main types of nucleic acids. They are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). | text | null |
L_0788 | carbon and living things | T_4096 | Nucleic acids consist of chains of small molecules called nucleotides. The structure of a nucleotide is shown in Figure 9.23. Each nucleotide contains a phosphate group (PO4 ), a sugar (C5 H8 O4 ) in DNA, and a nitrogen- containing base. (A base is a compound that is not neither acidic nor neutral.) There are four different nitrogenous bases in DNA. They are adenine, thymine, guanine, and cytosine. In RNA, the only difference is that thymine is replaced with a different base, uracil. DNA consists of two long chains of nucleotides. Nitrogen bases on the two chains form hydrogen bonds with each other. Adenine always bonds with thymine, and guanine always bonds with cytosine. These bonds hold the two chains together and give DNA is characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 9.24. Sugars and phosphate groups form the "backbone" of each chain of DNA. The bonded bases are called base pairs. RNA, in contrast to DNA, consists of just one chain of nucleotides. Determining the structure of DNA was a big scientific breakthrough. You can read the interesting story of its discovery at the URL below. | text | null |
L_0788 | carbon and living things | T_4096 | Nucleic acids consist of chains of small molecules called nucleotides. The structure of a nucleotide is shown in Figure 9.23. Each nucleotide contains a phosphate group (PO4 ), a sugar (C5 H8 O4 ) in DNA, and a nitrogen- containing base. (A base is a compound that is not neither acidic nor neutral.) There are four different nitrogenous bases in DNA. They are adenine, thymine, guanine, and cytosine. In RNA, the only difference is that thymine is replaced with a different base, uracil. DNA consists of two long chains of nucleotides. Nitrogen bases on the two chains form hydrogen bonds with each other. Adenine always bonds with thymine, and guanine always bonds with cytosine. These bonds hold the two chains together and give DNA is characteristic double helix, or spiral, shape. You can see the shape of the DNA molecule in Figure 9.24. Sugars and phosphate groups form the "backbone" of each chain of DNA. The bonded bases are called base pairs. RNA, in contrast to DNA, consists of just one chain of nucleotides. Determining the structure of DNA was a big scientific breakthrough. You can read the interesting story of its discovery at the URL below. | text | null |
L_0788 | carbon and living things | T_4097 | DNA stores genetic information in the cells of all living things. It contains the genetic code. This is the code that instructs cells how to make proteins. The instructions are encoded in the sequence of nitrogen bases in the nucleotide chains of DNA. RNA "reads" the genetic code in DNA and is involved in the synthesis of proteins based on the code. This video shows how: (2:51). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0789 | biochemical reactions | T_4098 | Most of the energy used by living things comes either directly or indirectly from the sun. Sunlight provides the energy for photosynthesis. This is the process in which plants and certain other organisms (see Figure 9.26) synthesize glucose (C6 H12 O6 ). The process uses carbon dioxide and water and also produces oxygen. The overall chemical equation for photosynthesis is: 6CO2 + 6H2 O + Light Energy ! C6 H12 O6 + 6O2 Photosynthesis changes light energy to chemical energy. The chemical energy is stored in the bonds of glucose molecules. Glucose is used for energy by the cells of almost all living things. Plants make their own glucose. Other organisms get glucose by consuming plants (or organisms that consume plants). How do living things get energy from glucose? The answer is cellular respiration. | text | null |
L_0789 | biochemical reactions | T_4098 | Most of the energy used by living things comes either directly or indirectly from the sun. Sunlight provides the energy for photosynthesis. This is the process in which plants and certain other organisms (see Figure 9.26) synthesize glucose (C6 H12 O6 ). The process uses carbon dioxide and water and also produces oxygen. The overall chemical equation for photosynthesis is: 6CO2 + 6H2 O + Light Energy ! C6 H12 O6 + 6O2 Photosynthesis changes light energy to chemical energy. The chemical energy is stored in the bonds of glucose molecules. Glucose is used for energy by the cells of almost all living things. Plants make their own glucose. Other organisms get glucose by consuming plants (or organisms that consume plants). How do living things get energy from glucose? The answer is cellular respiration. | text | null |
L_0789 | biochemical reactions | T_4099 | Cellular respiration is the process in which the cells of living things break down glucose with oxygen to produce carbon dioxide, water, and energy. The overall chemical equation for cellular respiration is: C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O + Heat and Chemical Energy Cellular respiration releases some of the energy in glucose as heat. It uses the rest of the energy to form many, even smaller molecules. The smaller molecules contain just the right amount of energy to power chemical reactions inside cells. You can look at cellular respiration in more detail at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0789 | biochemical reactions | T_4100 | Human body temperature must remain within a narrow range around 37C (98.6F). At this temperature, most biochemical reactions would occur too slowly to keep us alive. Thats where enzymes come in. Enzymes are biochemical catalysts. They speed up biochemical reactions, not only in humans but in virtually all living things. Most enzymes are proteins. Two are described in Figure 9.27. | text | null |
L_0790 | acceleration | T_4101 | Acceleration is a measure of the change in velocity of a moving object. It measures the rate at which velocity changes. Velocity, in turn, is a measure of the speed and direction of motion, so a change in velocity may reflect a change in speed, a change in direction, or both. Both velocity and acceleration are vectors. A vector is any measurement that has both size and direction. People commonly think of acceleration as in increase in speed, but a decrease in speed is also acceleration. In this case, acceleration is negative and called deceleration. A change in direction without a change in speed is acceleration as well. Q: Can you think of an example of acceleration that doesnt involve a change in speed? A: Driving at a constant speed around a bend in a road is one example. Use your imagination to think of others. | text | null |
L_0790 | acceleration | T_4102 | You can see several examples of acceleration in the pictures from the Figure 1.1. In each example, velocity is changing but in different ways. For example, direction may be changing but not speed, or vice versa. Figure out what is moving and how its moving in each of the photos. Q: Describe how velocity is changing in each of the motions you identified from the Figure 1.1. A: You should describe how both direction and speed are changing. For example, the boy on the carousel is moving up and down and around in a circle, so his direction is constantly changing, but his speed changes only at the beginning and end of the ride. The skydiver is falling straight down toward the ground so her direction isnt changing, but her speed keeps increasing as she falls until she opens her parachute. | text | null |
L_0790 | acceleration | T_4103 | If you are accelerating, you may be able to feel the change in velocity. This is true whether the change is in speed, direction, or both. You often feel acceleration when you ride in a car. As the car speeds up, you feel as though you are being pressed against the seat. When the car slows down, you feel like you are being pushed forward, especially if the change in speed is sudden. If the car changes direction and turns right, you feel as though you are being pushed to the left. With a left turn, you feel a push to the right. The next time you ride in a car, notice how it feels as the car accelerates in each of these ways. | text | null |
L_0791 | acceleration due to gravity | T_4104 | Gravity is a force that pulls objects down toward the ground. When objects fall to the ground, gravity causes them to accelerate. Acceleration is a change in velocity, and velocity, in turn, is a measure of the speed and direction of motion. Gravity causes an object to fall toward the ground at a faster and faster velocity the longer the object falls. In fact, its velocity increases by 9.8 m/s2, so by 1 second after an object starts falling, its velocity is 9.8 m/s. By 2 seconds after it starts falling, its velocity is 19.6 m/s (9.8 m/s + 9.8 m/s), and so on. The acceleration of a falling object due to gravity is illustrated in the Figure 1.1. Q: In this diagram, the boy drops the object at time t= 0 s. By t = 1 s, the object is falling at a velocity of 9.8 m/s. What is its velocity by t = 5 s? What will its velocity be at t = 6 s if it keeps falling? A: Its velocity at t = 5 s is 49.0 m/s, and at t = 6 s, it will be 58.8 m/s (49.0 m/s + 9.8 m/s). | text | null |
L_0791 | acceleration due to gravity | T_4105 | What if you were to drop a bowling ball and a soccer ball at the same time from the same distance above the ground? The bowling ball has greater mass than the basketball, so the pull of gravity on it is greater. Would it fall to the ground faster? No, the bowling ball and basketball would reach the ground at the same time. The reason? The more massive bowling ball is also harder to move because of its greater mass, so it ends up moving at the same acceleration as the soccer ball. This is true of all falling objects. They all accelerate at the same rate due to gravity, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leafs greater surface area. Q: If a leaf and an acorn were to fall to the ground in the absence of air (that is, in a vacuum), how would this affect their acceleration due to gravity? A: They would both accelerate at the same rate and reach the ground at the same time. | text | null |
L_0792 | accuracy and precision | T_4106 | The accuracy of a measurement is how close the measurement is to the true value. If you were to hit four different golf balls toward an over-sized hole, all of them might land in the hole. These shots would all be accurate because they all landed in the hole. This is illustrated in the sketch below. | text | null |
L_0792 | accuracy and precision | T_4107 | As you can see from the sketch above, the four golf balls did not land as close to one another as they could have. Each one landed in a different part of the hole. Therefore, these shots are not very precise. The precision of measurements is how close they are to each other. If you make the same measurement twice, the answers are precise if they are the same or at least very close to one another. The golf balls in the sketch below landed quite close together in a cluster, so they would be considered precise. However, they are all far from the hole, so they are not accurate. Q: If you were to hit four golf balls toward a hole and your shots were both accurate and precise, where would the balls land? A: All four golf balls would land in the hole (accurate) and also very close to one another (precise). | text | null |
L_0793 | acid base neutralization | T_4108 | An acid is a compound that produces positive hydrogen ions (H+ ) and negative nonmetal ions when it dissolves in water. (Ions are atoms that have become charged by losing or gaining electrons.) Hydrochloric acid (HCl) is an example of an acid. When it dissolves in water, it produces positive hydrogen ions and negative chloride ions (Cl ). This can be represented by the chemical equation: H O 2 HCl H+ + Cl A base is a compound that produces negative hydroxide ions (OH ) and positive metal ions when it dissolves in water. For example, when the base sodium hydroxide (NaOH) dissolves in water, it produces negative hydroxide ions and positive sodium ions (Na+ ). This can be represented by the chemical equation: H O 2 NaOH OH + Na+ Q: If you were to combine acid and base solutions, what products do you think would be produced? A: Combining acid and base solutions produces water and a neutral ionic compound. | text | null |
L_0793 | acid base neutralization | T_4109 | When an acid and a base react, the reaction is called a neutralization reaction. Thats because the reaction produces neutral products. Water is always one product, and a salt is also produced. A salt is a neutral ionic compound. Lets see how a neutralization reaction produces both water and a salt, using as an example the reaction between solutions of hydrochloric acid and sodium hydroxide. The overall equation for this reaction is: NaOH + HCl H2 O and NaCl Now lets break this reaction down into two parts to see how each product forms. Positive hydrogen ions from HCl and negative hydroxide ions from NaOH combine to form water. This part of the reaction can be represented by the equation: H+ + OH H2 O Positive sodium ions from NaOH and negative chloride ions from HCL combine to form the salt sodium chloride (NaCl), commonly called table salt. This part of the reaction can be represented by the equation: Na+ + Cl NaCl Another example of a neutralization reaction can be seen in the Figure 1.1. Q: What products are produced when antacid tablets react with hydrochloric acid in the stomach? A: The products are water and the salt calcium chloride (CaCl2 ). Carbon dioxide (CO2 ) is also produced. The reaction is represented by the chemical equation: CaCO3 + 2HCl H2 O + CaCl2 + CO2 | text | null |
L_0794 | activation energy | T_4110 | Chemical reactions also need energy to be activated. They require a certain amount of energy just to get started. This energy is called activation energy. For example, activation energy is needed to start a car engine. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas wont occur without the spark of energy to begin the reaction. Q: Why is activation energy needed? Why wont a reaction occur without it? A: A reaction wont occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants. | text | null |
L_0794 | activation energy | T_4111 | Some chemical reactions need a constant input of energy to take place. They are called endothermic reactions. Other chemical reactions release energy when they occur, so they can keep going without any added energy. They are called exothermic reactions. Q: It makes sense that endothermic reactions need activation energy. But do exothermic reactions also need activation energy? A: All chemical reactions need energy to get started, even exothermic reactions. Look at the Figure 1.1. They compare energy changes that occur during endothermic and exothermic reactions. From the graphs, you can see that both types of reactions need the same amount of activation energy in order to get started. Only after it starts does the exothermic reaction produce more energy than it uses. | text | null |
L_0794 | activation energy | T_4112 | You have probably used activation energy to start a chemical reaction. For example, if youve ever struck a match to light it, then you provided the activation energy needed to start a combustion reaction. When you struck the match on the box, the friction started the match head burning. Combustion is exothermic. Once a match starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, the match wont burst into flames on its own. | text | null |
L_0796 | alkaline earth metals | T_4116 | Barium (Ba) is one of six elements in group 2 of the periodic table, which is shown in Figure 1.1. Elements in this group are called alkaline Earth metals. These metals are silver or gray in color. They are relatively soft and low in density, although not as soft and lightweight as alkali metals. | text | null |
L_0796 | alkaline earth metals | T_4117 | All alkaline Earth metals have similar properties because they all have two valence electrons. They readily give up their two valence electrons to achieve a full outer energy level, which is the most stable arrangement of electrons. As a result, they are very reactive, although not quite as reactive as the alkali metals in group 1. For example, alkaline Earth metals will react with cold water, but not explosively as alkali metals do. Because of their reactivity, alkaline Earth metals never exist as pure substances in nature. Instead, they are always found combined with other elements. The reactivity of alkaline Earth metals increases from the top to the bottom of the group. Thats because the atoms get bigger from the top to the bottom, so the valence electrons are farther from the nucleus. When valence electrons are farther from the nucleus, they are attracted less strongly by the nucleus and more easily removed from the atom. This makes the atom more reactive. Q: Alkali metals have just one valence electron. Why are alkaline Earth metals less reactive than alkali metals? A: It takes more energy to remove two valence electrons from an atom than one valence electron. This makes alkaline Earth metals with their two valence electrons less reactive than alkali metals with their one valence electron. | text | null |
L_0796 | alkaline earth metals | T_4118 | For a better understanding of alkaline Earth metals, lets take a closer look at two of them: calcium (Ca) and strontium (Sr). Calcium is a soft, gray, nontoxic alkaline Earth metal. Although pure calcium doesnt exist in nature, calcium compounds are very common in Earths crust and in sea water. Calcium is also the most abundant metal in the human body, occurring as calcium compounds such as calcium phosphate and calcium carbonate. These calcium compounds are found in bones and make them hard and strong. The skeleton of the average adult contains about a kilogram of calcium. Because calciumlike bariumabsorbs x-rays, bones show up white in x-ray images. Calcium is an important component of a healthy human diet. Good food sources of calcium are pictured in Figure Q: What health problems might result from a diet low in calcium? A: Children who dont get enough calcium while their bones are forming may develop a deficiency disease called rickets, in which their bones are softer than normal and become bent and stunted. Adults who dont get enough calcium may develop a condition called osteoporosis, in which the bones lose calcium and become weak and brittle. People with osteoporosis are at high risk of bone fractures. Strontium is a silver-colored alkaline Earth metal that is even softer than calcium. Strontium compounds are quite common and have a variety of usesfrom fireworks to cement to toothpaste. In fireworks, strontium compounds produce deep red explosions. In toothpaste, the compound strontium chloride reduces tooth sensitivity. | text | null |
L_0797 | alloys | T_4119 | An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Alloys generally have more useful properties than pure metals. Several examples of alloys are described and pictured below. If you have braces on your teeth, you might even have this alloy in your mouth! Click image to the left or use the URL below. URL: | text | null |
L_0797 | alloys | T_4120 | Most metal objects are made of alloys rather than pure metals. Objects made of four different alloys are shown in the Figure 1.1. Brass saxophone: Brass is an alloy of copper and zinc. It is softer than bronze and easier to shape. Its also very shiny. Notice the curved pieces in this shiny brass saxophone. Brass is used for shap- ing many other curved objects, such as doorknobs and plumbing fixtures. Stain- less steel sink: Stainless steel is a type of steel that contains nickel and chromium in addition to carbon and iron. It is shiny, strong, and resistant to rusting. This makes it useful for sinks, eating utensils, and other objects that are exposed to wa- ter. "Gold" bracelet: Pure gold is relatively soft, so it is rarely used for jewelry. Most "gold" jewelry is actually made of an alloy of gold, copper and silver. Bronze statue: Bronze was the first alloy ever made. The earliest bronze dates back many thou- sands of years. Bronze is a mixture of copper and tin. Both copper and tin are relatively soft metals, but mixed together in bronze they are much harder. Bronze has been used for statues, coins, and other objects. Q: Sterling silver is an alloy that is used to make fine jewelry. What elements do you think sterling silver contains? What properties might sterling silver have that make it more useful than pure silver? A: Most sterling silver is about 93 percent silver and about 7 percent copper. Sterling silver is harder and stronger than pure silver, while retaining the malleability and luster of pure silver. | text | null |
L_0798 | alpha decay | T_4121 | Radioactive elements and isotopes have unstable nuclei. To become more stable, the nuclei undergo radioactive decay. In radioactive decay, the nuclei give off, or emit, radiation in the form of energy and often particles as well. There are several types of radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only alpha and beta decay also emit particles. | text | null |
L_0798 | alpha decay | T_4122 | Alpha decay occurs when a nucleus is unstable because it has too many protons. The Figure 1.1 shows what happens during alpha decay. The nucleus emits an alpha particle and energy. An alpha particle consists of two protons and two neutrons, which is actually a helium nucleus. Losing the protons and neutrons makes the nucleus more stable. | text | null |
L_0798 | alpha decay | T_4123 | Radioactive nuclei and particles are represented by nuclear symbols that indicate their numbers of protons and neutrons. For example, an alpha particle (helium nucleus) is represented by the symbol 42 He, where He is the chemical symbol for helium, the subscript 2 is the number of protons, and the superscript 4 is the mass number (2 protons + 2 neutrons). Nuclear symbols are used to write nuclear equations for radioactive decay. Lets consider an example. Uranium-238 undergoes alpha decay to become thorium-234. (The numbers following the chemical names refer to the number of protons plus neutrons.) In this reaction, uranium-238 loses two protons and two neutrons to become the element thorium-234. The reaction can be represented by this nuclear equation: 238 U 92 4 234 90 Th + 2 He + Energy If you count the number of protons (subscripts) as well as the number of protons plus neutrons (superscripts), youll see that the total numbers are the same on both sides of the arrow. This means that the equation is balanced. The thorium-234 produced in this reaction is also unstable, so it will undergo radioactive decay as well. The alpha particle (42 He) produced in the reaction can join with two free electrons to form the element helium. This is how most of Earths helium formed. Q: Fill in the missing subscript and superscript to balance the following nuclear equation for alpha decay of Polonium-210. 210 Po 84 ?? Pb + 42 He + Energy A: The subscript of Pb is 82, and the superscript is 206. This means that the new element produced in the reaction has 82 protons. You can find the element with this number of protons in the periodic table. It is the element lead (Pb). The new element also has 124 neutrons (206 - 82 protons = 124 neutrons). | text | null |
L_0798 | alpha decay | T_4124 | All types of radioactive decay pose risks to living things, but alpha decay is the least dangerous. Thats because alpha particles are relatively heavy, so they can travel only a few centimeters through the air. They also are not very penetrating. For example, they cant pass through a sheet of paper or thin layer of clothing. They may burn the skin, but they cant penetrate to the tissues underneath the skin. However, if alpha particles are emitted inside the body, they can do more damage. One way this can happen is by inhaling cigarette smoke. People who smoke actually inhale the radioactive element polonium-210. It undergoes alpha decay in the lungs. Over time, exposure to alpha particles may cause lung cancer. | text | null |
L_0800 | archimedes law | T_4128 | Did you ever notice when you get into a bathtub of water that the level of the water rises? More than 2000 years ago, a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a tub cant occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume of the submerged object. So more water is displaced by a bigger body than a smaller one. Q: If you jump into swimming pool, how much water does your body displace? A: The water displaced by your body is equal to your bodys volume. Depending on your size, this volume might be about 0.07 m3 . | text | null |
L_0800 | archimedes law | T_4129 | Objects such as ships may float in a fluid like water because of buoyant force. This is an upward force that a fluid exerts on any object that is placed in it. Archimedes discovered that the buoyant force acting on an object equals the weight of the fluid displaced by the object. This is known as Archimedes law (or Archimedes principle). | text | null |
L_0800 | archimedes law | T_4130 | Archimedes law explains why some objects float in fluids even though they are very heavy. It all depends on how much fluid they displace. The cruise ship pictured in the opening image is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship were placed in water, it would sink to the bottom. This is modeled in the Figure 1.1. The reason the ball sinks is that its shape is very compact, so it displaces relatively little water. The volume of water displaced by the steel ball weighs less than the ball itself, so the buoyant force is not as great as the force of gravity pulling down on the ball. Thus, the ball sinks. Now look at the ships hull in the Figure 1.1. Its shape causes the ship to displace much more water than the ball. In fact, the weight of the displaced water is greater than the weight of the ship. As a result, the buoyant force is greater than the force of gravity acting on the ship, so the ship floats. Q: Why might you be more likely to float in water if you stretch out your body rather than curl up into a ball? A: You would displace more water by stretching out your body, so there would be more buoyant force acting on it. Therefore, you would be more likely to float in this position. | text | null |
L_0801 | artificial light | T_4131 | If youre like most people, you dont give it a thought when you flick a switch to turn on a lightat least not until the power goes out and youre left in the dark! When you flick on a light switch, electricity normally flows through the light, and some type of light bulb converts the electrical energy to visible light. This can happen in various ways, depending on the type of light bulb. Several different types of light bulbs are described below. All of them are examples of artificial light, as opposed to natural light from the sun or other sources in nature. | text | null |
L_0801 | artificial light | T_4132 | An incandescent light bulb like the one pictured in the Figure 1.1 produces visible light by incandescence. Incan- descence occurs when something gets so hot that it glows. An incandescent light bulb contains a thin wire filament made of tungsten. When electric current passes through the filament, it gets extremely hot and emits light. | text | null |
L_0801 | artificial light | T_4133 | A fluorescent light bulb produces visible light by fluorescence. Fluorescence occurs when a substance absorbs shorter-wavelength ultraviolet light and then gives off the energy as visible light. The compact fluorescent light bulb (CFL) in the Figure 1.2 contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. Phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. | text | null |
L_0801 | artificial light | T_4133 | A fluorescent light bulb produces visible light by fluorescence. Fluorescence occurs when a substance absorbs shorter-wavelength ultraviolet light and then gives off the energy as visible light. The compact fluorescent light bulb (CFL) in the Figure 1.2 contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. Phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. | text | null |
L_0801 | artificial light | T_4134 | A neon light produces visible light by electroluminescence. In this process, neon or some other gas gives off light when an electric current passes through it. Other halogen gases besides neonincluding krypton and argonalso produce light in this way. The word OPEN in the sign 1.3 is a neon light. It is a long glass tube that contains neon gas. When electricity passes through the gas, it excites electrons of neon atoms, and the electrons jump to a higher energy level. As the excited electrons return to their original energy level, they give off visible light. Neon produces red light. Other gases produce light of different colors. For example, krypton produces violet light, and argon produces blue light. | text | null |
L_0801 | artificial light | T_4135 | A vapor light also produces visible light by electroluminescence The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like the streetlight pictured in the Figure 1.4, produce yellowish light. Mercury vapor lights produce bluish light. In addition to lighting city streets, vapor lights are used to light highways and stadiums. The bulbs are very bright and long lasting so they are a good choice for these places. | text | null |
L_0801 | artificial light | T_4136 | LED stands for light-emitting diode. An LED light contains a material called a semi-conductor, which gives off visible light when an electric current flows through it. LED lights are used for traffic lights (see Figure 1.5) and also indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable. Q: Some light bulbs produce a lot of heat in addition to visible light, so they waste energy. Other bulbs produce much less heat, so they use energy more efficiently. Which light bulbs described above would you place in each category? A: Incandescent light bulbs, which produce light by incandescence, give off a lot of heat as well as light, so they waste energy. The other light bulbs produce light by some type of luminescence, in which light is produced without heat. These light bulbs use energy more efficiently. Which types of light bulbs do you use? | text | null |
L_0802 | atomic forces | T_4137 | Electromagnetic force is a force of attraction or repulsion between all electrically charged particles. This force is transferred between charged particles of matter by fundamental force-carrying particles called photons. Because of electromagnetic force, particles with opposite charges attract each other and particles with the same charge repel each other. Inside the atom, two types of subatomic particles have electric charge: electrons, which have an electric charge of -1, and protons, which have an opposite but equal electric charge of +1. The model of an atom in the Figure 1.1 shows both types of charged particles. Protons are found inside the nucleus at the center of the atom, and they give the nucleus a positive charge. (There are also neutrons in the nucleus, but they have no electric charge.) Negative electrons stay in the area surrounding the positive nucleus because of the electromagnetic force of attraction between them. Q: Why do you think protons cluster together in the nucleus of the atom instead of repelling each other because of their like charges? A: The electromagnetic force of repulsion between positively charged protons is overcome by a stronger force, called the strong nuclear force. | text | null |
L_0802 | atomic forces | T_4138 | The strong nuclear force is a force of attraction between fundamental particles called quarks, which have a type of charge called color charge. The strong nuclear force is transferred between quarks by fundamental force-carrying particles called gluons. Both protons and neutrons consist of quarks. The exchange of gluons holds quarks together within a proton or neutron. Excess, or residual, strong force holds together protons and neutrons in the nucleus. The strong nuclear force is strong enough to overcome the electromagnetic force of repulsion pushing protons apart. Both forces are represented in the Figure 1.2. The strong nuclear force works only over very short distances. As a result, it isnt effective if the nucleus gets too big. As more protons are added to the nucleus, the electromagnetic force of repulsion between them gets stronger, while the strong nuclear force of attraction between them gets weaker. This puts an upper limit on the number of protons an atom can have and remain stable. If atoms have more than 83 protons, the electromagnetic repulsion between them is greater than the strong nuclear force of attraction between them. This makes the nucleus unstable, or radioactive, so it breaks down. The following video discusses the strong nuclear force and its role in the atom. The types of quarks found in protons and neutrons are called up quarks (u) and down quarks (d). Each proton consists of two up quarks and one down quark (uud), and each neutron consists of one up quark and two down quarks (udd). This diagram represents two protons. Click image to the left or use the URL below. URL: | text | null |
L_0802 | atomic forces | T_4139 | The weak nuclear force is transferred by the exchange of force-carrying fundamental particles called W and Z bosons. This force is also a very short-range force that works only within the nucleus of the atom. It is much weaker than the strong force or electromagnetic force that are also at work inside the atom. Unlike these other two forces, the weak nuclear force does not bind subatomic particles together in an atom. Instead, it changes subatomic particles from one type to another. The Figure 1.3 shows one way this can happen. In this figure, an up quark in a proton is changed by the weak force to a down quark. This changes the proton (uud) to a neutron (udd). Q: If the weak force causes a proton to change to a neutron, how does this change the atom? A: The resulting atom represents a different element. Thats because each element has a unique number of protons. For example, all atoms of helium have two protons. If one of the protons in a helium atom changes to a neutron, the resulting atom would have just one proton, so the atom would no longer be a helium atom. Instead it would be a hydrogen atom, because all hydrogen atoms have a single proton. | text | null |
L_0803 | atomic nucleus | T_4140 | The nucleus (plural, nuclei) is a positively charged region at the center of the atom. It consists of two types of subatomic particles packed tightly together. The particles are protons, which have a positive electric charge, and neutrons, which are neutral in electric charge. Outside of the nucleus, an atom is mostly empty space, with orbiting negative particles called electrons whizzing through it. The Figure 1.1 shows these parts of the atom. | text | null |
L_0803 | atomic nucleus | T_4141 | The nucleus of the atom is extremely small. Its radius is only about 1/100,000 of the total radius of the atom. If an atom were the size of a football stadium, the nucleus would be about the size of a pea! Click image to the left or use the URL below. URL: Electrons have virtually no mass, but protons and neutrons have a lot of mass for their size. As a result, the nucleus has virtually all the mass of an atom. Given its great mass and tiny size, the nucleus is very dense. If an object the size of a penny had the same density as the nucleus of an atom, its mass would be greater than 30 million tons! Click image to the left or use the URL below. URL: | text | null |
L_0803 | atomic nucleus | T_4142 | Particles with opposite electric charges attract each other. This explains why negative electrons orbit the positive nucleus. Particles with the same electric charge repel each other. This means that the positive protons in the nucleus push apart from one another. So why doesnt the nucleus fly apart? An even stronger forcecalled the strong nuclear forceholds protons and neutrons together in the nucleus. Click image to the left or use the URL below. URL: Q: Can you guess why an atomic bomb releases so much energy when it explodes? A: When an atomic bomb explodes, the nuclei of atoms undergo a process called fission, in which they split apart. This releases the huge amount of energy that was holding together subatomic particles in the nucleus. | text | null |
L_0804 | atomic number | T_4143 | Its often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. Its important to be able to distinguish atoms of one element from atoms of other elements. Elements are pure substances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples in the Figure 1.1. Q: The table shown above is called the periodic table of the elements. Each symbol stands for a different element. What do you think the symbol K stands for? A: The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which is kalium. The symbols in the table above would be more useful if they revealed more information about the atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. Thats where atomic number and mass number come in. | text | null |
L_0804 | atomic number | T_4144 | The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the elements symbol, like in the Figure 1.2 for helium. Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atoms properties. | text | null |
L_0804 | atomic number | T_4145 | There is another number in the box above for helium. That number is the mass number, which is the mass of the atom in a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 1027 kilograms, which is an extremely small mass. A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units. Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the box above. Q: What is the mass number of a helium atom that has three neutrons? A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5. Q: How would you represent this isotope of helium to show its atomic number and mass number? A: You would represent it by the elements symbol and both numbers, with the mass number on top and the atomic number on the bottom: 5 2 He | text | null |
L_0806 | balancing chemical equations | T_4153 | A chemical equation represents the changes that occur during a chemical reaction. A chemical equation has the general form: Reactants Products An example of a simple chemical reaction is the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to produce water (H2 O). In this reaction, the reactants are hydrogen and oxygen and the product is water. To write the chemical equation for this reaction, you would start by writing the reactants on the left and the product on the right, with an arrow between them to show the direction in which the reaction occurs: Equation 1: H2 + O2 H2 O Q: Look closely at equation 1. Theres something wrong with it. Do you see what it is? A: All chemical equations must be balanced. This means that there must be the same number of each type of atom on both sides of the arrow. Thats because mass is always conserved in chemical reactions. Count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced. | text | null |
L_0806 | balancing chemical equations | T_4154 | Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2 H2 , and two molecules of water would be written 2 H2 O. A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2 H2 + O2 2 H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms and so do the two molecules of water. Therefore, there are now four hydrogen atoms in both reactants and products. Q: Is equation 2 balanced? A: Count the oxygen atoms to find out. There are two oxygen atoms in the one molecule of oxygen in the reactants. There are also two oxygen atoms in the products, one in each of the two water molecules. Therefore, equation 2 is balanced. | text | null |
L_0806 | balancing chemical equations | T_4155 | Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Place coefficients, as needed, in front of the symbols or formulas to increase the number of atoms or molecules of the substances. Use the smallest coefficients possible. Warning! Never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. 3. Repeat steps 1 and 2 until the equation is balanced. Q: Balance this chemical equation for the reaction in which nitrogen (N2 ) and hydrogen (H2 ) combine to form ammonia (NH3 ): N2 + H2 NH3 A: First count the nitrogen atoms on both sides of the arrow. There are two nitrogen atoms in the reactants so there must be two in the products as well. Place the coefficient 2 in front of NH3 to balance nitrogen: N2 + H2 2 NH3 Now count the hydrogen atoms on both sides of the arrow. There are six hydrogen atoms in the products so there must also be six in the reactants. Place the coefficient 3 in front of H2 to balance hydrogen: N2 + 3 H2 2 NH3 | text | null |
L_0808 | beta decay | T_4158 | Atoms with unstable nuclei are radioactive. To become more stable, the nuclei undergo radioactive decay. In radioactive decay, the nuclei emit energy and usually particles of matter as well. There are several types of radioactive decay, including alpha, beta, and gamma decay. Energy is emitted in all three types of decay, but only alpha and beta decay also emit particles. | text | null |
L_0808 | beta decay | T_4159 | Beta decay occurs when an unstable nucleus emits a beta particle and energy. A beta particle is either an electron or a positron. An electron is a negatively charged particle, and a positron is a positively charged electron (or anti- electron). When the beta particle is an electron, the decay is called beta-minus decay. When the beta particle is a positron, the decay is called beta-plus decay. Beta-minus decay occurs when a nucleus has too many neutrons relative to protons, and beta-plus decay occurs when a nucleus has too few neutrons relative to protons. Q: Nuclei contain only protons and neutrons, so how can a nucleus emit an electron in beta-minus decay or a positron in beta-plus decay? A: Beta decay begins with a proton or neutron. You can see how in the Figure 1.1. Q: How does beta decay change an atom to a different element? A: In beta-minus decay an atom gains a proton, and it beta-plus decay it loses a proton. In each case, the atom becomes a different element because it has a different number of protons. | text | null |
L_0808 | beta decay | T_4160 | Radioactive nuclei and particles are represented by nuclear symbols.. For example, a beta-minus particle (electron) is represented by the symbol 01 e. The subscript -1 represents the particles charge, and the superscript 0 shows that the particle has virtually no mass (no protons or neutrons). Another example is the radioactive nucleus of thorium-234. It is represented by the symbol 234 90 Th, where the subscript 90 stands for the number of protons and the superscript 234 for the number of protons plus neutrons. Nuclear symbols are used to write nuclear equations for radioactive decay. Lets consider the example of the beta- minus decay of thorium-234 to protactinium-234. This reaction is represented by the equation: 234 Th 90 0 234 91 Pa + 1 e + energy The equation shows that thorium-234 becomes protactinium-234 and loses a beta particle and energy. The protactinium- 234 produced in the reaction is also radioactive, so it will decay as well. A nuclear equation is balanced if the total numbers of protons and neutrons are the same on both sides of the arrow. If you compare the subscripts and superscripts on both sides of the equation above, youll see that they are the same. Q: What happens to the electron produced in the reaction above? A: Along with another electron, it can combine with an alpha particle to form a helium atom. An alpha particle, which is emitted during alpha decay, consists of two protons and two neutrons. Q: Try to balance the following nuclear equation for beta-minus decay by filling in the missing subscript and superscript. 131 I 53 ?? Xe + 01 e + energy A: The subscript of Xe is 54, and the superscript is 131. | text | null |
L_0808 | beta decay | T_4161 | Beta particles can travel about a meter through air. They can pass through a sheet of paper or a layer of cloth but not through a sheet of aluminum or a few centimeters of wood. They can also penetrate the skin and damage underlying tissues. They are even more harmful if they are ingested or inhaled. | text | null |
L_0809 | biochemical compound classification | T_4162 | Glucose is an example of a biochemical compound. The prefix bio- comes from the Greek word that means life. A biochemical compound is any carbon-based compound that is found in living things. Biochemical compounds make up the cells and tissues of living things. They are also involved in all life processes, including making and using food for energy. Given their diversity of functions, its not surprising that there are millions of different biochemical compounds. Q: Plants make food in the process of photosynthesis. What biochemical compound is synthesized in photosynthe- sis? A: Glucose is synthesized in photosynthesis. Virtually all living things use glucose for energy, but glucose is just one of many examples of biochemical compounds that are found in most or all living things. In fact the similarity in biochemical compounds between living things provides some of the best evidence for the evolution of species from common ancestors. A classic example is the biochemical compound called cytochrome c. It is found in all living organisms because it performs essential life functions. Only slight variations in the molecule exist between closely related species, as you can see in the Figure and the single-celled tetrahymena (pictured in the Figure 1.1), the cytochrome c molecule is nearly 50 percent the same. | text | null |
L_0809 | biochemical compound classification | T_4163 | All biochemical molecules contain hydrogen and oxygen as well as carbon. They may also contain nitrogen, phosphorus, and/or sulfur. Almost all biochemical compounds are polymers. Polymers are large molecules that consist of many smaller, repeating molecules, called monomers. Glucose is a monomer of biochemical compounds called starches. In starches and all other biochemical polymers, monomers are joined together by covalent bonds, in which atoms share pairs of valence electrons. Click image to the left or use the URL below. URL: | text | null |
L_0809 | biochemical compound classification | T_4164 | Most biochemical molecules are macromolecules. The prefix macro- means large, and many biochemical molecules are very large indeed. They may contain thousands of monomer molecules. The largest known biochemical molecule is called titin. It plays an important role in muscle contraction. The human form of the molecule contains more than 34,000 monomers. Its chemical formula is C169723 H270464 N45688 O52243 S912 . Its chemical name contains almost 190,000 letters, and it has been called the longest word in any language. | text | null |
L_0809 | biochemical compound classification | T_4165 | Although there are millions of biochemical compounds, all of them can be grouped into just four main classes: carbohydrates, proteins, lipids, and nucleic acids. The classes are summarized in the Table 1.1. Class Carbohydrates Elements carbon hydrogen oxygen Examples sugars starches cellulose Proteins carbon hydrogen oxygen nitrogen sulfur carbon hydrogen oxygen carbon hydrogen oxygen nitrogen phosphorus enzymes hormones Lipids Nucleic acids Functions provide energy to cells store energy in plants makes up the cell walls of plants speed up biochemical re- actions regulate life processes fats oils store energy in animals store energy in plants DNA RNA stores genetic information in cells helps cells make proteins Q: In which class of biochemical compounds would you place glucose? A: Glucose is a sugar in the class carbohydrates. Like other carbohydrates, it contains only carbon, hydrogen, and oxygen. It provides energy to the cells of living things. Q: Look back at the chemical formula for titin. In which class of biochemical compounds should it be placed? A: Titin is a protein. You can tell because it contains sulfur, and proteins are the only biochemical compounds that contain this element. | text | null |
L_0810 | biochemical reaction chemistry | T_4166 | Chemical reactions that take place inside living things are called biochemical reactions (bio- means life). Its not just for energy that living things depend on biochemical reactions. Every function and structure of a living organism depends on thousands of biochemical reactions taking place in each cell. The sum of all these biochemical reactions is called metabolism. | text | null |
L_0810 | biochemical reaction chemistry | T_4167 | Biochemical reactions of metabolism can be divided into two general categories: catabolic reactions and anabolic reactions. Catabolic reactions involve breaking bonds. Larger molecules are broken down to smaller ones. For example, complex carbohydrates are broken down to simple sugars. Catabolic reactions release energy, so they are exothermic. Anabolic reactions involve forming bonds. Smaller molecules are combined to form larger ones. For example, simple sugars are combined to form complex carbohydrates. Anabolic reactions require energy, so they are endothermic. Q: Imagine! Each of the trillions of cells in your body is continuously performing thousands of catabolic and anabolic reactions. Thats an amazing number of biochemical reactionsfar more than the number of reactions that might take place in a lab or factory. How can so many biochemical reactions take place simultaneously in our cells? A: So many reactions can occur because biochemical reactions are amazingly fast. Q: In a lab or factory, reactants can be heated to very high temperatures or placed under great pressure so they will react very quickly. These ways of speeding up chemical reactions cant occur inside the delicate cells of living things. So how do cells speed up biochemical reactions? A: The answer is enzymes. | text | null |
L_0810 | biochemical reaction chemistry | T_4168 | Enzymes are proteins that increase the rate of chemical reactions by reducing the amount of activation energy needed for reactants to start reacting. Enzymes are synthesized in the cells that need them, based on instructions encoded in the cells DNA. Enzymes arent changed or used up in the reactions they catalyze, so they can be used to speed up the same reaction over and over again. Enzymes are highly specific for certain chemical reactions, so they are very effective. A reaction that would take years to occur without its enzyme might occur in a split second with the enzyme. Enzymes are also very efficient, so waste products rarely form. | text | null |
L_0810 | biochemical reaction chemistry | T_4169 | Some of the most important biochemical reactions are the reactions involved in photosynthesis and cellular respira- tion. Together, these two processes provide energy to almost all of Earths organisms. The two processes are closely related, as you can see in the Figure 1.1. In photosynthesis, light energy from the sun is converted to stored chemical energy in glucose. In cellular respiration, stored energy is released from glucose and stored in smaller amounts that cells can use. A: In photosynthesis, carbon dioxide (CO2 ) and water (H2 O) are the reactants. They combine using energy from light to produce oxygen (O2 ) and glucose (C6 H12 O6 ). Oxygen and glucose, in turn, are the reactants in cellular respiration. They combine to produce carbon dioxide, water, and energy. | text | null |
L_0811 | bohrs atomic model | T_4170 | The existence of the atom was first demonstrated around 1800 by John Dalton. Then, close to a century went by before J.J. Thomson discovered the first subatomic particle, the negatively charged electron. Because atoms are neutral in charge, Thomson thought that they must consist of a sphere of positive charge with electrons scattered through it. In 1910, Ernest Rutherford showed that this idea was incorrect. He demonstrated that all of the positive charge of an atom is actually concentrated in a tiny central region called the nucleus. Rutherford surmised that electrons move around the nucleus like planets around the sun. Rutherfords idea of atomic structure was an improvement on Thomsons model, but it wasnt the last word. Rutherford focused on the nucleus and didnt really clarify where the electrons were in the empty space surrounding the nucleus. The next major advance in atomic history occurred in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. His model identified more clearly where electrons could be found. Although later scientists would develop more refined atomic models, Bohrs model was basically correct and much of it is still accepted today. It is also a very useful model because it explains the properties of different elements. Bohr received the 1922 Nobel prize in physics for his contribution to our understanding of the structure of the atom. You can see a picture of Bohr 1.1. | text | null |
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