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L_0967 | newtons third law | T_4662 | The forces involved in actions and reactions can be represented with arrows. The way an arrow points shows the direction of the force, and the size of the arrow represents the strength of the force. Look at the skateboarders in the Figure 1.1. In the top row, the arrows represent the forces with which the skateboarders push against each other. This is the action. In the bottom row, the arrows represent the forces with which the skateboarders move apart. This is the reaction. Compare the top and bottom arrows. They point in different directions, but they are the same size. This shows that the reaction forces are equal and opposite to the action forces. | text | null |
L_0967 | newtons third law | T_4663 | Because action and reaction forces are equal and opposite, you might think they would cancel out, as balanced forces do. But you would be wrong. Balanced forces are equal and opposite forces that act on the same object. Thats why they cancel out. Action-reaction forces are equal and opposite forces that act on different objects, so they dont cancel out. In fact, they often result in motion. Think about Jerod again. He applies force with his foot to the ground, whereas the ground applies force to Jerod and the skateboard, causing them to move forward. Q: Actions and reactions occur all the time. Can you think of an example in your daily life? A: Heres one example. If you lean on something like a wall or your locker, you are applying force to it. The wall or locker applies an equal and opposite force to you. If it didnt, you would go right through it or else it would tip over. | text | null |
L_0968 | noble gases | T_4664 | Noble gases are nonreactive, nonmetallic elements in group 18 of the periodic table. As you can see in the periodic table below, noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). All noble gases are colorless and odorless. They also have low boiling points, explaining why they are gases at room temperature. Radon, at the bottom of the group, is radioactive, so it constantly decays to other elements. Click image to the left or use the URL below. URL: Q: Based on their position in the periodic table (Figure 1.1), how many valence electrons do you think noble gases have? A: The number of valence electrons starts at one for elements in group 1. It then increases by one from left to right across each period (row) of the periodic table for groups 1-2 and 13-18 (numbered 3-0 in the table above). Therefore, noble gases have eight valence electrons. | text | null |
L_0968 | noble gases | T_4665 | Noble gases are the least reactive of all known elements. Thats because with eight valence electrons, their outer energy levels are full. The only exception is helium, which has just two electrons. But helium also has a full outer energy level, because its only energy level (energy level 1) can hold a maximum of two electrons. A full outer energy level is the most stable arrangement of electrons. As a result, noble gases cannot become more stable by reacting with other elements and gaining or losing valence electrons. Therefore, noble gases are rarely involved in chemical reactions and almost never form compounds with other elements. | text | null |
L_0968 | noble gases | T_4666 | Because the noble gases are the least reactive of all elements, their eight valence electrons are used as the standard for nonreactivity and to explain how other elements interact. This is stated as the octet (group of eight) rule. According to this rule, atoms react to form compounds that allow them to have a group of eight valence electrons like the noble gases. For example, sodium (with one valence electron) reacts with chlorine (with seven valence electrons) to form the stable compound sodium chloride (table salt). In this reaction, sodium donates an electron and chlorine accepts it, giving each element an octet of valence electrons. | text | null |
L_0968 | noble gases | T_4667 | Did you ever get a birthday balloon like the one pictured 1.2? The balloon is filled with the noble gas helium. The gas is pumped from a tank into a Mylar balloon. Unlike a balloon filled with air, a balloon filled with helium needs to be weighted down so it wont float away. Q: Why does a helium balloon float away if its not weighted down? A: Helium atoms have just two protons, two neutrons, and two electrons, so they have less mass than any other atoms except hydrogen. As a result, helium is lighter than air, explaining why a helium balloon floats up into the air unless weighted down. Early incandescent light bulbs, like the one pictured in the Figure 1.3, didnt last very long. The filaments quickly burned out. Although air was pumped out of the bulb, it wasnt a complete vacuum. Oxygen in the small amount of air remaining inside the light bulb reacted with the metal filament. This corroded the filament and caused dark deposits on the glass. Filling a light bulb with argon gas prevents these problems. Thats why modern light bulbs are filled with argon. A: As a noble gas with eight electrons, argon doesnt react with the metal in the filament. This protects the filament and keeps the glass blub free of deposits. Noble gases are also used to fill the glass tubes of lighted signs like the one in the Figure 1.4. Although noble gases are chemically nonreactive, their electrons can be energized by sending an electric current through them. When this happens, the electrons jump to a higher energy level. When the electrons return to their original energy level, they give off energy as light. Different noble gases give off light of different colors. Neon gives off reddish-orange light, like the word Open in the sign below. Krypton gives off violet light and xenon gives off blue light. | text | null |
L_0968 | noble gases | T_4667 | Did you ever get a birthday balloon like the one pictured 1.2? The balloon is filled with the noble gas helium. The gas is pumped from a tank into a Mylar balloon. Unlike a balloon filled with air, a balloon filled with helium needs to be weighted down so it wont float away. Q: Why does a helium balloon float away if its not weighted down? A: Helium atoms have just two protons, two neutrons, and two electrons, so they have less mass than any other atoms except hydrogen. As a result, helium is lighter than air, explaining why a helium balloon floats up into the air unless weighted down. Early incandescent light bulbs, like the one pictured in the Figure 1.3, didnt last very long. The filaments quickly burned out. Although air was pumped out of the bulb, it wasnt a complete vacuum. Oxygen in the small amount of air remaining inside the light bulb reacted with the metal filament. This corroded the filament and caused dark deposits on the glass. Filling a light bulb with argon gas prevents these problems. Thats why modern light bulbs are filled with argon. A: As a noble gas with eight electrons, argon doesnt react with the metal in the filament. This protects the filament and keeps the glass blub free of deposits. Noble gases are also used to fill the glass tubes of lighted signs like the one in the Figure 1.4. Although noble gases are chemically nonreactive, their electrons can be energized by sending an electric current through them. When this happens, the electrons jump to a higher energy level. When the electrons return to their original energy level, they give off energy as light. Different noble gases give off light of different colors. Neon gives off reddish-orange light, like the word Open in the sign below. Krypton gives off violet light and xenon gives off blue light. | text | null |
L_0969 | nonmetals | T_4668 | Nonmetals are elements that generally do not conduct electricity. They are one of three classes of elements (the other two classes are metals and metalloids.) Nonmetals are the second largest of the three classes after metals. They are the elements located on the right side of the periodic table. Q: From left to right across each period (row) of the periodic table, each element has atoms with one more proton and one more electron than the element before it. How might this be related to the properties of nonmetals? A: Because nonmetals are on the right side of the periodic table, they have more electrons in their outer energy level than elements on the left side or in the middle of the periodic table. The number of electrons in the outer energy level of an atom determines many of its properties. | text | null |
L_0969 | nonmetals | T_4669 | As their name suggests, nonmetals generally have properties that are very different from the properties of metals. Properties of nonmetals include a relatively low boiling point, which explains why many of them are gases at room temperature. However, some nonmetals are solids at room temperature, including the three pictured above, and one nonmetalbromineis a liquid at room temperature. Other properties of nonmetals are illustrated and described in the Figure 1.1. | text | null |
L_0969 | nonmetals | T_4670 | Reactivity is how likely an element is to react chemically with other elements. Some nonmetals are extremely reactive, whereas others are completely nonreactive. What explains this variation in nonmetals? The answer is their number of valence electrons. These are the electrons in the outer energy level of an atom that are involved in interactions with other atoms. Lets look at two examples of nonmetals, fluorine and neon. Simple atomic models of these two elements are shown in the Figure 1.2. Q: Which element, fluorine or neon, do you predict is more reactive? A: Fluorine is more reactive than neon. Thats because it has seven of eight possible electrons in its outer energy level, whereas neon already has eight electrons in this energy level. Although neon has just one more electron than fluorine in its outer energy level, that one electron makes a huge difference. Fluorine needs one more electron to fill its outer energy level in order to have the most stable arrangement of electrons. Therefore, fluorine readily accepts an electron from any element that is equally eager to give one up, Click image to the left or use the URL below. URL: | text | null |
L_0969 | nonmetals | T_4670 | Reactivity is how likely an element is to react chemically with other elements. Some nonmetals are extremely reactive, whereas others are completely nonreactive. What explains this variation in nonmetals? The answer is their number of valence electrons. These are the electrons in the outer energy level of an atom that are involved in interactions with other atoms. Lets look at two examples of nonmetals, fluorine and neon. Simple atomic models of these two elements are shown in the Figure 1.2. Q: Which element, fluorine or neon, do you predict is more reactive? A: Fluorine is more reactive than neon. Thats because it has seven of eight possible electrons in its outer energy level, whereas neon already has eight electrons in this energy level. Although neon has just one more electron than fluorine in its outer energy level, that one electron makes a huge difference. Fluorine needs one more electron to fill its outer energy level in order to have the most stable arrangement of electrons. Therefore, fluorine readily accepts an electron from any element that is equally eager to give one up, Click image to the left or use the URL below. URL: | text | null |
L_0969 | nonmetals | T_4671 | Like most other nonmetals, fluorine cannot conduct electricity, and its electrons explain this as well. An electric current is a flow of electrons. Elements that readily give up electrons (the metals) can carry electric current because their electrons can flow freely. Elements that gain electrons instead of giving them up cannot carry electric current. They hold onto their electrons so they cannot flow. | text | null |
L_0970 | nuclear fission | T_4672 | Nuclear fission is the splitting of the nucleus of a radioactive atom into two smaller nuclei. This type of reaction releases a great deal of energy from a very small amount of matter. Fission of a tiny pellet of radioactive uranium- 235, like the one pictured in the Figure 1.1, releases as much energy as burning 1,000 kilograms of coal! Q: What causes the nucleus of uranium-235 atom to fission? A: Another particle collides with it. | text | null |
L_0970 | nuclear fission | T_4673 | The Figure 1.2 shows how nuclear fission of uranium-235 occurs. It begins when a uranium nucleus gains a neutron. This can happen naturally when a free neutron strikes it, or it can occur deliberately when a neutron is crashed into it in a nuclear power plant. In either case, the nucleus of uranium-235 becomes extremely unstable with the extra neutron. As a result, it splits into two smaller nuclei, krypton-92 and barium-141. The reaction also releases three neutrons and a great deal of energy. It can be represented by this nuclear equation: 235 U 92 141 + 1 neutron 92 36 Kr + 56 Ba + 3 neutrons + energy Note that the subscripts of the element symbols represent numbers of protons and the superscripts represent numbers of protons plus neutrons. | text | null |
L_0970 | nuclear fission | T_4674 | The neutrons released when uranium-235 fissions may crash into other uranium nuclei and cause them to fission as well. This can start a nuclear chain reaction. You can see how this happens in the Figure 1.3. In a chain reaction, one fission reaction leads to others, which lead to others, and so on. A nuclear chain reaction is similar to a pile of wood burning. If you start one piece of wood burning, enough heat is produced by the burning wood to start the rest of the pile burning without any further help from you. Click image to the left or use the URL below. URL: | text | null |
L_0970 | nuclear fission | T_4675 | If a nuclear chain reaction is uncontrolled, it produces a lot of energy all at once. This is what happens in an atomic bomb. However, if a nuclear chain reaction is controlled, it produces energy much more slowly. This is what occurs in a nuclear power plant. The reaction is controlled by inserting rods of nonfissioning material into the fissioning material. You can see this in the Figure 1.4. The radiation from the controlled fission is used to heat water and turn it to steam. The steam is under pressure and causes a turbine to spin. The spinning turbine runs a generator, which produces electricity. | text | null |
L_0970 | nuclear fission | T_4675 | If a nuclear chain reaction is uncontrolled, it produces a lot of energy all at once. This is what happens in an atomic bomb. However, if a nuclear chain reaction is controlled, it produces energy much more slowly. This is what occurs in a nuclear power plant. The reaction is controlled by inserting rods of nonfissioning material into the fissioning material. You can see this in the Figure 1.4. The radiation from the controlled fission is used to heat water and turn it to steam. The steam is under pressure and causes a turbine to spin. The spinning turbine runs a generator, which produces electricity. | text | null |
L_0970 | nuclear fission | T_4676 | In the U.S., the majority of electricity is produced by burning coal or other fossil fuels. This causes air pollution that harms the health of living things. The air pollution also causes acid rain and contributes to global warming. In addition, fossil fuels are nonrenewable resources, so if we keep using them, they will eventually run out. The main advantage of nuclear energy is that it doesnt release air pollution or cause the other environmental problems associated with the burning of fossil fuels. On the other other hand, radioactive elements are nonrenewable like fossil fuels and could eventually be used up. The main concern over the use of nuclear energy is the risk of radiation. Accidents at nuclear power plants can release harmful radiation that endangers people and other living things. Even without accidents, the used fuel that is left after nuclear fission reactions is still radioactive and very dangerous. It takes thousands of years for it to decay until it no longer releases harmful radiation. Therefore, used fuel must be stored securely to protect people and other living things. Click image to the left or use the URL below. URL: | text | null |
L_0971 | nuclear fusion | T_4677 | In nuclear fusion, two or more small nuclei combine to form a single, larger nucleus. You can see an example in the Figure 1.1. In this example, nuclei of two hydrogen isotopes (tritium and deuterium) fuse to form a helium nucleus. A neutron and a tremendous amount of energy are also released. | text | null |
L_0971 | nuclear fusion | T_4678 | Nuclear fusion of hydrogen to form helium occurs naturally in the sun and other stars. It takes place only at extremely high temperatures. Thats because a great deal of energy is needed to overcome the force of repulsion between the positively charged nuclei. The suns energy comes from fusion in its core, shown in the Figure 1.2. In the core, temperatures reach millions of degrees Kelvin. Click image to the left or use the URL below. URL: The Sun Q: Why doesnt nuclear fusion occur naturally on Earth? A: Nuclear fusion doesnt occur naturally on Earth because it requires temperatures far higher than Earth tempera- tures. | text | null |
L_0971 | nuclear fusion | T_4679 | Scientists are searching for ways to create controlled nuclear fusion reactions on Earth. Their goal is develop nuclear fusion power plants, where the energy from fusion of hydrogen nuclei can be converted to electricity. You can see how this might work in the Figure 1.3. In the thermonuclear reactor, radiation from fusion is used to heat water and produce steam. The steam can then be used to turn a turbine and generate electricity. The use of nuclear fusion for energy has several pros. Unlike nuclear fission, which involves dangerous radioactive elements, nuclear fusion involves just hydrogen and helium. These elements are harmless. Hydrogen is also very plentiful. There is a huge amount of hydrogen in ocean water. The hydrogen in just a gallon of water could produce as much energy by nuclear fusion as burning 1,140 liters (300 gallons) of gasoline! The hydrogen in the oceans would generate enough energy to supply all the worlds people for a very long time. Unfortunately, using energy from nuclear fusion is far from a reality. Scientists are a long way from developing the necessary technology. One problem is raising temperatures high enough for fusion to take place. Another problem is that matter this hot exists only in the plasma state. There are no known materials that can contain plasma, although a magnet might be able to do it. Thats because plasma consists of ions and responds to magnetism. Click image to the left or use the URL below. URL: | text | null |
L_0972 | nucleic acid classification | T_4680 | Nucleic acids are one of four classes of biochemical compounds. (The other three classes are carbohydrates, proteins, and lipids.) Nucleic acids include RNA (ribonucleic acid) as well as DNA (deoxyribonucleic acid). Both types of nucleic acids contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus. Q: Which of the elements in DNA is not identified with any other class of biochemical compounds? A: All biochemical compounds contain carbon, hydrogen, and oxygen; and proteins as well as nucleic acids contain nitrogen. Phosphorus is the only element that is identified with nucleic acids. | text | null |
L_0972 | nucleic acid classification | T_4681 | Nucleic acids consist of chains of small molecules called nucleotides, which are held together by covalent bonds. The structure of a nucleotide is shown in the Figure 1.1. Each nucleotide consists of: 1. a phosphate group, which contains phosphorus and oxygen (PO4 ). 2. a sugar, which is deoxyribose (C5 H8 O4 ) in DNA and ribose (C5 H10 O5 ) in RNA. 3. one of four nitrogen-containing bases. (A base is a compound that is not neither acidic nor neutral.) In DNA, the bases are adenine, thymine, guanine, and cytosine. RNA has the base uracil instead of thymine, but the other three bases are the same. | text | null |
L_0972 | nucleic acid classification | T_4682 | RNA consists of just one chain of nucleotides. DNA consists of two chains. Nitrogen bases on the two chains of DNA form hydrogen bonds with each other. Hydrogen bonds are relatively weak bonds that form between a positively charged hydrogen atom in one molecule and a negatively charged atom in another molecule. Hydrogen bonds form only between adenine and thymine, and between guanine and 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 the Figure 1.2. Sugars and phosphate groups form the backbone of each chain of DNA. The bonded bases are called base pairs. Determining the structure of DNA was a huge scientific breakthrough. Q: Compare the structure of DNA to a spiral staircase. What part of the molecule do the stair steps represent? A: The steps represent the base pairs. | text | null |
L_0972 | nucleic acid classification | T_4683 | 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 copies and interprets the genetic code in DNA and is also involved in the synthesis of proteins based on the code. Click image to the left or use the URL below. URL: Q: DNA is found only in the nucleus of cells, but proteins are synthesized in the cytoplasm of cells, outside of the nucleus. How do you think the instructions encoded in DNA reach the cytoplasm so they can be used to make proteins? A: After RNA copies the instructions in DNA, it carries them from the nucleus to a site of protein synthesis in the cytoplasm, where the instructions are translated into a protein. | text | null |
L_0976 | optical instruments | T_4691 | Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Knowledge of light was needed for the invention of optical instruments such as microscopes, telescopes, and cameras, in addition to optical fibers. These instruments use mirrors and lenses to reflect and refract light and form images. Q: What is an image? A: An image is a copy of an object created by the reflection or refraction of visible light. | text | null |
L_0976 | optical instruments | T_4692 | A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one shown in the Figure lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewers eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! Q: How has the microscope advanced scientific knowledge? A: The microscope has revealed secrets of the natural world like no other single invention. The microscope let scientists see entire new worlds, leading to many discoveriesespecially in biology and medicinethat could not have been made without it. Some examples include the discovery of cells and the identification of bacteria and other single-celled organisms. With the development of more powerful microscopes, viruses were discovered and even atoms finally became visible. These discoveries changed our ideas about the human body and the nature of life itself. | text | null |
L_0976 | optical instruments | T_4693 | Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objectssuch as distant starsthat only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in the Figure 1.2. They differ in how they collect light, but both use convex lenses to form enlarged images. Click image to the left or use the URL below. URL: | text | null |
L_0976 | optical instruments | T_4694 | A camera is an optical instrument that forms and records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as shown in the Figure 1.3. Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture. As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus. The shutter controls the amount of light that actually strikes the film (or sensor). It stays open longer in dim light to let more light in. | text | null |
L_0976 | optical instruments | T_4695 | Did you ever see a cat chase after a laser light, like the one in Figure 1.4? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram in Figure 1.4 shows why a beam of laser light is so focused compared with ordinary light from a flashlight. The following Figure 1.5 provides a closer look at the tube where laser light is created. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. Click image to the left or use the URL below. URL: | text | null |
L_0976 | optical instruments | T_4695 | Did you ever see a cat chase after a laser light, like the one in Figure 1.4? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram in Figure 1.4 shows why a beam of laser light is so focused compared with ordinary light from a flashlight. The following Figure 1.5 provides a closer look at the tube where laser light is created. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. Click image to the left or use the URL below. URL: | text | null |
L_0976 | optical instruments | T_4696 | Besides entertaining a cat, laser light has many other uses. One use is carrying communication signals in optical fibers. Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optical fiber at the same time, as you can see in the Figure Q: When lasers were invented in 1960, they were called "a solution looking for a problem. Since then, they have been put to thousands of different uses. Can you name other ways that lasers are used? A: The first widespread use of lasers was the supermarket barcode scanner, introduced in 1974. The compact disc (CD) player was the first laser-equipped device commonly used by consumers, starting in 1982. The CD player was quickly followed by the laser printer. Some other uses of lasers include bloodless surgery, cutting and welding of metals, guiding missiles, thermometers, laser light shows, and acne treatments. The optical fiber in the diagram is much larger than a real optical fiber, which is only about as wide as a human hair. | text | null |
L_0977 | orbital motion | T_4697 | Earth and many other bodiesincluding asteroids, comets, and the other planetsmove around the sun in curved paths called orbits. Generally, the orbits are elliptical, or oval, in shape. You can see the shape of Earths orbit in the Figure 1.1. Because of the suns relatively strong gravity, Earth and the other bodies constantly fall toward the sun, but they stay far enough away from the sun because of their forward velocity to fall around the sun instead of into it. As a result, they keep orbiting the sun and never crash to its surface. The motion of Earth and the other bodies around the sun is called orbital motion. Orbital motion occurs whenever an object is moving forward and at the same time is pulled by gravity toward another object. | text | null |
L_0977 | orbital motion | T_4698 | Just as Earth orbits the sun, the moon also orbits Earth. The moon is affected by Earths gravity more than it is by the gravity of the sun because the moon is much closer to Earth. The gravity between Earth and the moon pulls the moon toward Earth. At the same time, the moon has forward velocity that partly counters the force of Earths gravity. So the moon orbits Earth instead of falling down to the surface of the planet. The Figure 1.2 shows the forces involved in the moons orbital motion around Earth. In the diagram, v represents the forward velocity of the moon, and a represents the acceleration due to gravity between Earth and the moon. The line encircling Earth shows the moons actual orbit, which results from the combination of v and a. | text | null |
L_0977 | orbital motion | T_4698 | Just as Earth orbits the sun, the moon also orbits Earth. The moon is affected by Earths gravity more than it is by the gravity of the sun because the moon is much closer to Earth. The gravity between Earth and the moon pulls the moon toward Earth. At the same time, the moon has forward velocity that partly counters the force of Earths gravity. So the moon orbits Earth instead of falling down to the surface of the planet. The Figure 1.2 shows the forces involved in the moons orbital motion around Earth. In the diagram, v represents the forward velocity of the moon, and a represents the acceleration due to gravity between Earth and the moon. The line encircling Earth shows the moons actual orbit, which results from the combination of v and a. | text | null |
L_0979 | ph concept | T_4703 | Acids are ionic compounds that produce positively charged hydrogen ions (H+ ) when dissolved in water. Acids taste sour and react with metals. Bases are ionic compounds that produce negatively charged hydroxide ions (OH ) when dissolved in water. Bases taste bitter and do not react with metals. Examples of acids are vinegar and battery acid. The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through skin. Examples of bases include those in antacid tablets and drain cleaner. Bases in antacid tablets are weak enough to take for an upset stomach. Bases in drain cleaner are strong enough to cause serious burns. Q: What do you think causes these differences in the strength of acids and bases? A: The strength of an acid or a base depends on how much of it breaks down into ions when it dissolves in water. | text | null |
L_0979 | ph concept | T_4704 | The strength of an acid depends on how many hydrogen ions it produces when it dissolves in water. A stronger acid produces more hydrogen ions than a weaker acid. For example, sulfuric acid (H2 SO4 ), which is found in car batteries, is a strong acid because nearly all of it breaks down into ions when it dissolves in water. On the other hand, acetic acid (CH3 CO2 H), which is the acid in vinegar, is a weak acid because less than 1 percent of it breaks down into ions in water. The strength of a base depends on how many hydroxide ions it produces when it dissolves in water. A stronger base produces more hydroxide ions than a weaker base. For example, sodium hydroxide (NaOH), a base in drain cleaner, is a strong base because all of it breaks down into ions when it dissolves in water. Calcium carbonate (CaCO3 ), a base in antacids, is a weak base because only a small percentage of it breaks down into ions in water. | text | null |
L_0979 | ph concept | T_4705 | The strength of acids and bases is measured on a scale called the pH scale, which is shown in the Figure 1.1. By definition, pH represents the acidity, or hydrogen ion (H+ ) concentration, of a solution. Pure water, which is neutral, has a pH of 7. With a higher the concentration of hydrogen ions, a solution is more acidic and has a lower pH. Acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. Its important to realize that the pH scale is based on powers of ten. For example, a solution with a pH of 8 is 10 times more basic than a solution with a pH of 7, and a solution with a pH of 9 is 100 times more basic than a solution with a pH of 7. Q: How much more acidic is a solution with a pH of 4 than a solution with a pH of 7? A: A solution with a pH of 4 is 1000 (10 10 10, or 103 ) times more acidic than a solution with a pH of 7. Q: Which solution on the pH scale in the Figure 1.1 is the weakest acid? Which solution is the strongest base? A: The weakest acid on the scale is milk, which has a pH value between 6.5 and 6.8. The strongest base on the scale is liquid drain cleaner, which has a pH of 14. | text | null |
L_0979 | ph concept | T_4706 | Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish may also need a pH between 6 and 7. Certain air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower. The pH chart in the Figure lowers the pH of surface waters such as ponds and lakes. As a result, the water may become too acidic for fish and other water organisms to survive. Acid fog and acid rain killed the trees in this forest. Even normal (clean) rain is somewhat acidic. Thats because carbon dioxide (CO2 ) in the air dissolves in raindrops, producing a weak acid called carbonic acid (H2 CO3 ), which has a pH of about 5.5. When rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms underground caves. Q: How do you think acid rain might affect buildings and statues made of stone? A: Acid rain dissolves and damages stone buildings and statues. The Figure 1.3 shows a statue that has been damaged by acid rain. | text | null |
L_0979 | ph concept | T_4706 | Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish may also need a pH between 6 and 7. Certain air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower. The pH chart in the Figure lowers the pH of surface waters such as ponds and lakes. As a result, the water may become too acidic for fish and other water organisms to survive. Acid fog and acid rain killed the trees in this forest. Even normal (clean) rain is somewhat acidic. Thats because carbon dioxide (CO2 ) in the air dissolves in raindrops, producing a weak acid called carbonic acid (H2 CO3 ), which has a pH of about 5.5. When rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms underground caves. Q: How do you think acid rain might affect buildings and statues made of stone? A: Acid rain dissolves and damages stone buildings and statues. The Figure 1.3 shows a statue that has been damaged by acid rain. | text | null |
L_0980 | photosynthesis reactions | T_4707 | Most of the energy used by living things comes either directly or indirectly from the sun. Thats because sunlight provides the energy for photosynthesis. This is the process in which plants and certain other organisms 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, in turn, is used for energy by the cells of almost all living things. Photosynthetic organisms such as plants make their own glucose. Other organisms get glucose by consuming plants (or organisms that consume plants). Q: How do living things get energy from glucose? A: They break bonds in glucose and release the stored energy in the process of cellular respiration. | text | null |
L_0980 | photosynthesis reactions | T_4708 | The organisms pictured in the Figures 1.1, 1.2, and 1.3 all use sunlight to make glucose in the process of photo- synthesis. In addition to plants, they include bacteria and algae. All of these organisms contain the green pigment chlorophyll, which is needed to capture light energy. A tremendous amount of photosynthesis takes place in the plants of this lush tropi- cal rainforest. | text | null |
L_0980 | photosynthesis reactions | T_4708 | The organisms pictured in the Figures 1.1, 1.2, and 1.3 all use sunlight to make glucose in the process of photo- synthesis. In addition to plants, they include bacteria and algae. All of these organisms contain the green pigment chlorophyll, which is needed to capture light energy. A tremendous amount of photosynthesis takes place in the plants of this lush tropi- cal rainforest. | text | null |
L_0985 | position time graphs | T_4725 | The motion of an object can be represented by a position-time graph like Graph 1 in the Figure 1.1. In this type of graph, the y-axis represents position relative to the starting point, and the x-axis represents time. A position-time graph shows how far an object has traveled from its starting position at any given time since it started moving. Q: In the Figure 1.1, what distance has the object traveled from the starting point by the time 5 seconds have elapsed? A: The object has traveled a distance of 50 meters. | text | null |
L_0985 | position time graphs | T_4726 | In a position-time graph, the velocity of the moving object is represented by the slope, or steepness, of the graph line. If the graph line is horizontal, like the line after time = 5 seconds in Graph 2 in the Figure 1.2, then the slope is zero and so is the velocity. The position of the object is not changing. The steeper the line is, the greater the slope of the line is and the faster the objects motion is changing. | text | null |
L_0985 | position time graphs | T_4726 | In a position-time graph, the velocity of the moving object is represented by the slope, or steepness, of the graph line. If the graph line is horizontal, like the line after time = 5 seconds in Graph 2 in the Figure 1.2, then the slope is zero and so is the velocity. The position of the object is not changing. The steeper the line is, the greater the slope of the line is and the faster the objects motion is changing. | text | null |
L_0985 | position time graphs | T_4727 | Its easy to calculate the average velocity of a moving object from a position-time graph. Average velocity equals the change in position (represented by d) divided by the corresponding change in time (represented by t): velocity = d t For example, in Graph 2 in the Figure 1.2, the average velocity between 0 seconds and 5 seconds is: d t 25 m 0 m = 5 s0 s 25 m = 5s = 5 m/s velocity = | text | null |
L_0986 | potential energy | T_4728 | The diver has energy because of her position high above the pool. The type of energy she has is called potential energy. Potential energy is energy that is stored in a person or object. Often, the person or object has potential energy because of its position or shape. Q: What is it about the divers position that gives her potential energy? A: Because the diver is high above the water, she has the potential to fall toward Earth because of gravity. This gives her potential energy. | text | null |
L_0986 | potential energy | T_4729 | Potential energy due to the position of an object above Earths surface is called gravitational potential energy. Like the diver on the diving board, anything that is raised up above Earths surface has the potential to fall because of gravity. You can see another example of people with gravitational potential energy in the Figure 1.1. Gravitational potential energy depends on an objects weight and its height above the ground. It can be calculated with the equation: Gravitational potential energy (GPE) = weight height Consider the little girl on the sled, pictured in the Figure 1.1. She weighs 140 Newtons, and the top of the hill is 4 meters higher than the bottom of the hill. As she sits at the top of the hill, the childs gravitational potential energy is: GPE = 140 N 4 m = 560 N m Notice that the answer is given in Newton meters (N m), which is the SI unit for energy. A Newton meter is the energy needed to move a weight of 1 Newton over a distance of 1 meter. A Newton meter is also called a joule (J). Q: The gymnast on the balance beam pictured in the Figure 1.1 weighs 360 Newtons. If the balance beam is 1.2 meters above the ground, what is the gymnasts gravitational potential energy? A: Her gravitational potential energy is: GPE = 360 N 1.2 m = 432 N m, or 432 J | text | null |
L_0986 | potential energy | T_4730 | Potential energy due to an objects shape is called elastic potential energy. This energy results when an elastic object is stretched or compressed. The farther the object is stretched or compressed, the greater its potential energy is. A point will be reached when the object cant be stretched or compressed any more. Then it will forcefully return to its original shape. Look at the pogo stick in the Figure 1.2. Its spring has elastic potential energy when it is pressed down by the boys weight. When it cant be compressed any more, it will spring back to its original shape. The energy it releases will push the pogo stickand the boyoff the ground. Q: The girl in the Figure 1.3 is giving the elastic band of her slingshot potential energy by stretching it. Shes holding a small stone against the stretched band. What will happen when she releases the band? A: The elastic band will spring back to its original shape. When that happens, watch out! Some of the bands elastic potential energy will be transferred to the stone, which will go flying through the air. | text | null |
L_0986 | potential energy | T_4731 | All of the examples of potential energy described above involve movement or the potential to move. The form of energy that involves movement is called mechanical energy. Other forms of energy also involve potential energy, including chemical energy and nuclear energy. Chemical energy is stored in the bonds between the atoms of compounds. For example, food and batteries both contain chemical energy. Nuclear energy is stored in the nuclei of atoms because of the strong forces that hold the nucleus together. Nuclei of radioactive elements such as uranium are unstable, so they break apart and release the stored energy. | text | null |
L_0986 | potential energy | T_4731 | All of the examples of potential energy described above involve movement or the potential to move. The form of energy that involves movement is called mechanical energy. Other forms of energy also involve potential energy, including chemical energy and nuclear energy. Chemical energy is stored in the bonds between the atoms of compounds. For example, food and batteries both contain chemical energy. Nuclear energy is stored in the nuclei of atoms because of the strong forces that hold the nucleus together. Nuclei of radioactive elements such as uranium are unstable, so they break apart and release the stored energy. | text | null |
L_0987 | power | T_4732 | Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules (J) and time is measured in seconds (s), so power is expressed in joules per second (J/s). This is the SI unit for power, also known as the watt (W). A watt equals 1 joule of work per second. Youre probably already familiar with watts. Light bulbs and small appliances such as microwave ovens are labeled with the watts of power they provide. For example, the package of light bulbs in the Figure 1.1 is labeled 14 watts. Q: Assume you have two light bulbs of the same type, such as two compact fluorescent light bulbs like the one pictured in the Figure 1.1. If one light bulb is a 25-watt bulb and the other is a 60-watt bulb, which bulb produces brighter light? A: The 60-watt bulb is more powerful, so it produces brighter light. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave oven, a high-power microwave oven can cook more food in the same time or the same amount of food in less time. | text | null |
L_0987 | power | T_4733 | Power can be calculated using the formula above if the amount of work and time are known. For example, assume that a microwave oven does 24,000 joules of work in 30 seconds. Then the power of the microwave is: 24000 J Power = Work Time = 30 s = 800 J/s, or 800 W Q: Another microwave oven does 5,000 joules of work in 5 seconds. What is its power? A: The power of the other microwave oven is: J Power = 5000 5 s = 1000 J/s, or 1000 W Q: Which microwave oven will heat the same amount of food in less time? A: The 1000-watt microwave oven has more power, so it will heat the same amount of food in less time. | text | null |
L_0987 | power | T_4734 | You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power Time For example, if you use a 1000-watt microwave oven for 20 seconds, how much work does it do? First express 1000 watts in J/s and then substitute this value for power the work equation: Work = 1000 J/s 20 s = 20,000 J | text | null |
L_0987 | power | T_4735 | Sometimes power is measured in a unit called the horsepower. For example, the power of car engines is usually expressed in horsepowers. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. Compare the horsepowers in the Figure 1.2 to the other Figure 1.3. This team of three horses provides 3 horsepowers of power. This big tractor provides 180 horsepowers of power. Q: If the team of horses and the tractor do the same amount of work plowing a field, which will get the job done faster? A: The tractor will get the job done faster because it has more power. In fact, because the tractor has 30 times the power of the six-horse team, ideally it can do the same work 30 times faster! | text | null |
L_0987 | power | T_4735 | Sometimes power is measured in a unit called the horsepower. For example, the power of car engines is usually expressed in horsepowers. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. Compare the horsepowers in the Figure 1.2 to the other Figure 1.3. This team of three horses provides 3 horsepowers of power. This big tractor provides 180 horsepowers of power. Q: If the team of horses and the tractor do the same amount of work plowing a field, which will get the job done faster? A: The tractor will get the job done faster because it has more power. In fact, because the tractor has 30 times the power of the six-horse team, ideally it can do the same work 30 times faster! | text | null |
L_0989 | projectile motion | T_4741 | When the archer releases the bowstring, the arrow will be flung forward toward the top of the target where shes aiming. But another force will also act on the arrow in a different direction. The other force is gravity, and it will pull the arrow down toward Earth. The two forces combined will cause the arrow to move in the curved path shown in the Figure 1.1. This type of motion is called projectile motion. It occurs whenever an object curves down toward the ground because it has both a horizontal force and the downward force of gravity acting on it. Because of projectile motion, to hit the bulls eye of a target with an arrow, you actually have to aim for a spot above the bulls eye. You can see in theFigure 1.2 what happens if you aim at the bulls eye instead of above it. | text | null |
L_0989 | projectile motion | T_4742 | You can probably think of other examples of projectile motion. One is shown in the Figure 1.3. The cannon shoots a ball straight ahead, giving it horizontal motion. At the same time, gravity pulls the ball down toward the ground. Q: How would you show the force of gravity on the cannon ball in the Figure 1.3? A: You would add a line pointing straight down from the cannon to the ground. | text | null |
L_0989 | projectile motion | T_4742 | You can probably think of other examples of projectile motion. One is shown in the Figure 1.3. The cannon shoots a ball straight ahead, giving it horizontal motion. At the same time, gravity pulls the ball down toward the ground. Q: How would you show the force of gravity on the cannon ball in the Figure 1.3? A: You would add a line pointing straight down from the cannon to the ground. | text | null |
L_0989 | projectile motion | T_4742 | You can probably think of other examples of projectile motion. One is shown in the Figure 1.3. The cannon shoots a ball straight ahead, giving it horizontal motion. At the same time, gravity pulls the ball down toward the ground. Q: How would you show the force of gravity on the cannon ball in the Figure 1.3? A: You would add a line pointing straight down from the cannon to the ground. | text | null |
L_0990 | properties of acids | T_4743 | Acids are ionic compounds that produce positive hydrogen ions (H+ ) when dissolved in water. Ionic compounds are compounds that contain positive metal ions and negative nonmetal ions held together by ionic bonds. (Ions are atoms that have become charged particles by gaining or losing electrons.) An example of an acid is hydrogen chloride (HCl). When it dissolves in water, it separates into positive hydrogen ions and negative chloride ions (Cl ). This is represented by the chemical equation: H O 2 HCl H+ + Cl | text | null |
L_0990 | properties of acids | T_4744 | You already know that a sour taste is one property of acids. (Warning: Never taste an unknown substance to see whether it is an acid!) Acids have certain other properties as well. For example, acids can conduct electricity when dissolved in water because they consist of charged particles in solution. (Electric current is a flow of charged particles.) Acids can also react with metals, and when they do they produce hydrogen gas. An example of this type of reaction is hydrochloric acid reacting with the metal zinc (Zn). The reaction is pictured in the Figure 1.1. It can be represented by the chemical equation: Zn + 2HCl H2 + ZnCl2 Q: What sign indicates that a gas is being produced in this reaction? A: The bubbles are hydrogen gas rising through the acid. Q: Besides hydrogen gas, what else is produced in this reaction? A: This reaction also produces zinc chloride ZnCl2 , which is a neutral ionic compound called a salt. | text | null |
L_0990 | properties of acids | T_4745 | Certain compounds, called indicators, change color when acids come into contact with them, so indicators can be used to detect acids. An example of an indicator is the compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of acid on a strip of blue litmus paper, the paper will turn red. You can see this in the Figure 1.2. Litmus isnt the only indicator for detecting acids. Red cabbage juice also works well, as you can see in this entertaining video. Click image to the left or use the URL below. URL: Drawing of blue litmus paper turning red in acid. | text | null |
L_0990 | properties of acids | T_4746 | The strength of acids is measured on a scale called the pH scale. The pH value of a solution represents its concentration of hydrogen ions. A pH value of 7 indicates a neutral solution, and a pH value less than 7 indicates an acidic solution. The lower the pH value is, the greater is the concentration of hydrogen ions and the stronger the acid. The strongest acids, such as battery acid, have pH values close to zero. | text | null |
L_0990 | properties of acids | T_4747 | Acids have many important uses, especially in industry. For example, sulfuric acid is used to manufacture a variety of different products, including paper, paint, and detergent. Some other uses of acids are be seen in the Figure 1.3. | text | null |
L_0991 | properties of bases | T_4748 | Bases are ionic compounds that produce negative hydroxide ions (OH ) when dissolved in water. An ionic com- pound contains positive metal ions and negative nonmetal ions held together by ionic bonds. (Ions are atoms that have become charged particles because they have either lost or gained electrons.) An example of a base is sodium hydroxide (NaOH). When it dissolves in water, it produces negative hydroxide ions and positive sodium ions (Na+ ). This can be represented by the equation: H O 2 NaOH OH + Na+ | text | null |
L_0991 | properties of bases | T_4749 | All bases share certain properties, including a bitter taste. (Warning: Never taste an unknown substance to see whether it is a base!) Bases also feel slippery. Think about how slippery soap feels. Thats because its a base. In addition, bases conduct electricity when dissolved in water because they consist of charged particles in solution. (Electric current is a flow of charged particles.) Q: Bases are closely related to compounds called acids. How are their properties similar? How are they different? A: A property that is shared by bases and acids is the ability to conduct electricity when dissolved in water. Some ways bases and acids are different is that acids taste sour whereas bases taste bitter. Also, acids but not bases react with metals. | text | null |
L_0991 | properties of bases | T_4750 | Certain compounds, called indicators, change color when bases come into contact with them, so they can be used to detect bases. An example of an indicator is a compound called litmus. It is placed on small strips of paper that may be red or blue. If you place a few drops of a base on a strip of red litmus paper, the paper will turn blue. You can see this in the Figure 1.1. Litmus isnt the only detector of bases. Red cabbage juice can also detect bases, as you can see in this video. Click image to the left or use the URL below. URL: Drawing of red litmus paper turning blue in a base. | text | null |
L_0991 | properties of bases | T_4751 | The strength of bases is measured on a scale called the pH scale, which ranges from 0 to 14. On this scale, a pH value of 7 indicates a neutral solution, and a pH value greater than 7 indicates a basic solution. The higher the pH value is, the stronger the base. The strongest bases, such as drain cleaner, have a pH value close to 14. | text | null |
L_0991 | properties of bases | T_4752 | Bases are used for a variety of purposes. For example, soaps contain bases such as potassium hydroxide (KOH). Other uses of bases can be seen in the Figure 1.2. | text | null |
L_0992 | properties of electromagnetic waves | T_4753 | All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is about 300 million meters per second (3.0 x 108 m/s). Nothing else in the universe is known to travel this fast. The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! | text | null |
L_0992 | properties of electromagnetic waves | T_4754 | Although all electromagnetic waves travel at the same speed across space, they may differ in their wavelengths, frequencies, and energy levels. Wavelength is the distance between corresponding points of adjacent waves (see the Figure 1.1). Wavelengths of electromagnetic waves range from longer than a soccer field to shorter than the diameter of an atom. Wave frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electromagnetic waves range from thousands of waves per second to trillions of waves per second. The energy of electromagnetic waves depends on their frequency. Low-frequency waves have little energy and are normally harmless. High-frequency waves have a lot of energy and are potentially very harmful. Q: Which electromagnetic waves do you think have higher frequencies: visible light or X rays? A: X rays are harmful but visible light is harmless, so you can infer that X rays have higher frequencies than visible light. | text | null |
L_0992 | properties of electromagnetic waves | T_4755 | The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation: Speed = Wavelength Frequency The equation for wave speed can be rewritten as: Speed Speed Frequency = Wavelength or Wavelength = Frequency Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag- netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0 108 meters per second. Its frequency can be found by substituting these values into the frequency equation: Frequency = 3.0108 m/s 3.0 m = 1.0 108 waves/s, or 1.0 108 Hz Q: What is the wavelength of an electromagnetic wave that has a frequency of 3.0 108 hertz? A: Use the wavelength equation: Wavelength = 3.0108 m/s 3.0108 waves/s = 1.0 m | text | null |
L_0994 | protein classification | T_4759 | Hemoglobin is a compound in the class of compounds called proteins. Proteins are one of four classes of biochemi- cal compounds, which are compounds in living things. (The other three classes are carbohydrates, lipids, and nucleic acids.) Proteins contain carbon, hydrogen, oxygen, nitrogen, and sulfur. Protein molecules consist of one or more chains of small molecules called amino acids. | text | null |
L_0994 | protein classification | T_4760 | Amino acids are the building blocks of proteins. There are 20 different amino acids. The structural formula of the simplest amino acid, called glycine, is shown in the Figure 1.1. Other amino acids have slightly different structures. A protein molecule is made from one or more long chains of amino acids, each linked to its neighbors by covalent bonds. If a protein has more than one chain, the chains are held together by weaker bonds, such as hydrogen bonds. The sequence of amino acids in chains and the number of chains in a protein determine the proteins shape. The shape of a protein, in turn, determines its function. Shapes may be very complex. Click image to the left or use the URL below. URL: Q: What do you think the ribbons in the colorful hemoglobin molecule pictured in the opening image represent? A: The ribbons represent chains of amino acids. | text | null |
L_0994 | protein classification | T_4761 | Proteins are the most numerous and diverse biochemical compounds, and they have many different functions. Some of their functions include: making up tissues as components of muscle. speeding up biochemical reactions as enzymes. regulating life processes as hormones. helping to defend against infections as antibodies. carrying materials around the body as transport proteins (see the example of hemoglobin in the Figure 1.2). | text | null |
L_0995 | protons | T_4762 | A proton is one of three main particles that make up the atom. The other two particles are the neutron and electron. Protons are found in the nucleus of the atom. This is a tiny, dense region at the center of the atom. Protons have a positive electrical charge of one (+1) and a mass of 1 atomic mass unit (amu), which is about 1.67 1027 kilograms. Together with neutrons, they make up virtually all of the mass of an atom. Click image to the left or use the URL below. URL: Q: How do you think the sun is related to protons? A: The suns tremendous energy is the result of proton interactions. In the sun, as well as in other stars, protons from hydrogen atoms combine, or fuse, to form nuclei of helium atoms. This fusion reaction releases a huge amount of energy and takes place in nature only at the extremely high temperatures of stars such as the sun. | text | null |
L_0995 | protons | T_4763 | All protons are identical. For example, hydrogen protons are exactly the same as protons of helium and all other elements, or pure substances. However, atoms of different elements have different numbers of protons. In fact, atoms of any given element have a unique number of protons that is different from the numbers of protons of all other elements. For example, a hydrogen atom has just one proton, whereas a helium atom has two protons. The number of protons in an atom determines the electrical charge of the nucleus. The nucleus also contains neutrons, but they are neutral in charge. The one proton in a hydrogen nucleus, for example, gives it a charge of +1, and the two protons in a helium nucleus give it a charge of +2. | text | null |
L_0995 | protons | T_4764 | Protons are made of fundamental particles called quarks and gluons. As you can see in the Figure 1.1, a proton contains three quarks (colored circles) and three streams of gluons (wavy white lines). Two of the quarks are called up quarks (u), and the third quark is called a down quark (d). The gluons carry the strong nuclear force between quarks, binding them together. This force is needed to overcome the electric force of repulsion between positive protons. Although protons were discovered almost 100 years ago, the quarks and gluons inside them were discovered much more recently. Scientists are still learning more about these fundamental particles. | text | null |
L_0996 | pulley | T_4765 | A pulley is a simple machine that consists of a rope and grooved wheel. The rope fits into the groove in the wheel, and pulling on the rope turns the wheel. Pulleys are generally used to lift objects, especially heavy objects. The object lifted by a pulley is called the load. The force applied to the pulley is called the effort. Q: Can you guess what the pulley pictured above is used for? A: The pulley is used to lift heavy buckets full of water out of the well. | text | null |
L_0996 | pulley | T_4766 | Some pulleys are attached to a beam or other secure surface and remain fixed in place. They are called fixed pulleys. Other pulleys are attached to the object being moved and are moveable themselves. They are called moveable pulleys. Sometimes, fixed and moveable pulleys are used together. They make up a compound pulley. The three types of pulleys are compared in the Table 1.1. Q: Which type of pulley is the old pulley in the opening image? A: The old pulley is a single fixed pulley. It is securely attached to the beam above it. Type of Pulley How It Works Example Single fixed pul- ley Flagpole pulley No. of Rope Segments Pulling Up 1 Ideal Mechani- cal Advantage 1 Change Direction Force? yes Single moveable pulley Zip-line pulley 2 2 no Compound pulley (fixed & moveable pulleys) Crane pulley 2 2 varies in of | text | null |
L_0996 | pulley | T_4767 | The mechanical advantage of a simple machine such as a pulley is the factor by which the machine changes the force applied to it. The ideal mechanical advantage of a machine is its mechanical advantage in the absence of friction. All machines must overcome friction, so the ideal mechanical advantage is always somewhat greater than the actual mechanical advantage of the machine as it is used in the real world. In a pulley, the ideal mechanical advantage is equal to the number of rope segments pulling up on the object. The more rope segments that are helping to do the lifting work, the less force that is needed for the job. Look at the table of types of pulleys. It gives the ideal mechanical advantage of each type. In the single fixed pulley, only one rope segment pulls up on the load, so the ideal mechanical advantage is 1. In other words, this type of pulley doesnt increase the force that is applied to it. However, it does change the direction of the force. This allows you to use your weight to pull on one end of the rope and more easily raise the load attached to the other end. In the single moveable pulley, two rope segments pull up on the load, so the ideal mechanical advantage is 2. This type of pulley doesnt change the direction of the force applied to it, but it increases the force by a factor of 2. In a compound pulley, two or more rope segments pull up on the load, so the ideal mechanical advantage is 2 or greater than 2. This type of pulley may or may not change the direction of the force applied to itit depends on the number and arrangement of pulleysbut the increase in force may be great. Q: If a compound pulley has four rope segments pulling up on the load, by what factor does it multiply the force applied to the pulley? A: With four rope segments, the ideal mechanical advantage is 4. This means that the compound pulley multiplies the force applied to it by a factor of 4. For example if 400 Newtons of force were applied to the pulley, the pulley would apply 1600 Newtons of force to the load. | text | null |
L_0997 | radio waves | T_4768 | Electromagnetic waves consist of vibrating electric and magnetic fields. They transfer energy across space as well as through matter. Electromagnetic waves vary in their wavelengths and frequencies, and higher-frequency waves have more energy. The full range of wavelengths of electromagnetic waves is called the electromagnetic spectrum. It is outlined in the following Figure 1.1. | text | null |
L_0997 | radio waves | T_4769 | Electromagnetic waves on the left side of the Figure 1.1 are called radio waves. Radio waves are electromagnetic waves with the longest wavelengths. They may have wavelengths longer than a soccer field. They are also the electromagnetic waves with the lowest frequencies. With their low frequencies, they have the least energy of all electromagnetic waves. Nonetheless, radio waves are very useful. They are used for radio and television broadcasts and many other purposes. Click image to the left or use the URL below. URL: Q: Based on the electromagnetic spectrum Figure 1.1, what is the range of frequencies of radio waves? A: The range of frequencies of radio waves is between 105 and 1012 Hz, or waves per second. | text | null |
L_0997 | radio waves | T_4770 | In radio broadcasts, sounds are encoded in radio waves, and then the waves are sent out through the atmosphere from a radio tower. A radio receiver detects the waves and changes them back to sounds. You may have listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts. AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the am- plitude, or maximum height, of radio waves. AM broadcasts use longer wavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in the Figure 1.2. Because the waves are reflected, they can reach radio receivers that are very far away from the radio tower. FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually produce clearer sounds than AM broadcasts. However, the relatively short wavelengths of FM waves means that they dont reflect off the ionosphere as AM waves do. Instead, FM waves pass through the ionosphere and out into space. This is also shown in the Figure 1.2. As a result, FM waves cannot reach very distant receivers. Q: The composition of the ionosphere changes somewhat from day to night. The changes make the nighttime ionosphere even better at reflecting AM radio waves. How do you think this might affect the distance AM radio waves travel at night? A: With greater reflection off the ionosphere, AM waves can travel even farther at night than they can during the day. Radio receivers can often pick up radio broadcasts at night from cities that are hundreds of miles away. | text | null |
L_0997 | radio waves | T_4771 | Television broadcasts also use radio waves (see Figure 1.2). For TV broadcasts, sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded waves are broadcast from a TV tower. When the waves are received by television sets, they are decoded and changed back to sounds and pictures. | text | null |
L_0998 | radioactive decay | T_4772 | Radioactive decay is the process in which the nuclei of radioactive atoms emit charged particles and energy, which are called by the general term radiation. Radioactive atoms have unstable nuclei, and when the nuclei emit radiation, they become more stable. Radioactive decay is a nuclearrather than chemicalreaction because it involves only the nuclei of atoms. In a nuclear reaction, one element may change into another. Click image to the left or use the URL below. URL: | text | null |
L_0998 | radioactive decay | T_4773 | There are several types of radioactive decay, including alpha, beta, and gamma decay. In all three types, nuclei emit radiation, but the nature of the radiation differs. The Table 1.1 shows the radiation emitted in each type of decay. Type Alpha decay Beta decay Gamma decay Radiation Emitted alpha particle (2 protons and 2 neutrons) + energy beta particle (1 electron or 1 positron) + energy energy (gamma ray) | text | null |
L_0998 | radioactive decay | T_4774 | Both alpha and beta decay change the number of protons in an atoms nucleus, thereby changing the atom to a different element. In alpha decay, the nucleus loses two protons. In beta decay, the nucleus either loses a proton or gains a proton. In gamma decay, no change in proton number occurs, so the atom does not become a different element. Q: If the radioactive element polonium (Po) undergoes alpha decay, what element does it become? A: From the periodic table, the atomic number of polonium is 84, so it has 84 protons. If it loses two protons through alpha decay, it will have 82 protons. Atoms with 82 protons are the element lead (Pb). | text | null |
L_0998 | radioactive decay | T_4775 | The charged particles and energy emitted during radioactive decay can harm living things, but the three types of radioactive decay arent equally dangerous. Thats because they differ in how far they can travel and what they can penetrate. You can see this in the Figure 1.1. | text | null |
L_0999 | radioactivity | T_4776 | For an atom of one element to change into a different element, the number of protons in its nucleus must change. Thats because each element has a unique number of protons. For example, lead atoms always have 82 protons, and gold atoms always have 79 protons. Q: So how can one element change into another? A: The starting element must be radioactive, and its nuclei must gain or lose protons. | text | null |
L_0999 | radioactivity | T_4777 | Radioactivity is the ability of an atom to emit, or give off, charged particles and energy from its nucleus. The charged particles and energy are called by the general term radiation. Only unstable nuclei emit radiation. They are unstable because they have too much energy, too many protons, or an unstable ratio of protons to neutrons. For example, all elements with more than 83 protonssuch as uranium, radium, and poloniumhave unstable nuclei. They are called radioactive elements. The nuclei of these elements must lose protons to become more stable. When they do, they become different elements. | text | null |
L_0999 | radioactivity | T_4778 | Radioactivity was discovered in 1896 by a French physicist named Antoine Henri Becquerel, who is pictured 1.1. Becquerel was experimenting with uranium, which was known to glow after being exposed to sunlight. Becquerel wanted to see if the glow was caused by rays of energy, like rays of light or X-rays. He placed a bit of uranium on a photographic plate after exposing the uranium to sunlight. The plate was similar to the film that is used today to take X-rays, and Becquerel expected the uranium to leave an image on the plate. The next day, there was an image on the plate, just as Becquerel expected. This meant that uranium gives off rays after being exposed to sunlight. Becquerel was a good scientist, so he wanted to repeat his experiment to confirm his results. He placed more uranium on another photographic plate. However, the day had turned cloudy, so he tucked the plate and uranium in a drawer to try again another day. He wasnt expecting the uranium to leave an image on the plate without first being exposed to sunlight. To his surprise, there was an image on the plate in the drawer the next day. Becquerel had discovered that uranium gives off rays of energy on its own. He had discovered radioactivity, for which he received a Nobel prize. Another scientist, who worked with Becquerel, actually came up with the term radioactivity. The other scientist was the French chemist Marie Curie. She went on to discover the radioactive elements polonium and radium. She won two Nobel Prizes for her discoveries. | text | null |
L_1000 | radioisotopes | T_4779 | All the atoms of a given element have the same number of protons in their nucleus, but they may have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Many elements have one or more isotopes that are radioactive. These isotopes are called radioisotopes. Their nuclei are unstable, so they break down, or decay, and emit radiation. Q: What makes the nucleus of a radioisotope unstable? A: The nucleus may be unstable because it has too many protons or an unstable ratio of protons to neutrons. For a nucleus with a small number of protons to be stable, the ratio of protons to neutrons should be 1:1. For a nucleus with a large number of protons to be stable, the ratio should be about 1:1.5. | text | null |
L_1000 | radioisotopes | T_4780 | Find carbon in the Figure 1.1, and youll see that its atomic number is 6. This means that all carbon atoms have 6 protons per nucleus. Almost all carbon atoms also have 6 neutrons per nucleus. These carbon atoms are called carbon-12, where 12 is the number of protons (6) plus neutrons (6). This gives carbon-12 nuclei a 1:1 ratio of protons to neutrons, so carbon-12 nuclei are stable. Some carbon atoms have more than 6 neutrons, either 7 or 8. Carbon atoms with 8 neutrons are called carbon-14 (6 protons + 8 neutrons). The nuclei of carbon-14 atoms are unstable because they have too many neutrons relative to protons, so they gradually decay. Q: What is the proton-to-neutron ratio of carbon-14 nuclei? A: With six protons and 8 neutrons, the ratio is 6:8, or 1:1.3. Q: How is carbon-14 used to estimate the ages of fossils? A: Living things take in carbon, including tiny amounts of carbon-14, throughout life. The carbon-14 constantly decays, but more carbon-14 is taken in all the time to replace it. After living things die, no new carbon-14 is taken in, and the carbon-14 they already have keeps decaying. The older a fossil is, the less carbon-14 it still has, so the remaining amount can be measured to estimate the fossils age. Click image to the left or use the URL below. URL: Periodic Table of the Elements | text | null |
L_1000 | radioisotopes | T_4781 | In elements with more than 83 protons, all of the isotopes are radioactive. In the Figure 1.1, these are the elements with a yellow background. The force of repulsion among all those protons makes the nuclei unstable. Elements with more than 92 protons have such unstable nuclei that they dont even exist in nature. They have only been created in labs. | text | null |
L_1002 | reactants and products | T_4786 | All chemical reactionsincluding a candle burninginvolve reactants and products. Reactants are substances that start a chemical reaction. Products are substances that are produced in the reaction. When a candle burns, the reactants are fuel (the candlewick and wax) and oxygen (in the air). The products are carbon dioxide gas and water vapor. | text | null |
L_1002 | reactants and products | T_4787 | The relationship between reactants and products in a chemical reaction can be represented by a chemical equation that has this general form: Reactants Products The arrow () shows the direction in which the reaction occurs. In many reactions, the reaction also occurs in the opposite direction. This is represented with another arrow pointing in the opposite direction (). Q: Write a general chemical equation for the reaction that occurs when a fuel such as candle wax burns. A: The burning of fuel is a combustion reaction. The general equation for this type of reaction is: Fuel + O2 CO2 + H2 O Q: How do the reactants in a chemical reaction turn into the products? A: Bonds break in the reactants, and new bonds form in the products. | text | null |
L_1002 | reactants and products | T_4788 | The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms end up in different combinations in the products. This makes the products new substances that are chemically different from the reactants. Consider the example of water forming from hydrogen and oxygen. Both hydrogen and oxygen gases exist as diatomic (two-atom) molecules. These molecules are the reactants in the reaction. The Figure 1.1 shows that bonds must break to separate the atoms in the hydrogen and oxygen molecules. Then new bonds must form between hydrogen and oxygen atoms to form water molecules. The water molecules are the products of the reaction. Q: Watch the animation of a similar chemical reaction at the following URL. Can you identify the reactants and the product in the reaction? Click image to the left or use the URL below. URL: A: The reactants are hydrogen (H2 ) and fluorine (F2 ), and the product is hydrogen fluoride (HF). | text | null |
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