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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 |
L_1003 | recognizing chemical reactions | T_4789 | A change in color is just one of several potential signs that a chemical reaction has occurred. Other potential signs include: Change in temperature-Heat is released or absorbed during the reaction. Production of a gas-Gas bubbles are released during the reaction. Production of a solid-A solid settles out of a liquid solution. The solid is called a precipitate. Click image to the left or use the URL below. URL: | text | null |
L_1003 | recognizing chemical reactions | T_4790 | Look carefully at the Figures 1.1, 1.2, and 1.3. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. A burning campfire can warm you up on a cold day. Dissolving an antacid tablet in water produces a fizzy drink. Adding acid to milk produces solid curds of cottage cheese. Q: Did you ever make a volcano by pouring vinegar over a mountain of baking soda? If you did, you probably saw the mixture bubble up and foam over. Did a chemical reaction occur? How do you know? A: Yes, a chemical reaction occurred. You know because the bubbles are evidence that a gas has been produced and production of a gas is a sign of a chemical reaction. | text | null |
L_1003 | recognizing chemical reactions | T_4790 | Look carefully at the Figures 1.1, 1.2, and 1.3. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. A burning campfire can warm you up on a cold day. Dissolving an antacid tablet in water produces a fizzy drink. Adding acid to milk produces solid curds of cottage cheese. Q: Did you ever make a volcano by pouring vinegar over a mountain of baking soda? If you did, you probably saw the mixture bubble up and foam over. Did a chemical reaction occur? How do you know? A: Yes, a chemical reaction occurred. You know because the bubbles are evidence that a gas has been produced and production of a gas is a sign of a chemical reaction. | text | null |
L_1003 | recognizing chemical reactions | T_4790 | Look carefully at the Figures 1.1, 1.2, and 1.3. All of the photos demonstrate chemical reactions. For each photo, identify a sign that one or more chemical reactions have taken place. A burning campfire can warm you up on a cold day. Dissolving an antacid tablet in water produces a fizzy drink. Adding acid to milk produces solid curds of cottage cheese. Q: Did you ever make a volcano by pouring vinegar over a mountain of baking soda? If you did, you probably saw the mixture bubble up and foam over. Did a chemical reaction occur? How do you know? A: Yes, a chemical reaction occurred. You know because the bubbles are evidence that a gas has been produced and production of a gas is a sign of a chemical reaction. | text | null |
L_1005 | replacement reactions | T_4794 | A replacement reaction occurs when elements switch places in compounds. This type of reaction involves ions (electrically charged versions of atoms) and ionic compounds. These are compounds in which positive ions of a metal and negative ions of a nonmetal are held together by ionic bonds. Generally, a more reactive element replaces an element that is less reactive, and the less reactive element is set free from the compound. There are two types of replacement reactions: single and double. Both types are described below. Q: Can you predict how single and double replacement reactions differ? A: One way they differ is that a single replacement reaction involves one reactant compound, whereas a double replacement reaction involves two reactant compounds. Keep reading to learn more about these two types of reactions. | text | null |
L_1005 | replacement reactions | T_4795 | A single replacement reaction occurs when one element replaces another in a single compound. This type of reaction has the general equation: A + BC B + AC In this equation, A represents a more reactive element and BC represents the original compound. During the reaction, A replaces B, forming the product compound AC and releasing the less reactive element B. An example of a single replacement reaction occurs when potassium (K) reacts with water (H2 O). A colorless solid compound named potassium hydroxide (KOH) forms, and hydrogen gas (H2 ) is set free. The equation for the reaction is: 2K + 2H2 O 2KOH + H2 In this reaction, a potassium ion replaces one of the hydrogen atoms in each molecule of water. Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. Q: Find potassium in the periodic table of the elements. What other element might replace hydrogen in water in a similar replacement reaction? A: Another group 1 element, such as lithium or sodium, might be involved in a similar replacement reaction with water. | text | null |
L_1005 | replacement reactions | T_4796 | A double replacement reaction occurs when two ionic compounds exchange ions. This produces two new ionic compounds. A double replacement reaction can be represented by the general equation: AB + CD AD + CB AB and CD are the two reactant compounds, and AD and CB are the two product compounds that result from the reaction. During the reaction, the ions B and D change places. Q: Could the product compounds be DA and BC? A: No, they could not. In an ionic compound, the positive metal ion is always written first, followed by the negative nonmetal ion. Therefore, A and C must always come first, followed by D or B. An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation: NaCl + AgF NaF + AgCl During the reaction, chloride and fluoride ions change places, so two new compounds are formed in the products: sodium fluoride (NaF) and silver chloride (AgCl). Q: When iron sulfide (FeS) and hydrogen chloride (HCl) react together, a double replacement reaction occurs. What are the products of this reaction? What is the chemical equation for this reaction? A: The products of the reaction are iron chloride (FeCl2 ) and hydrogen sulfide (H2 S). The chemical equation for this reaction is: FeS + 2HCl H2 S + FeCl2 | text | null |
L_1007 | rutherfords atomic model | T_4799 | In 1804, almost a century before the nucleus was discovered, the English scientist John Dalton provided evidence for the existence of the atom. Dalton thought that atoms were the smallest particles of matter, which couldnt be divided into smaller particles. He modeled atoms with solid wooden balls. In 1897, another English scientist, named J. J. Thomson, discovered the electron. It was first subatomic particle to be identified. Because atoms are neutral in electric charge, Thomson assumed that atoms must also contain areas of positive charge to cancel out the negatively charged electrons. He thought that an atom was like a plum pudding, consisting mostly of positively charged matter with negative electrons scattered through it. The nucleus of the atom was discovered next. It was discovered in 1911 by a scientist from New Zealand named Ernest Rutherford, who is pictured in Figure 1.1. Through his clever research, Rutherford showed that the positive charge of an atom is confined to a tiny massive region at the center of the atom, rather than being spread evenly throughout the pudding of the atom as Thomson had suggested. | text | null |
L_1007 | rutherfords atomic model | T_4800 | The way Rutherford discovered the atomic nucleus is a good example of the role of creativity in science. His quest actually began in 1899 when he discovered that some elements give off positively charged particles that can penetrate just about anything. He called these particles alpha () particles (we now know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered how he could use alpha particles to learn about the structure of the atom. He decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it can be pounded into sheets that are only 0.00004 cm thick. Surrounding the sheet of gold foil, he placed a screen that glowed when alpha particles struck it. It would be used to detect the alpha particles after they passed through the foil. A small slit in the screen allowed the beam of alpha particles to reach the foil from the particle emitter. You can see the setup for Rutherfords experiment in the Figure 1.2. Q: What would you expect to happen when the alpha particles strike the gold foil? A: The alpha particles would penetrate the gold foil. Alpha particles are positive, so they might be repelled by any areas of positive charge inside the gold atoms. Assuming a plum pudding model of the atom, Rutherford predicted that the areas of positive charge in the gold atoms would deflect, or bend, the path of all the alpha particles as they passed through. You can see what really happened in the Figure 1.2. Most of the alpha particles passed straight through the gold foil as though it wasnt there. The particles seemed to be passing through empty space. Only a few of the alpha particles were deflected from their straight path, as Rutherford had predicted. Surprisingly, a tiny percentage of the particles bounced back from the foil like a basketball bouncing off a backboard! Q: What can you infer from these observations? A: You can infer that most of the alpha particles were not repelled by any positive charge, whereas a few were repelled by a strong positive charge. | text | null |
L_1007 | rutherfords atomic model | T_4800 | The way Rutherford discovered the atomic nucleus is a good example of the role of creativity in science. His quest actually began in 1899 when he discovered that some elements give off positively charged particles that can penetrate just about anything. He called these particles alpha () particles (we now know they were helium nuclei). Like all good scientists, Rutherford was curious. He wondered how he could use alpha particles to learn about the structure of the atom. He decided to aim a beam of alpha particles at a sheet of very thin gold foil. He chose gold because it can be pounded into sheets that are only 0.00004 cm thick. Surrounding the sheet of gold foil, he placed a screen that glowed when alpha particles struck it. It would be used to detect the alpha particles after they passed through the foil. A small slit in the screen allowed the beam of alpha particles to reach the foil from the particle emitter. You can see the setup for Rutherfords experiment in the Figure 1.2. Q: What would you expect to happen when the alpha particles strike the gold foil? A: The alpha particles would penetrate the gold foil. Alpha particles are positive, so they might be repelled by any areas of positive charge inside the gold atoms. Assuming a plum pudding model of the atom, Rutherford predicted that the areas of positive charge in the gold atoms would deflect, or bend, the path of all the alpha particles as they passed through. You can see what really happened in the Figure 1.2. Most of the alpha particles passed straight through the gold foil as though it wasnt there. The particles seemed to be passing through empty space. Only a few of the alpha particles were deflected from their straight path, as Rutherford had predicted. Surprisingly, a tiny percentage of the particles bounced back from the foil like a basketball bouncing off a backboard! Q: What can you infer from these observations? A: You can infer that most of the alpha particles were not repelled by any positive charge, whereas a few were repelled by a strong positive charge. | text | null |
L_1007 | rutherfords atomic model | T_4801 | Rutherford made the same inferences. He concluded that all of the positive charge and virtually all of the mass of an atom are concentrated in one tiny area and the rest of the atom is mostly empty space. Rutherford called the area of concentrated positive charge the nucleus. He predictedand soon discoveredthat the nucleus contains positively charged particles, which he named protons. Rutherford also predicted the existence of neutral nuclear particles called neutrons, but he failed to find them. However, his student James Chadwick discovered them several years later. | text | null |
L_1007 | rutherfords atomic model | T_4802 | Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread evenly throughout an atom. Instead, it is all concentrated in the tiny nucleus. The rest of the atom is empty space except for the electrons scattered through it. In Rutherfords model of the atom, which is shown in the Figure 1.3, the electrons move around the massive nucleus like planets orbiting the sun. Thats why his model is called the planetary model. Rutherford didnt know exactly where or how electrons orbit the nucleus. That research would be undertaken by later scientists, beginning with Niels Bohr in 1913. New and improved atomic models would also be developed. Nonetheless, Rutherfords model is still often used to represent the atom. | text | null |
L_1009 | saturated hydrocarbons | T_4806 | Saturated hydrocarbons are hydrocarbons that contain only single bonds between carbon atoms. They are the simplest class of 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 the Figure 1.1. In this compound, named ethane, each carbon atom is bonded to three hydrogen atoms. In the structural formula, each dash (-) represents a single covalent bond, in which two atoms share one pair of valence electrons. Q: What is the chemical formula for ethane? A: The chemical formula is C2 H6 . | text | null |
L_1009 | saturated hydrocarbons | T_4807 | 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 other small alkanes are listed in the Table 1.1. The boiling and melting points of alkanes are determined mainly by the number of carbon atoms they have. Alkanes with more carbon atoms generally boil and melt at higher temperatures. 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 20 C) gas gas gas gas liquid liquid liquid liquid Q: The Table 1.1 shows only alkanes that have relatively few carbon atoms. Some alkanes have many more carbon atoms. What properties might larger alkanes have? A: Alkanes with more carbon atoms have higher boiling and melting points, so some of them are solids at room temperature. | text | null |
L_1009 | saturated hydrocarbons | T_4808 | Structural formulas are often used to represent hydrocarbon compounds because the molecules can have different shapes and a structural formula shows how the atoms are arranged. Hydrocarbons may form straight chains, A) In a straight-chain molecule, all the carbon atoms are lined up in a row like cars of a train. The carbon atoms form the backbone of the molecule. B) In a branched-chain molecule, at least one of the carbon atoms branches off from the backbone. C) In a cyclic molecule, the chain of carbon atoms is joined at the two ends to form a ring. Each ring usually contains just five or six carbon atoms, but rings can join together to form larger molecules. A cyclic molecule generally has higher boiling and melting points than straight-chain and branched- chain molecules. | text | null |
L_1012 | scientific graphing | T_4814 | Graphs are very useful tools in science. They can help you visualize a set of data. With a graph, you can actually see what all the numbers in a data table mean. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Each type of graph is suitable for showing a different type of data. | text | null |
L_1012 | scientific graphing | T_4815 | The data in Table 1.1 shows the average number of tornadoes per year for the ten U.S. cities that have the most tornadoes. The data were averaged over the time period 1950-2007. | text | null |
L_1012 | scientific graphing | T_4816 | Rank City 1 2 3 4 5 6 7 8 9 10 Clearwater, FL Oklahoma City, OK Tampa-St. Petersburg, FL Houston, TX Tulsa, OK New Orleans, LA Melbourne, FL Indianapolis, IN Fort Worth, TX Lubbock, TX Average Number of Tornadoes(per 1000 Square Miles) 7.4 2.2 2.1 2.1 2.1 2.0 1.9 1.7 1.7 1.6 Bar graphs are especially useful for comparing values for different things, such as the average numbers of tornadoes for different cities. Therefore, a bar graph is a good choice for displaying the data in theTable 1.1. The bar graph below shows one way that these data could be presented. Q: What do the two axes of this bar graph represent? A: The x-axis represents cities, and the y-axis represents average numbers of tornadoes. Q: Could you switch what the axes represent? If so, how would the bar graph look? A: Yes; the x-axis could represent average numbers of tornadoes, and the y-axis could represent cities. The bars of the graph would be horizontal instead of vertical. | text | null |
L_1012 | scientific graphing | T_4817 | The data in Table 1.2 shows the percent of all U.S. tornadoes by tornado strength for the years 1986 to 1995. In this table, tornadoes are rated on a scale called the F scale. On this scale, F0 tornadoes are the weakest and F5 tornadoes are the strongest. Tornado Scale(F-scale rating) F0 F1 F2 F3 F4 F5 Percent of all U.S. Tornadoes 55.0% 31.6% 10.0% 2.6% 0.7% 0.1% Circle graphs are used to show percents (or fractions) of a whole, such as the percents of F0 to F5 tornadoes out of all tornadoes. Therefore, a circle graph is a good choice for the data in the table. The circle graph below displays these data. Q: What if the Table 1.2 on tornado strength listed the numbers of tornadoes rather than the percents of tornadoes? Could a circle graph be used to display these data? A: No, a circle graph can only be used to show percents (or fractions) of a whole. However, the numbers could be used to calculate percents, which could then be displayed in a circle graph. | text | null |
L_1012 | scientific graphing | T_4818 | Consider the data in Table 1.3. It lists the number of tornadoes in the U.S. per month, averaged over the years 2009 to 2011. Month January February March April May June July August September October November December Average Number of Tornadoes 17 33 74 371 279 251 122 57 39 65 39 34 Line graphs are especially useful for showing changes over time, or time trends in data, such as how the average number of tornadoes varies throughout the year. Therefore, a line graph would be a good choice to display the data in the Table 1.3. The line graph below shows one way this could be done. Q: Based on the line graph above, describe the trend in tornado numbers by month throughout the course of a year. A: The number of tornadoes rises rapidly from a low in January to a peak in April. This is followed by a relatively slow decline throughout the rest of the year. | text | null |
L_1016 | scientific modeling | T_4828 | A model is a representation of an object, system, or process. For example, a road map is a representation of an actual system of roads on the ground. Models are very useful in science. They provide a way to investigate things that are too small, large, complex, or distant to investigate directly. To be useful, a model must closely represent the real thing in important ways, but it must be simpler and easier to understand than the real thing. Q: What might be examples of things that would be modeled in physical science because they are difficult to investigate directly? A: Examples include extremely small things such as atoms, very distant objects such as stars, and complex systems such as the electric grid that carries electricity throughout the country. Q: What are ways that these things might be modeled? A: Types of models include two-dimensional diagrams, three-dimensional structures, mathematical formulas, and computer simulations. Examples of simple two-dimensional models in physical science are described below. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_1016 | scientific modeling | T_4829 | The diagram below is a simple two-dimensional model of a water molecule. This is the smallest particle of water that still has the properties of water. The model shows that each molecule of water consists of one atom of oxygen and two atoms of hydrogen. Q: What else can you learn about water molecules from this model? A: The model shows the number of atomic particlesprotons, neutrons, and electronsin each type of atom. It also shows that each hydrogen atom in a water molecule shares its electron with the oxygen atom. Q: Do you think this water molecule model satisfies the criteria of a useful model? In other words, does it represent a real water molecule in important ways while being simpler and easier to understand than a real water molecule? A: The model shows the basic structure of a water molecule and how the atoms in the molecule share electrons. These features of the water molecule explain important properties of water. The model is also simpler and easier to understand than a real water molecule. In a real molecule, electrons spin around the nuclei at the center of the atoms in a cloud, rather than in neat, circular orbits, as shown in the model. The atoms of a real water molecule also contain even smaller particles than protons, neutrons, and electrons. For many purposes, however, its not necessary to represent these more complex features of a real water molecule. The diagram below shows another example of a simple model in physical science. This diagram is a model of an electric circuit. It represents the main parts of the circuit with simple symbols. Horizontal lines with + and - signs represent a battery. The parts labeled R1 , R2 , and R3 are devices that use electricity provided by the battery. For example, these parts might be a series of three light bulbs. Q: In the electric circuit diagram, what do the black lines connecting the battery and electric devices represent? A: The black lines represent electric wires. The wires are necessary to carry electric current from the battery to the electric devices and back to the battery again. Q: How is a circuit diagram simpler and easier to understand than an actual electric circuit? A: A circuit diagram shows only the parts of the circuit that carry electric current, and it uses simple symbols to represent them. | text | null |
L_1019 | scope of chemistry | T_4836 | Chemistry is the study of matter and energy and how they interact, mainly at the level of atoms and molecules. Basic concepts in chemistry include chemicals, which are specific types of matter, and chemical reactions. In a chemical reaction, atoms or molecules of certain types of matter combine chemically to form other types of matter. All chemical reactions involve energy. Q: How do you think chemistry explains why the copper on the Statue of Liberty is green instead of brownish red? A: The copper has become tarnished. The tarnishalso called patinais a compound called copper carbonate, which is green. Copper carbonate forms when copper undergoes a chemical reaction with carbon dioxide in moist air. The green patina that forms on copper actually preserves the underlying metal. Thats why its not removed from the statue. Some people also think that the patina looks attractive. | text | null |
L_1019 | scope of chemistry | T_4837 | Chemistry can help you understand the world around you. Everything you touch, taste, or smell is made of chemicals, and chemical reactions underlie many common changes. For example, chemistry explains how food cooks, why laundry detergent cleans your clothes, and why antacid tablets relieve an upset stomach. Other examples are illustrated in the Figure 1.1. Chemistry even explains you! Your body is made of chemicals, and chemical changes constantly take place within it. Each of these pictures represents a way that chemicals and chemical reactions af- fect our lives. | text | null |
L_1021 | scope of physics | T_4840 | Physics is the study of energy, matter, and their interactions. Its a very broad field because it is concerned with matter and energy at all levelsfrom the most fundamental particles of matter to the entire universe. Some people would even argue that physics is the study of everything! Important concepts in physics include motion, forces such as magnetism and gravity, and forms of energy such as light, sound, and electrical energy. Q: How do you think physics explains the distorted images formed by a funhouse mirror? A: Physics explains how energy interacts with matter. In this case, for example, physics explains how visible light reflects from mirrors to form images. Most mirrors, such as bathroom mirrors, have a flat surface. Light reflected from a flat mirror forms an image that looks the same as the object in front of it. Funhouse mirrors, like the one pictured above, are different. They have a curved surface that reflects light at different angles. This explains why the images they form are distorted. | text | null |
L_1021 | scope of physics | T_4841 | Physics can help you understand just about everything in the world around you. Thats because everything around you consists of matter and energy. Several examples of matter and energy interacting are pictured in the Figure 1.1. Read how physics explains each example. Examples of how matter and energy interact. Q: Based on the examples in Figure 1.1, what might be other examples of energy and matter interacting? A: Like the strings of cello, anything that vibrates produces waves of energy that travel through matter. For example, when you throw a pebble into a pond, waves of energy travel from the pebble through the water in all directions. Like an incandescent light bulb, anything that glows consists of matter that produces light energy. For example, fireflies use chemicals to produce light energy. Like a moving tennis racket, anything that moves has energy because it is moving, including your eyes as they read this sentence. | text | null |
L_1022 | screw | T_4842 | A screw is a simple machine that consists of an inclined plane wrapped around a central cylinder. No doubt you are familiar with screws like the wood screw in the left-hand side of the Figure 1.1. The cap of the bottle pictured on the right is another example of a screw. Screws move objects to a greater depth (or higher elevation) by increasing the force applied to the screw. Many screws are used to hold things together, such as two pieces of wood or a screw cap and bottle. When you use a screw, you apply force to turn the inclined plane. The screw, in turn, applies greater force to the object, such as the wood or bottle top. Q: Can you identify the inclined plane in each example of a screw pictured in the Figure 1.1? A: The inclined plane of the screw on the left consists of the ridges, or threads, that wrap around the central cylinder of the screw. The inclined plane of the cap on the right consists of the ridges that wrap around the inner sides of the cap. | text | null |
L_1022 | screw | T_4843 | The mechanical advantage of a simple machine is the factor by which it multiplies the force applied to the machine. It is the ratio of the output force to the input force. The force applied by the screw (output force) is always greater than the force applied to the screw (input force). Therefore, the mechanical advantage of a screw is always greater than 1. Look at the two screws in the Figure 1.2. In the screw on the right, the threads of the inclined plane are closer together. This screw has a greater mechanical advantage and is easier to turn than the screw on the left, so it takes less force to penetrate the wood with the right screw. The trade-off is that more turns of the screw are needed to do the job because the distance over which the input force must be applied is greater. Q: Why is it harder to turn a screw with more widely spaced threads? A: The screw moves farther with each turn when the threads are more widely space, so more force must be applied to turn the screw and cover the greater distance. | text | null |
L_1024 | significant figures | T_4847 | In any measurement, the number of significant figures is the number of digits thought to be correct by the person doing the measuring. It includes all digits that can be read directly from the measuring device plus one estimated digit. Look at the sketch of a beaker below. How much blue liquid does the beaker contain? The top of the liquid falls between the mark for 40 mL and 50 mL, but its closer to 50 mL. A reasonable estimate is 47 mL. In this measurement, the first digit (4) is known for certain and the second digit (7) is an estimate, so the measurement has two significant figures. Now look at the graduated cylinder sketched below. How much blue liquid does it contain? First, its important to note that you should read the amount of liquid at the bottom of its curved surface. This falls about half way between the mark for 36 mL and the mark for 37 mL, so a reasonable estimate would be 36.5 mL. Q: How many significant figures does this measurement have? A: There are three significant figures in this measurement. You know that the first two digits (3 and 6) are accurate. The third digit (5) is an estimate. | text | null |
L_1024 | significant figures | T_4848 | The examples above show that its easy to count the number of significant figures when you are making a measure- ment. But what if someone else has made the measurement? How do you know which digits are known for certain and which are estimated? How can you tell how many significant figures there are in the measurement? There are several rules for counting significant figures: Leading zeros are never significant. For example, in the number 006.1, only the 6 and 1 are significant. Zeros within a number between nonzero digits are always significant. For example, in the number 106.1, the zero is significant, so this number has four significant figures. Zeros that show only where the decimal point falls are not significant. For example, the number 470,000 has just two significant figures (4 and 7). The zeros just show that the 4 represents hundreds of thousands and the 7 represents tens of thousands. Therefore, these zeros are not significant. Trailing zeros that arent needed to show where the decimal point falls are significant. For example, 4.00 has three significant figures. Q: How many significant figures are there in each of these numbers: 20,080, 2.080, and 2000? A: Both 20,080 and 2.080 contain four significant figures, but 2000 has just one significant figure. | text | null |
L_1024 | significant figures | T_4849 | When measurements are used in a calculation, the answer cannot have more significant figures than the measurement with the fewest significant figures. This explains why the homework answer above is wrong. It has more significant figures than the measurement with the fewest significant figures. As another example, assume that you want to calculate the volume of the block of wood shown below. The volume of the block is represented by the formula: Volume = length width height Therefore, you would do the following calculation: Volume = 1.2 cm 1.0 cm 1 cm = 1.2 cm3 Q: Does this answer have the correct number of significant figures? A: No, it has too many significant figures. The correct answer is 1 cm3 . Thats because the height of the block has just one significant figure. Therefore, the answer can have only one significant figure. | text | null |
L_1024 | significant figures | T_4850 | To get the correct answer in the volume calculation above, rounding was necessary. Rounding is done when one or more ending digits are dropped to get the correct number of significant figures. In this example, the answer was rounded down to a lower number (from 1.2 to 1). Sometimes the answer is rounded up to a higher number. How do you know which way to round? Follow these simple rules: If the digit to be rounded (dropped) is less than 5, then round down. For example, when rounding 2.344 to three significant figures, round down to 2.34. If the digit to be rounded is greater than 5, then round up. For example, when rounding 2.346 to three significant figures, round up to 2.35. If the digit to be rounded is 5, round up if the digit before 5 is odd, and round down if digit before 5 is even. For example, when rounding 2.345 to three significant figures, round down to 2.34. This rule may seem arbitrary, but in a series of many calculations, any rounding errors should cancel each other out. | text | null |
L_1025 | simple machines | T_4851 | A machine is any device that makes work easier by changing a force. Work is done whenever a force moves an object over a distance. The amount of work done is represented by the equation: Work = Force x Distance When you use a machine, you apply force to the machine. This force is called the input force. The machine, in turn, applies force to an object. This force is called the output force. The output force may or may not be the same as the input force. The force you apply to the machine is applied over a given distance, called the input distance. The force applied by the machine to the object is also applied over a distance, called the output distance. The output distance may or may not be the same as the input distance. | text | null |
L_1025 | simple machines | T_4852 | Contrary to popular belief, machines do not increase the amount of work that is done. They just change how the work is done. Machines make work easier by increasing the amount of force that is applied, increasing the distance over which the force is applied, or changing the direction in which the force is applied. Q: If a machine increases the force applied, what does this tell you about the distance over which the force is applied by the machine: A: The machine must apply the force over a shorter distance. Thats because a machine doesnt change the amount of work and work equals force times distance. Therefore, if force increases, distance must decrease. For the same reason, if a machine increases the distance over which the force is applied, it must apply less force. | text | null |
L_1025 | simple machines | T_4853 | Examples of machines that increase force are steering wheels and pliers (see Figure 1.1). Read below to find out how both of these machines work. In each case, the machine applies more force than the user applies to the machine, but the machine applies the force over a shorter distance. | text | null |
L_1025 | simple machines | T_4854 | Examples of machines that increase the distance over which force is applied are leaf rakes and hammers (see Figure which the force is applied, but it reduces the strength of the force. | text | null |
L_1025 | simple machines | T_4855 | Some machines change the direction of the force applied by the user. They may or may not also change the strength of the force or the distance over which the force is applied. Two examples of machines that work this way are the claw ends of hammers and flagpole pulleys. You can see in the Figure 1.3 how each of these machines works. In both cases, the direction of the force applied by the user is reversed by the machine. Q: If the pulley only changes the direction of the force, how does it make the work of raising the flag easier? A: The pulley makes it easier to lift the flag because it allows a person to pull down on the rope and add his or her own weight to the effort, rather than simply lifting the load. | text | null |
L_1025 | simple machines | T_4856 | There are six types of simple machines that are the basis of all other machines. They are the inclined plane, lever, wedge, screw, pulley, and wheel and axle. The six types are pictured in the Figure 1.4. Youve probably used some of these simple machines yourself. Most machines are combinations of two or more simple machines. These machines are called compound machines. An example of a compound machine is a wheelbarrow (see bottom of Figure 1.4). It consists of two simple machines: a lever and a wheel and axle. Many compound machines are much more complex and consist of many simple machines. Examples include washing machines and cars. | text | null |
L_1025 | simple machines | T_4856 | There are six types of simple machines that are the basis of all other machines. They are the inclined plane, lever, wedge, screw, pulley, and wheel and axle. The six types are pictured in the Figure 1.4. Youve probably used some of these simple machines yourself. Most machines are combinations of two or more simple machines. These machines are called compound machines. An example of a compound machine is a wheelbarrow (see bottom of Figure 1.4). It consists of two simple machines: a lever and a wheel and axle. Many compound machines are much more complex and consist of many simple machines. Examples include washing machines and cars. | text | null |
L_1025 | simple machines | T_4856 | There are six types of simple machines that are the basis of all other machines. They are the inclined plane, lever, wedge, screw, pulley, and wheel and axle. The six types are pictured in the Figure 1.4. Youve probably used some of these simple machines yourself. Most machines are combinations of two or more simple machines. These machines are called compound machines. An example of a compound machine is a wheelbarrow (see bottom of Figure 1.4). It consists of two simple machines: a lever and a wheel and axle. Many compound machines are much more complex and consist of many simple machines. Examples include washing machines and cars. | text | null |
L_1032 | sound waves | T_4875 | In science, sound is defined as the transfer of energy from a vibrating object in waves that travel through matter. Most people commonly use the term sound to mean what they hear when sound waves enter their ears. The tree above generated sound waves when it fell to the ground, so it made sound according to the scientific definition. But the sound wasnt detected by a persons ears if there was nobody in the forest. So the answer to the riddle is both yes and no! | text | null |
L_1032 | sound waves | T_4876 | All sound waves begin with vibrating matter. Look at the first guitar string on the left in the Figure 1.1. Plucking the string makes it vibrate. The diagram below the figure shows the wave generated by the vibrating string. The moving string repeatedly pushes against the air particles next to it, which causes the air particles to vibrate. The vibrations spread through the air in all directions away from the guitar string as longitudinal waves. In longitudinal waves, particles of the medium vibrate back and forth parallel to the direction that the waves travel. Q: If there were no air particles to carry the vibrations away from the guitar string, how would sound reach the ear? A: It wouldnt unless the vibrations were carried by another medium. Sound waves are mechanical waves, so they can travel only though matter and not through empty space. | text | null |
L_1032 | sound waves | T_4877 | The fact that sound cannot travel through empty space was first demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still ticking, but the ticking sound could no longer be heard. Thats because the sound couldnt travel away from the clock without air particles to pass the sound energy along. Click image to the left or use the URL below. URL: | text | null |
L_1032 | sound waves | T_4878 | Most of the sounds we hear reach our ears through the air, but sounds can also travel through liquids and solids. If you swim underwateror even submerge your ears in bathwaterany sounds you hear have traveled to your ears through the water. Some solids, including glass and metals, are very good at transmitting sounds. Foam rubber and heavy fabrics, on the other hand, tend to muffle sounds. They absorb rather than pass on the sound energy. Q: How can you tell that sounds travel through solids? A: One way is that you can hear loud outdoor sounds such as sirens through closed windows and doors. You can also hear sounds through the inside walls of a house. For example, if you put your ear against a wall, you may be able to eavesdrop on a conversation in the next roomnot that you would, of course. | text | null |
L_1033 | sources of visible light | T_4879 | Visible light includes all the wavelengths of light that the human eye can detect. It allows us to see objects in the world around us. Without visible light, we would only be able to sense most objects by sound, touch, or smell. Like humans, most other organisms also depend on visible light, either directly or indirectly. Many animalsincluding predators of jellyfishuse visible light to see. Plants and certain other organisms use visible light to make food in the process of photosynthesis. Without this food, most other organisms would not be able to survive. Q: Do you think that some animals might be able to see light that isnt visible to humans? A: Some animals can see light in the infrared or ultraviolet range of wavelengths. For example, mosquitoes can see infrared light, which is emitted by warm objects. By seeing infrared light, mosquitoes can tell where the warmest, blood-rich areas of the body are located. | text | null |
L_1033 | sources of visible light | T_4880 | Most of the visible light on Earth comes from the sun. The sun and other stars produce light because they are so hot. They glow with light due to their extremely high temperatures. This way of producing light is called incandescence. Incandescent light bulbs also produce light in this way. When electric current passes through a wire filament inside an incandescent bulb, the wire gets so hot that it glows. Do you see the glowing filament inside the incandescent light bulb in the Figure 1.1? Q: What are some other sources of incandescent light? A: Flames also produce incandescent light. For example, burning candles, oil lamps, and bonfires produce light in this way. | text | null |
L_1033 | sources of visible light | T_4881 | Some objects produce light without becoming very hot. They generate light through chemical reactions or other processes. Producing light without heat is called luminescence. Luminescence, in turn, can occur in several different ways: One type of luminescence is called fluorescence. In this process, a substance absorbs shorter-wavelength ultraviolet light and then gives off light in the visible range of wavelengths. Certain minerals produce light in this way, including gemstones such as amethyst, diamond, and emerald. Another type of luminescence is called electroluminescence. In this process, a substance gives off light when an electric current passes through it. Gases such as neon, argon, and krypton produce light by this means. The car dash lights in the Figure 1.2 are produced by electroluminescence. A third type of luminescence is called bioluminescence. This is the production of light by living things as a result of chemical reactions. The jellyfish in the opening photo above produces light by bioluminescence. So does the firefly in the Figure 1.3. Fireflies give off visible light to attract mates. | text | null |
L_1033 | sources of visible light | T_4882 | Many other objects appear to produce their own light, but they actually just reflect light from another source. Being lit by another source is called illumination. The moon in the Figure 1.4 is glowing so brightly that you can see shadows under the trees. It appears to glow from its own light, but its really just illuminated by light from the sun. Everything you can see that doesnt produce its own light is illuminated by light from some other source. | text | null |
L_1033 | sources of visible light | T_4882 | Many other objects appear to produce their own light, but they actually just reflect light from another source. Being lit by another source is called illumination. The moon in the Figure 1.4 is glowing so brightly that you can see shadows under the trees. It appears to glow from its own light, but its really just illuminated by light from the sun. Everything you can see that doesnt produce its own light is illuminated by light from some other source. | text | null |
L_1035 | speed | T_4885 | How fast or slow something moves is its speed. Speed determines how far something travels in a given amount of time. The SI unit for speed is meters per second (m/s). Speed may be constant, but often it varies from moment to moment. | text | null |
L_1035 | speed | T_4886 | Even if speed varies during the course of a trip, its easy to calculate the average speed by using this formula: speed = distance time For example, assume you go on a car trip with your family. The total distance you travel is 120 miles, and it takes 3 hours to travel that far. The average speed for the trip is: 120 mi 3h = 40 mi/h speed = Q: Terri rode her bike very slowly to the top of a big hill. Then she coasted back down the hill at a much faster speed. The distance from the bottom to the top of the hill is 3 kilometers. It took Terri 41 hour to make the round trip. What was her average speed for the entire trip? (Hint: The round-trip distance is 6 km.) A: Terris speed can be calculated as follows: 6 km 0.25 h = 24 km/h speed = | text | null |
L_1035 | speed | T_4887 | When you travel by car, you usually dont move at a constant speed. Instead you go faster or slower depending on speed limits, traffic lights, the number of vehicles on the road, and other factors. For example, you might travel 65 miles per hour on a highway but only 20 miles per hour on a city street (see the pictures in the Figure 1.1.) You might come to a complete stop at traffic lights, slow down as you turn corners, and speed up to pass other cars. Therefore, your speed at any given instant, or your instantaneous speed, may be very different than your speed at other times. Instantaneous speed is much more difficult to calculate than average speed. Cars race by in a blur of motion on an open highway but crawl at a snails pace when they hit city traffic. | text | null |
L_1035 | speed | T_4888 | If you know the average speed of a moving object, you can calculate the distance it will travel in a given period of time or the time it will take to travel a given distance. To calculate distance from speed and time, use this version of the average speed formula given above: distance = speed time For example, if a car travels at an average speed of 60 km/h for 5 hours, then the distance it travels is: distance = 60 km/h 5 h = 300 km To calculate time from speed and distance, use this version of the formula: time = distance speed Q: If you walk 6 km at an average speed of 3 km/h, how much time does it take? A: Use the formula for time as follows: distance speed 6 km = 3 km/h =2h time = | text | null |
L_1036 | speed of sound | T_4889 | The speed of sound is the distance that sound waves travel in a given amount of time. Youll often see the speed of sound given as 343 meters per second. But thats just the speed of sound under a certain set of conditions, specifically, through dry air at 20 C. The speed of sound may be very different through other matter or at other temperatures. | text | null |
L_1036 | speed of sound | T_4890 | Sound waves are mechanical waves, and mechanical waves can only travel through matter. The matter through which the waves travel is called the medium (plural, media). The Table 1.1 gives the speed of sound in several different media. Generally, sound waves travel most quickly through solids, followed by liquids, and then by gases. Particles of matter are closest together in solids and farthest apart in gases. When particles are closer together, they can more quickly pass the energy of vibrations to nearby particles. Medium (20 C) Dry Air Speed of Sound Waves (m/s) 343 Medium (20 C) Water Wood Glass Aluminum Speed of Sound Waves (m/s) 1437 3850 4540 6320 Q: The table gives the speed of sound in dry air. Do you think that sound travels more or less quickly through air that contains water vapor? (Hint: Compare the speed of sound in water and air in the table.) A: Sound travels at a higher speed through water than air, so it travels more quickly through air that contains water vapor than it does through dry air. | text | null |
L_1036 | speed of sound | T_4891 | The speed of sound also depends on the temperature of the medium. For a given medium, sound has a slower speed at lower temperatures. You can compare the speed of sound in dry air at different temperatures in the following Table 1.2. At a lower temperature, particles of the medium are moving more slowly, so it takes them longer to transfer the energy of the sound waves. Temperature of Air 0 C 20 C 100 C Speed of Sound Waves (m/s) 331 343 386 Q: What do you think the speed of sound might be in dry air at a temperature of -20 C? A: For each 1 degree Celsius that temperature decreases, the speed of sound decreases by 0.6 m/s. So sound travels through dry, -20 C air at a speed of 319 m/s. | text | null |
L_1038 | static electricity and static discharge | T_4895 | Static electricity is a buildup of electric charges on objects. Charges build up when negative electrons are transferred from one object to another. The object that gives up electrons becomes positively charged, and the object that accepts the electrons becomes negatively charged. This can happen in several ways. One way electric charges can build up is through friction between materials that differ in their ability to give up or accept electrons. When you wipe your rubber-soled shoes on the wool mat, for example, electrons rub off the mat onto your shoes. As a result of this transfer of electrons, positive charges build up on the mat and negative charges build up on you. Once an object becomes electrically charged, it is likely to remain charged until it touches another object or at least comes very close to another object. Thats because electric charges cannot travel easily through air, especially if the air is dry. Q: Youre more likely to get a shock in the winter when the air is very dry. Can you explain why? A: When the air is very dry, electric charges are more likely to build up objects because they cannot travel easily through the dry air. This makes a shock more likely when you touch another object. | text | null |
L_1038 | static electricity and static discharge | T_4896 | What happens when you have become negatively charged and your hand approaches the metal doorknocker? Your negatively charged hand repels electrons in the metal, so the electrons move to the other side of the knocker. This makes the side of the knocker closest to your hand positively charged. As your negatively charged hand gets very close to the positively charged side of the metal, the air between your hand and the knocker also becomes electrically charged. This allows electrons to suddenly flow from your hand to the knocker. The sudden flow of electrons is static discharge. The discharge of electrons is the spark you see and the shock you feel. | text | null |
L_1038 | static electricity and static discharge | T_4897 | Another example of static discharge, but on a much larger scale, is lightning. You can see how it occurs in the following diagram (Figure 1.1). During a rainstorm, clouds develop regions of positive and negative charge due to the movement of air molecules, water drops, and ice particles. The negative charges are concentrated at the base of the clouds, and the positive charges are concentrated at the top. The negative charges repel electrons on the ground beneath them, so the ground below the clouds becomes positively charged. At first, the atmosphere prevents electrons from flowing away from areas of negative charge and toward areas of positive charge. As more charges build up, however, the air between the oppositely charged areas also becomes charged. When this happens, static electricity is discharged as bolts of lightning. | text | null |
L_1040 | surface wave | T_4900 | A surface wave is a wave that travels along the surface of a medium. The medium is the matter through which the wave travels. Ocean waves are the best-known examples of surface waves. They travel on the surface of the water between the ocean and the air. Q: What do you think causes ocean waves? A: Most ocean waves are caused by wind blowing across the water. Moving air molecules transfer some of their energy to molecules of ocean water. The energy travels across the surface of the water in waves. The stronger the winds are blowing, the larger the waves are and the more energy they have. | text | null |
L_1040 | surface wave | T_4901 | A surface wave is a combination of a transverse wave and a longitudinal wave. A transverse wave is a wave in which particles of the medium move up and down perpendicular to the direction of the wave. A longitudinal wave is a wave in which particles of the medium move parallel to the direction of the wave. In a surface wave, particles of the medium move up and down as well as back and forth. This gives them an overall circular motion. You can see how the particles move in the Figure 1.1. Click image to the left or use the URL below. URL: | text | null |
L_1040 | surface wave | T_4902 | In deep water, particles of water just move in circles. They dont actually move closer to shore with the energy of the waves. However, near the shore where the water is shallow, the waves behave differently. Look at the Figure 1.2. You can see how the waves start to drag on the bottom in shallow water. This creates friction that slows down the bottoms of the waves, while the tops of the waves keep moving at the same speed. The difference in speed causes the waves to get steeper until they topple over and break. The crashing waves carry water onto the shore as surf. Q: In this diagram of a wave breaking near shore, where do you think a surfer would try to catch the wave? A: The surfer would try to catch the wave where it starts to steepen and lean forward toward the shore. | text | null |
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