Energy Simply stated energy is the ability to do work or cause change. The energy of motion is called kinetic energy. Particles within matter are in constant motion. The amount of motion of these particles depends on the kinetic energy they possess.
1. Standards—8.3.9: Demonstrate, using drawings and models, the movement of atoms in a solid, liquid, and gaseous state. Explain that atoms and molecules are perpetually in motion. 8.3.10: Explain that increased tem- perature means that atoms have a greater average energy of motion and that most gases expand when heated. Also covers: 8.2.8 (Detailed standards begin on page IN8.) Changes of State Thermal Energy and Heat Shards of ice fly from the sculptor’s chisel. As the crowd looks on, a swan slowly emerges from a massive block of ice. As the day wears on, however, drops of water begin to fall from the ■ Define and compare thermal sculpture. Drip by drip, the sculpture is transformed into a pud- energy and temperature. dle of liquid water. What makes matter change from one state to ■ Relate changes in thermal another? To answer this question, you need to think about the energy to changes of state. ■ Explore energy and temperature particles that make up matter. changes on a graph. Energy Simply stated, energy is the ability to do work or cause change. The energy of motion is called kinetic energy. Particles Matter changes state as it heats up within matter are in constant motion. The amount of motion of or cools down. these particles depends on the kinetic energy they possess. Particles with more kinetic energy move faster and farther apart. Particles Review Vocabulary with less energy move more slowly and stay closer together. energy: the ability to do work or The total kinetic energy of all the particles in a sample of cause change matter is called thermal energy. Thermal energy, an extensive New Vocabulary property, depends on the number of particles in a substance as well as the amount of energy each particle has. If either the •• thermal energy temperature number of particles or the amount of energy in each particle •• heat melting changes, the thermal energy of the sample changes. With identi- cally sized samples, the warmer substance has the greater ther- •• freezing vaporization mal energy. In Figure 7, the particles of hot water from the hot spring have more thermal energy than the particles of snow on • condensation the surrounding ground. Figure 7 These girls are enjoy- ing the water from the hot spring. Infer why the girls appear to be comfortable in the hot spring while there is snow on the ground. SECTION 2 Changes of State 99 Annie Griffiths Belt/CORBIS
2. Figure 8 The particles in hot tea move faster than those in iced tea. The temperature of hot tea is higher than the temperature of iced tea. Identify which tea has the higher kinetic energy. M642-06C-MS Indiana Academic Standard Check Temperature Not all of the particles in a sample of matter have 8.3.10: Explain that increased the same amount of energy. Some have more energy than others. temperature means that atoms The average kinetic energy of the individual particles is the have a greater average energy of temperature, an intensive property, of the substance. You can find motion . . . an average by adding up a group of numbers and dividing the Which tea has a lower average total by the number of items in the group. For example, the aver- kinetic energy—iced tea or hot tea? age of the numbers 2, 4, 8, and 10 is (2 ⫹ 4 ⫹ 8 ⫹ 10) ⫼ 4 ⫽ 6. Temperature is different from thermal energy because thermal energy is a total and temperature is an average. You know that the iced tea is colder than the hot tea, as shown in Figure 8. Stated differently, the temperature of iced tea is lower than the temperature of hot tea. You also could say that the average kinetic energy of the particles in the iced tea is less Types of Energy Thermal than the average kinetic energy of the particles in the hot tea. energy is one of several dif- ferent forms of energy. Heat When a warm object is brought near a cooler object, ther- Other forms include the chemical energy in chemical mal energy will be transferred from the warmer object to the compounds, the electrical cooler one. The movement of thermal energy from a substance energy used in appliances, at a higher temperature to one at a lower temperature is called the electromagnetic energy heat. When a substance is heated, it gains thermal energy. of light, and the nuclear Therefore, its particles move faster and its temperature rises. energy stored in the When a substance is cooled, it loses thermal energy, which causes nucleus of an atom. Make a list of examples of energy its particles to move more slowly and its temperature to drop. that you are familiar with. How is heat related to temperature? 100 CHAPTER 4 States of Matter Amanita Pictures
3. Specific Heat As you study more science, you will discover that water has many unique properties. One of those is the amount of heat required to increase the temper- ature of water as compared to most other substances. The specific heat of a substance is the amount of heat required to raise the temperature of 1 g of a sub- stance 1°C. Substances that have a low specific heat, such as most metals and the sand in Figure 9, heat up and cool down quickly because they require only small amounts Figure 9 The specific heat of of heat to cause their temperatures to rise. A substance with a high water is greater than that of sand. specific heat, such as the water in Figure 9, heats up and cools The energy provided by the Sun down slowly because a much larger quantity of heat is required to raises the temperature of the sand cause its temperature to rise or fall by the same amount. much faster than the water. Changes Between the Solid and Liquid States Figure 10 Rather than melting Matter can change from one state to another when thermal into a liquid, glass gradually soft- energy is absorbed or released. This change is known as change of ens. Glass blowers use this charac- state. The graph in Figure 11 shows the changes in temperature as teristic to shape glass into thermal energy is gradually added to a container of ice. beautiful vases while it is hot. Melting As the ice in Figure 11 is heated, it absorbs thermal energy and its temperature rises. At some point, the temperature stops rising and the ice begins to change into liquid water. The change from the solid state to the liquid state is called melting. The temperature at which a substance changes from a solid to a liquid is called the melting point. The melting point of water is 0°C. Amorphous solids, such as rub- ber and glass, don’t melt in the same way as crystalline solids. Because they don’t have crystal structures to break down, these solids get softer and softer as they are heated, as you can see in Figure 10. SECTION 2 Changes of State 101 (t)David Weintraub/Stock Boston, (b)James L. Amos/Peter Arnold, Inc.
4. VISUALIZING STATES OF MATTER Figure 11 L ike most substances, water can exist in three distinct states—solid, liquid, or gas. At certain temperatures, water changes from one state VAPORIZATION When water reaches its to another. This diagram shows what changes occur boiling point of 100ºC, water molecules are as water is heated or cooled. moving so fast that they break free of the attractions that hold them together in the MELTING When ice melts, its temperature liquid state. The result is vaporization— remains constant until all the ice turns to the liquid be-comes a gas. The tempera- water. Continued heating of liquid water ture of boiling water remains constant causes the molecules to vibrate even until all of the liquid turns to steam. faster, steadily raising the temperature. FREEZING When liquid water freezes, it releases thermal energy and turns into Gas the solid state, ice. Vaporization 100°C Condensation Temperature Liquid CONDENSATION When steam is cooled, Melting 0°C it releases thermal energy and turns Freezing into its liquid state. This process is called condensation. Solid Thermal energy Solid state: ice Liquid state: water Gaseous state: steam 102 CHAPTER 4 States of Matter Dave King/DK Images
5. Freezing The process of melting a crystalline solid can be reversed if the liquid is cooled. The change from the liquid state to the solid state is called freezing. As the liquid cools, it loses thermal energy. As a result, its particles slow down and come Topic: Freezing Point closer together. Attractive forces begin to trap particles, and the Study crystals of a solid begin to form. As you can see in Figure 11, Visit in8.msscience.com for Web freezing and melting are opposite processes. links to information about freezing. The temperature at which a substance changes from the liq- Activity Make a list of several uid state to the solid state is called the freezing point. The freez- substances and the temperatures ing point of the liquid state of a substance is the same at which they freeze. Find out how temperature as the melting point of the solid state. For example, the freezing point affects how the solid water melts at 0°C and liquid water freezes at 0°C. substance is used. During freezing, the temperature of a substance remains constant while the particles in the liquid form a crystalline solid. Because particles in a liquid have more energy than particles in a solid, energy is released during freezing. This energy is released into the surroundings. After all of the liquid has become a solid, the temperature begins to decrease again. How can ice save oranges? uring the spring, Florida citrus D farmers carefully watch the fruit when temperatures drop close to freez- ing. When the temperatures fall below 0°C, the liquid in the cells of oranges can freeze and expand. This causes the cells to break, making the oranges mushy and the crop useless for sale. To prevent this, farmers spray the oranges with water just before the temperature reaches 0°C. How does spraying oranges with water protect them? Identifying the Problem Using the diagram in Figure 11, con- sider what is happening to the water at 0°C. Two things occur. What are they? Solving the Problem 1. What change of state and what energy changes occur when water freezes? 2. How does the formation of ice on the orange help the orange? SECTION 2 Changes of State 103 Joseph Sohm/ChromoSohm, Inc./CORBIS
6. Changes Between the Liquid and Gas States After an early morning rain, you and your friends enjoy Observing stomping through the puddles left behind. But later that after- Vaporization noon when you head out to run through the puddles once more, Procedure the puddles are gone. The liquid water in the puddles changed 1. Use a dropper to place one into a gas. Matter changes between the liquid and gas states drop of rubbing alcohol through vaporization and condensation. on the back of your hand. 2. Describe how your hand Vaporization As liquid water is heated, its temperature rises feels during the next until it reaches 100°C. At this point, liquid water changes into 2 min. 3. Wash your hands. water vapor. The change from a liquid to a gas is known as vaporization (vay puh ruh ZAY shun). You can see in Figure 11 that the temperature of the substance does not change during 1. What changes in the appearance of the rubbing vaporization. However, the substance absorbs thermal energy. alcohol did you notice? The additional energy causes the particles to move faster until 2. What sensation did you they have enough energy to escape the liquid as gas particles. feel during the 2 min? How Two forms of vaporization exist. Vaporization that takes can you explain this place below the surface of a liquid is called boiling. When a liq- sensation? uid boils, bubbles form within the liquid and rise to the surface, 3. Infer how sweating cools the body. as shown in Figure 12. The temperature at which a liquid boils is called the boiling point. The boiling point of water is 100°C. Vaporization that takes place at the surface of a liquid is called evaporation. Evaporation, which occurs at temperatures below the boiling point, explains how puddles dry up. Imagine that you could watch individual water molecules in a puddle. You would notice that the molecules move at different speeds. Although the temperature of the water is constant, remember that temperature is a measure of the average kinetic energy of the molecules. Some of the fastest-moving molecules overcome the attractive forces of other molecules and escape from the sur- face of the water. Figure 12 During boiling, liquid changes to gas, forming bubbles in the liquid that rise to the surface. Define the word that describes a liquid changing to the gas. 104 CHAPTER 4 States of Matter Michael Dalton/Fundamental Photographs
7. Figure 13 The drops of water on these glasses and pitcher of lemonade were formed when water vapor in the air lost enough energy to return to the liquid state. This process is called condensation. Location of Molecules It takes more than speed for water molecules to escape the liquid state. During evaporation, these faster molecules also must be near the surface, heading in the right direction, and they must avoid hitting other water mole- cules as they leave. With the faster particles evaporating from the surface of a liquid, the particles that remain are the slower, cooler ones. Evaporation cools the liquid and anything near the liquid. You experience this cooling effect when perspiration evaporates from your skin. Condensation Pour a nice, cold glass of lemonade and place Topic: Condensation Visit in8.msscience.com for Web it on the table for a half hour on a warm day. When you come links to information about how back to take a drink, the outside of the glass will be covered by condensation is involved in drops of water, as shown in Figure 13. What happened? As a gas weather. cools, its particles slow down. When particles move slowly Activity Find out how conden- enough for their attractions to bring them together, droplets of sation is affected by the tempera- liquid form. This process, which is the opposite of vaporization, ture as well as the amount of is called condensation. As a gas condenses to a liquid, it releases water in the air. the thermal energy it absorbed to become a gas. During this process, the temperature of the substance does not change. The decrease in energy changes the arrangement of particles. After the change of state is complete, the temperature continues to drop, as you saw in Figure 11. What energy change occurs during condensation? Condensation formed the droplets of water on the outside of your glass of lemonade. In the same way, water vapor in the atmosphere condenses to form the liquid water droplets in clouds. When the droplets become large enough, they can fall to the ground as rain. SECTION 2 Changes of State 105 Swarthout & Associates/The Stock Market/CORBIS
8. Changes Between the Solid and Gas States Some substances can change from the solid state to the gas state without ever becoming a liquid. During this process, known as sublimation, the sur- face particles of the solid gain enough energy to become a gas. One example of a substance that undergoes sublimation is dry ice. Dry ice is the solid form of carbon dioxide. It often is used to keep materials cold and dry. At room temperature and pressure, carbon dioxide does not exist as a liquid. Figure 14 The solid carbon Therefore, as dry ice absorbs thermal energy from the objects dioxide (dry ice) at the bottom of around it, it changes directly into a gas. When dry ice becomes this beaker of water is changing a gas, it absorbs thermal energy from water vapor in the air. As directly into gaseous carbon dioxide. a result, the water vapor cools and condenses into liquid water This process is called sublimation. droplets, forming the fog you see in Figure 14. Summary Self Check Thermal Energy and Heat 1. Describe how thermal energy and temperature are sim- • Thermal energy depends on the amount of the substance and the kinetic energy of parti- ilar. How are they different? 2. Explain how a change in thermal energy causes matter cles in the substance. to change from one state to another. Give two examples. • Heat is the movement of thermal energy from a warmer substance to a cooler one. 3. List the three changes of state during which energy is absorbed. 4. Describe the two types of vaporization. Specific Heat 5. Think Critically How can the temperature of a sub- • Specific heat is a measure of the amount of energy required to raise 1 g of a substance 1°C. stance remain the same even if the substance is absorb- ing thermal energy? Changes Between Solid and Liquid States 6. Write a paragraph in your Science Journal that explains • During all changes of state, the temperature of a substance stays the same. why you can step out of the shower into a warm bath- room and begin to shiver. Changes Between Liquid and Gas States • Vaporization is the change from the liquid state to a gaseous state. 7. Make and Use Graphs Use the data you collected in the Launch Lab to plot a temperature-time graph. • Condensation is the change from the gaseous state to the liquid state. Describe your graph. At what temperature does the graph level off? What was the liquid doing during this time period? Changes Between Solid and Gas States 8. Use Numbers If sample A requires 10 calories to raise • Sublimation is the process of a substance going from the solid state to the gas state the temperature of a 1-g sample 1°C, how many calo- ries does it take to raise a 5-g sample 10°C? without ever being in the liquid state. 106 CHAPTER 4 States of Matter in8.msscience.com/self_check_quiz Tony Freeman/PhotoEdit, Inc.
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