Thermal Energy and States of Matter

					Thermal Energy and States of Matter Throughout the day, temperatures at an orange grove drop steadily. The anxious farmer awaits the updated weather forecast. The news is not good. The temperature is expected to fall even further during the night. Low temperatures could wipe out the entire crop. He considers picking the crop early, but the oranges are not yet ripe. Instead, the farmer tells his workers to haul in long water hoses. He has them spray the orange trees with water. As it gets colder, the water turns to ice. The ice keeps the oranges warm! How does ice keep something warm? The answer has to do with how thermal energy is moved as water becomes ice. Three States of Matter What happens when you hold an ice cube in your hand? It melts. The solid and the liquid are both the same matter. All matter can exist in three states – solid, liquid, and gas. These three states are solid, liquid and gas. Although the chemical composition of a substance remains the same, the arrangement of the particles that make up the matter differ from one state to another Solids An ice cube, a coin, a book, and the crystal of fluorite shown above are all solids. The particles that make up a solid are packed together in relatively fixed positions. Particles in a solid cannot move out of their positions. They can only vibrate back and forth. This is why solids retain a fixed shape and volume. Liquids Water, orange juice, and the molten steel shown at the left are all liquids. The particles that make up a liquid are close together, but they are not held together as tightly as those of a solid. Even though the particles are close together they are not as packed together as the particles of a solid. Because liquid particles can move around, liquids don’t have a definite shape but they do have a definite volume. Gases Air, helium, and neon are three examples of gases. In a gas, the particles are moving so fast that they don’t even stay close together. Gases expand to fill all the space available. They do not have a fixed shape or volume. Changes of State The physical change from one state of matter to another is called a change of state. A change of state most often occurs between the solid and liquid states, and between the liquid and gas states The state of a substance depends on how much thermal energy it possesses.. The more thermal energy the substance has, the faster its particles move. A gas has more thermal energy than a liquid. Because of this, the particles of a gas move faster than the particles of the same substance that is a solid or liquid. Particles in a liquid state move faster than particles in a solid state. Matter will change from one state to another if thermal energy is absorbed or released. Figure 24 is a graph of changes of state. Thermal energy is shown on the horizontal axis and temperature is shown on the vertical axis. You can see that as thermal energy increases, a substance changes from a solid to a liquid and then to a gas. A substance changes from a gas to a liquid and then to a solid as thermal energy is removed from it. The flat regions of the graph show certain times when thermal energy is changing but temperature remains the same. Under these conditions matter is changing from one

state to another. During a change of state, the addition or loss of thermal energy changes the distances among the particles. However, the average kinetic energy of those particles does not change. Since temperature is a measure of average kinetic energy, the temperature does not change as a substance changes state. Solid-Liquid Changes of State On the lower left portion of the graph in Figure 24, matter goes through changes between the solid and liquid states of matter. These changes are known as melting and freezing. Melting The change of state from a solid to a liquid is called melting. Melting occurs when a solid absorbs thermal energy. As the thermal energy of the solid increases, the rigid structure of its particles begins to break down. The particles become freer to move around. The temperature at which a solid changes to a liquid is called the melting point. Freezing The change of state from a liquid to a solid is called freezing. Freezing is when a liquid loses thermal energy. The temperature at which a substance changes from a liquid to a solid is called its freezing point. For a given substance, the freezing point and the melting point are the same. The only difference between the two is whether the substance is gaining or releasing thermal energy. The fact that freezing involves a release of energy explains why the farmer had his workers spray the orange trees with water. The liquid water released thermal energy as it froze. Some of this thermal energy was transferred to the oranges, and kept them from freezing. Liquid-Gas Changes of State The upper right portion of Figure 24 on the previous page shows changes between the liquid and gas states of matter. These changes are known as vaporization and condensation. Vaporization The process by which matter changes from the liquid to the gas state is called vaporization. During this process, particles in a liquid absorb thermal energy. This causes the particles to move faster. Eventually they move fast enough to escape the liquid, as gas particles. If vaporization takes place at the surface of a liquid, it is called evaporation. At higher temperatures, vaporization can occur below the surface of a liquid as well. This is called boiling. When a liquid boils, gas bubbles formed within the liquid rise to the surface. The temperature at which a liquid boils is called its boiling point. Condensation You have probably noticed that beads of water appear on the outside of a cold drinking glass or on the bathroom mirror after you take a shower. Why do you think this occurs? The water vapor in the air loses thermal energy when it comes in contact with the cold glass. When a gas loses a sufficient amount of thermal energy, it will change into a liquid. A change from the gas state to the liquid state is called condensation. Thermal Expansion Have you ever loosened a tight jar lid by holding it under a stream of hot water? This works because the metal lid expands a little. Most substances expand when heated. As the thermal energy of a substance increases, its particles move faster and the substance expands. This is true even when the substance is not changing state. The expanding of matter when it is heated is known as thermal expansion. When a substance is cooled, thermal energy is released. This means that the motion of the particles slows down and the particles move closer together. So as a substance is cooled, it usually contracts, or decreases in size.

Thermometers You are already familiar with one application of thermal expansion- a thermometer. In a common thermometer, a liquid such as mercury or alcohol is sealed within a glass tube. As the liquid is heated, it expands and climbs up the tube. As the liquid is cooled, it contracts and flows down in the tube. Thermostats The principal of thermal expansion is used to make thermostats. Many thermostats contain bimetallic strips, which are strips of two different metals joined together. Different metals expand at different rates. When the bimetallic strip is heated, one side expands more then the other. This causes the strip to curve. The movement of the strip operates a switch. If the switch is connected to a furnace or other heating system, the thermostat will turn the heating system on and off. In addition to home heating systems, thermostats are used on such devices as air conditioners, ovens, toasters, and electric blankets. What are Waves? Far out to sea, the wind disturbs the calm surface of the water. A ripple forms. As the wind continues to blow, the ripple grows into a powerful wave that can travel a great distance. Near the beach, surfers wait eagerly. They quickly paddle into deeper water to catch the monstrous wave. Surfers enjoy the power of nature as they ride the wave to the shore. What are waves? How can they travel so far? Why are some waves more powerful than others? In this section, you will explore how waves begin and how they move. Waves-Matter and Energy Interacting Waves crashing on a beach show the tremendous energy waves can carry. A wave is a disturbance that transfers energy from place to place. In science, energy is defined as the ability to do work. To understand waves, think of a boat out on the ocean. If a wave disturbs the surface of the water, it will cause anything floating on the water to be disturbed, too. The energy carried by a wave can lift even a large ship as it passes. The disturbance caused by a wave is temporary. After the wave has passed, the water is calm again.


				
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