How Does the Sun s Energy Cause Rain?

1.2 Investigate 3.3 Read How Does the Sun s Energy Cause Rain? In the water-cycle simulation, you observed water change from a liquid to a gas, and then back to a liquid falling to the bottom of the container. In this section, you will read about how water can change from one form to another and back again. But first you will think about what you already know. Conference Imagine that it rained last night in your neighborhood, but this morning the Sun is shining. On your way to school, you pass by a puddle on the sidewalk. On your way home from school, you notice that the puddle is much smaller than it was earlier. Read Collaboratively Read along with one or more other students. You can either each read a passage aloud, or silently. After each passage, discuss what was just read as a group. This puddle was photographed three times in one day. In your group, discuss what happened to the puddle. Where did the water go? Share any ideas that you may have about where the water went and why it is no longer there. Listen to ideas from other group members. Use the questions to guide your discussion. 1. Where do you think the puddle came from? 2. Do you think the puddle grew smaller all at once or slowly over time? 3. What do you think happened to the water in the puddle? 4. Draw a sketch to show how you think the water moved and changed. Use arrows to show motion and add words to describe what the arrows mean. WW 141 WEATHER WATCH

Learning Set 3 Why Are There Differences in Precipitation? Communicate Share Your Ideas Share your answers to the questions on the previous page with the class. As you discuss what happens to the puddle, develop a set of class diagrams that show the different ideas class members have about how the water moved and changed. Identify what else you need to know so that you can decide which ideas match what actually happens to the puddle. Water as a liquid a solid and a gas. (The liquid water in the wet clothes turns to invisible water vapor as the clothes dry.) What Happens When Water Changes State? In the water-cycle simulation, you observed rain form on the underside of the lid. When you saw those water droplets form, you probably suspected that the water came from the liquid water at the bottom of the container. In between, the water was in the air as a gas. When water becomes a gas, it is called water vapor. The ice above the lid was solid water. Solid, liquid, and gas are three different states of matter. Water occurs naturally on Earth in all three states. As you read in Learning Set 2, all matter is made of tiny particles constantly moving. The particles that make up water are molecules. Whether water is a solid, a liquid, or a gas, it is still water because the molecules are the same. In each state, the molecules behave differently. Before you think more about how water gets into the atmosphere, you need to understand a bit about the differences in the behavior of the particles in solids, liquids, and gases. In gases, particles are far apart and are free to move around anywhere in space. A gas fills the container in which it is placed. However, if the container is not sealed, a gas can also escape from the container. Project-Based Inquiry Science WW 142

3.3 Read In liquids, particles are closer together, but they are still free to move around one another. However, their movement is limited by the volume of the liquid. Unlike a gas, which can expand or be compressed, a liquid has a definite volume. Although a liquid takes the shape of the container in which it is placed, it does not necessarily fill the container. Although there are differences between liquids and gases, the one thing they have in common is that the particles in liquids and gases are free to flow. Because they can flow, gases and liquids are both referred to as fluids. In solids, the particles are very close together. The particles of solids vibrate constantly, but they do not move from one place to another. Because particles do not move around in a solid, a solid is able to maintain its shape. The diagrams below show the movement of the particles of a substance in a solid, a liquid, and a gas. The arrows represent the speed of movement of each particle. The longer the arrow, the faster the particle moves. Notice that the arrows in the liquid are not all the same length and point in different directions. The same is true for the gas. All the particles in a gas and in a liquid do not move at the same speed or in the same direction. water vapor: the gas state of water. solid: matter that has a definite shape and volume and an organized arrangement of particles that remain very close together and vibrate. states of matter: the forms a substance can take. The three normal states of matter are solid, liquid, and gas. Stop and Think 1. Liquid water looks very different from ice. Water vapor is not even visible. How can you be sure that the three states of water are the same substance? 2. How does the movement of the particles of a substance differ in a gas, a liquid, and a solid? 3. Why can liquids and gases fl ow, but solids cannot? WW 143 WEATHER WATCH

Learning Set 3 Why Are There Differences in Precipitation? How Does Water Change State? Particles of matter are always moving, but the speed at which they move can change. The speed at which all the particles in an object are moving is related to its thermal energy. Remember that thermal energy is the total kinetic energy (energy of motion) of the atoms and molecules in an object. A system can increase its thermal energy through heating. You have read that heating can occur in three ways: conduction, convection, and radiation. This transfer of thermal energy to a substance increases the kinetic energy of its particles. For example, transferring thermal energy to solid ice makes the molecules vibrate faster. As more thermal energy is transferred, the molecules vibrate in place faster. Eventually, the molecules move fast enough to move away from their fixed positions in the solid. The molecules are now free to slide past one another, or fl ow. The water is now in the liquid state. If more thermal energy is transferred to the molecules, the process continues. The water molecules move faster and spread farther apart. With enough energy, the molecules escape from the liquid and change state again, becoming a gas. At what temperature do these changes of state occur? You have read that temperature is a measure of the average kinetic energy of the particles in a substance. When ice is heated and its particles begin to move faster, the average kinetic energy of the particles increases and its temperature rises. As ice is heated, its temperature will rise. At a given point, the temperature Project-Based Inquiry Science WW 144

3.3 Read stops rising. At this point, the transfer of more thermal energy no longer raises the temperature. Instead, the heat changes the ice to liquid water in a process called melting. The temperature at which a substance melts is called its melting point. At Earth s sea level, the melting point of water is 0 C (32 F). If the liquid water is heated further, the temperature rises once again. Eventually, the temperature again stops rising. At this point, the thermal energy that is transferred changes the liquid water to a gas that bubbles out of the liquid in a process called boiling. The temperature at which a substance boils is called the boiling point. At Earth s sea level, the boiling point of water is 100 C (212 F). condensation: the process of changing from vapor, or gas, to liquid. The whole process can be reversed if thermal energy is transferred out of a system. As a gas loses heat and cools, its thermal energy decreases. The gas molecules begin to slow down and become more organized. Eventually, the gas becomes a liquid again. This process is called condensation. You observed condensation in the water-cycle simulation. If the thermal energy continues to decrease, the particles of the liquid stop fl owing and can become very organized as the molecules become arranged in patterns. The substance is now a solid. Stop and Think 1. Describe how you know that water molecules in ice have less thermal energy than do water molecules in liquid water. 2. Describe what happens to substance at its melting point. Describe what happens to a substance at its boiling point. 3. What causes water molecules to change from the liquid state to the gas state? From the gas state to the liquid state? WW 145 WEATHER WATCH

Learning Set 3 Why Are There Differences in Precipitation? evaporation: the process of changing from liquid to vapor. evaporate: to change from the liquid state to the gas state without boiling. In the puddles, some molecules absorb enough energy from the Sun s radiation or from the air above to escape the puddles and enter the gas state. How Does the Water From the Puddle Get Into the Atmosphere? A pot of boiling water is very hot. A lot of thermal energy is needed to change water from the liquid state to the gas state. But water in a puddle on the street or pavement disappears even though it never has a chance to boil. You thought about how this happens at the beginning of this section. To disappear, the water in the puddle must have turned from liquid water that is visible to water vapor that is invisible. But how can that happen if the water does not boil? The process by which the water in a puddle changes to water vapor is called evaporation. In this process, liquid water changes to water vapor, or evaporates, without boiling. To understand how evaporation takes place, you need to think back to the kinetic theory of matter. It states that all matter is made of tiny particles, and that the tiny particles are always in motion. Another one of the main ideas of this theory that you have not yet considered is that the particles of matter are attracted to one another. The strength of this attraction depends on the type of material and also on the distance between the particles. In solids, the particles are close together and the attraction between particles is very strong. This means that in solids the attraction is strong enough that particles are held tightly in place. In liquids, because the molecules are not as close to one another, the attraction between the particles is weaker than in a solid. However, the attraction is still strong enough that the particles cannot move freely on their own. In gases, the molecules are moving faster, which tends to keep them farther apart. The attraction exerted between molecules is very weak, and the particles are free to move in all directions. When water is in the liquid state, there are forces between the water molecules that are strong enough to hold the molecules together so they cannot move freely on their own. As thermal energy is transferred to the water, molecules move faster. Some molecules move faster than others. Project-Based Inquiry Science WW 146

3.3 Read Sometimes, molecules that are near the surface move fast enough that they can overcome the attractive force that makes them stick to the other molecules. They can then move away from the surface of the liquid into the air. Although the temperature of the water does not heat to the boiling point, some water molecules have enough energy to escape and become a gas. This process of evaporation is an important part of the water cycle. This is what happens to water in a puddle. Even though the temperature of the water is not near the boiling point, some molecules absorb enough energy from the Sun s radiation or from the air above the puddle to escape the puddle and enter the gas state. You observed evaporation in the water-cycle simulation. In the simulation, the lamp heated the water by radiation. The water molecules moved faster as thermal energy from the lamp was transferred to the water. Some of the water molecules moved fast enough to escape from the water and become water vapor. Later you saw those molecules condense on the underside of the lid. In the water-cycle simulation, the energy that caused the water to evaporate came from the lamp. In Earth s water cycle, the energy that causes evaporation comes from the Sun. Without energy radiated by the Sun, water in a puddle would not evaporate. Even when water is not in direct sunlight, such as at night, indoors, or on cloudy days, the Sun still provides the energy for evaporation. The thermal energy that water absorbs, even at those times, has ultimately come from the Sun. The Sun plays a big role in the formation of rain. Without evaporation, there would not be water vapor in the atmosphere to form clouds and become liquid again. The Sun provides the energy needed to keep the water cycle going. What Are the Sources of Water Vapor in Earth s Atmosphere? The puddle is one small example of the way water enters the atmosphere. The oceans, which cover over 70 percent of Earth s surface, are the major sources of water vapor in Earth s atmosphere. The same evaporation process that changes water in a puddle into water vapor in the atmosphere also changes water from the oceans into water vapor in the atmosphere. Water also evaporates from other water sources on Earth, such as lakes, rivers, and ponds. The oceans are the major sources of water vapor in the atmosphere. In the rainforest, the plants release a lot of water molecules into the atmosphere as water vapor. WW 147 WEATHER WATCH

Learning Set 3 Why Are There Differences in Precipitation? Ice and snow are another source of water vapor. Ice and snow can turn directly into water vapor by a process called sublimation. Sublimation is the change of state directly from a solid to a gas without going through the liquid phase. Water can also enter the atmosphere from animals and plants. In plants, the process is called transpiration. In transpiration, plants release water molecules into the atmosphere as water vapor. Stop and Think 1. Draw a picture to show how water from a puddle gets into the atmosphere. Label liquid water and water vapor. 2. Name some sources of water in the atmosphere. 3. Update the diagram that you made of the water cycle. Add arrows and descriptive words to the picture. You may need to start with a new page. sublimation: the process of a substance changing directly from a solid to a gas. transpiration: the process by which plants release water vapor. 4. What else do you need to know to more completely understand the water cycle? What s the Point? In the water cycle, there are continuous changes as water moves among liquid, solid, and gas states. As water is heated by energy from the Sun, it becomes water vapor. This invisible vapor can move from one place to another in the atmosphere. When water vapor cools, it again becomes liquid. The liquid water forms drops that can fall as rain or other forms of precipitation. Back on Earth s surface, water can once again be heated by the Sun and return to the atmosphere. Cloud cover over Earth. Project-Based Inquiry Science WW 148