TEKS describe and illustrate the continuous movement of water above and on the surface of Earth through the water cycle and explain the role of the Sun as a major source of energy in this process The Water Cycle Water on Earth We can t live without water. Water covers more than three-fourths of Earth s surface. Approximately 97% of the water on Earth is salt water. We cannot use salt water to drink or water our crops. We would have to take the salt out of the water, and that is very expensive to do. The other 3% of Earth s water is fresh. However, two-thirds of that water is frozen in the ice caps and glaciers. It is not available for our use. This means that only about 1% of all the water on Earth s surface is usable for humans and land animals. This fresh water is found in lakes, rivers, streams, ponds, in the ground, and as humidity in the atmosphere. Recycled Water Earth has what we call a closed system for water. We do not gain water from anywhere else, and we do not lose water to anywhere. The water you drink has been on Earth for billions of years. States of Water Water exists as a solid, a liquid, and a gas at temperatures common on Earth s surface. Most of the water on the surface is liquid water. Water in the solid form is called ice. Water in the form of a gas is called water vapor. Phases Evaporation is the process in which liquid water changes into water vapor. The speed of evaporation depends on temperature. During the water cycle, the sun warms some of the water in the oceans, freshwater lakes, and rivers. The water on the surface evaporates. Plants absorb water from the soil and transfer the water through the stems to the leaves. Once the water reaches the leaves, some of it evaporates into the air as water vapor. This process of evaporation through plant leaves is called transpiration. Condensation is the opposite of evaporation. Water vapor turns back into a liquid in the atmosphere. We see the tiny drops of water in the air as clouds or fog. The water droplets form on tiny particles of dust, salt, and smoke in the air. As the droplets grow, they become heavier and start to fall. Precipitation is water in its liquid or solid form falling from the atmosphere to Earth s surface. Rain, snow, sleet, and hail are some forms of precipitation. Much of the water that returns to Earth as precipitation runs off the surface of the land and is collected in streams, rivers, ponds, and lakes. Small streams flow into larger streams, then into rivers, and later into the ocean. Through surface runoff, much of the water returns to the oceans, where a great deal of evaporation occurs. and Assessment TEKS Page 1 of 6
Some precipitation soaks into the ground. Water that is underground is called groundwater. Groundwater trickles slowly down through the soil until it reaches rock. There, underground streams may form. Some groundwater returns to the surface as springs or in low spots on Earth s surface. Movement of Water The water cycle is a continuous circulation of water and water vapor between Earth and the atmosphere. It is an ongoing process that has no beginning and no ending. The heat and energy for the cycle come from the sun. Water vapor rises from oceans, lakes, rivers, forests, fields, plants, and animals by the process of evaporation. The evaporated water is carried into the atmosphere, where it cools and develops into clouds and fog through condensation. It falls back to Earth as precipitation, completing the cycle. The Water Cycle Water vapor in the air condenses, forming clouds. Water returns to Earth s surface as precipitation. Evaporation from the leaves of plants is called transpiration. Water evaporates from the ocean and other bodies of water. The sun is the source of energy that causes evaporation and drives the water cycle. Some precipitation soaks into the soil and becomes groundwater. Some water stays on the surface in streams, ponds, lakes, and rivers. Water on Earth s surface flows toward the ocean. Trace the cycle of water in this diagram with your finger. and Assessment TEKS Page 2 of 6
Hands-On Activity Model the Water Cycle In this activity, you will make a model of the water cycle. Then you will use the model to make inferences about what happens during the water cycle. Materials graduated cylinder water small plastic cup zip-top plastic bag Procedure: 1. Using the graduated cylinder, measure and pour 100 ml of water into the cup. 2. Open a plastic bag, and carefully put the cup inside. Then seal the bag. Be careful not to spill any water from the cup. 3. Place the sealed bag near a sunny window. Predict what will happen to the water in the cup. 4. Leave the bag near the window for 3 or 4 days. Observe the cup and the bag each day. Record what you see. 5. Remove the cup from the bag. Measure the water in the cup only by pouring it back into the graduated cylinder. Use the numbers you recorded to find any difference between the amount of water poured into the cup and the amount of water removed from the cup. Discussion Questions: 1. What did you observe during the time the cup was in the bag? 2. What happened to the water in the cup? 3. Scientists often infer the causes of what they observe. What can you infer about where the water in the bag came from? 4. If you could accurately measure the volume of water in the bag, what do you predict it would be? Explain. and Assessment TEKS Page 3 of 6
The Water Cycle Label the parts of the water cycle shown in each image below. What role does the sun play in the water cycle? List four types of precipitation. Why is the water cycle important for living things on land? and Assessment TEKS Page 4 of 6
TEKS Assessment Fill in the letter of the best choice. Which is the process in which liquid water turns into water vapor? evaporation condensation transpiration precipitation Which is the process in which water evaporates from plants and goes into the atmosphere? Where does most of the water that evaporates on Earth come from? lakes ponds oceans creeks Which of the following shows the source of energy for the water cycle? evaporation condensation transpiration precipitation What are clouds made of? wind cotton sunlight tiny drops of water Which of the following is the term for water that falls to the ground? evaporation condensation transpiration precipitation and Assessment TEKS Page 5 of 6
Answer Key Hands-On Activity (p. 3) 1. The amount of water in the cup dropped. Some drops of water formed on the inside of the bag. 2. The water in the cup evaporated. 3. The water in the bag must have come from the water that started out in the cup. 4. It would equal the amount of water lost from the cup. Student Response Activity (p. 4) 1. precipitation; evaporation; condensation 2. The sun is the energy source that drives the water cycle. It causes water to evaporate from Earth s surface and to transpire from the leaves of plants. 3. rain, snow, sleet, and hail 4. All living things need water to survive. Precipitation brings fresh water to the land. Without precipitation, all water on land would eventually drain to the ocean. TEKS Assessment (p. 5) 1. A 2. C 3. D 4. D 5. C 6. A and Assessment TEKS Page 6 of 6