LESSON 4 - ACTIVITY THREE - THE WATER CYCLE
Student will understand the stages of the water cycle.
The water cycle, also called the hydrologic cycle, is the movement of water in Earth's hydrosphere. It involves changing the state of water between a liquid, a solid, and a gas. The hydrologic cycle is the movement of water between the atmosphere, land, and surface/subsurface waters. The hydrologic cycle has five main components: evaporation, precipitation, infiltration, runoff, and subsurface flow:
Evaporation is the movement of water from surface water to the atmosphere. In evaporation water is changed from a liquid into a gas. Transpiration is another form of evaporation and occurs in plants. 90% of the water found in the atmosphere is there as a result of evaporation. The other 10% comes from transpiration.
Precipitation is moisture in the atmosphere that forms clouds. Water has changed from a gas to a solid or liquid and eventually falls to the Earth in the form of rain, snow, sleet, etc.
Infiltration into the ground is the transition from surface water to groundwater. The infiltration rate will depend upon soil or rock permeability as well as other factors. Infiltrated water may reach another compartment known as groundwater. Groundwaters tend to move slowly, so the water may return as surface water after a period of time that can amount to thousands of years. Water returns to the land surface at a lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures.
Runoff includes surface water that moves down a slope to the ocean. Water flowing in streams and rivers may be delayed for a time in lakes. Not all precipitated water returns to the sea as runoff. Much of it evaporates before reaching the ocean or becomes groundwater.
Subsurface flow incorporates movement of water within the earth. After infiltrating, subsurface water may return to the surface or eventually seep into the ocean.
1. Students will conduct this experiment to observe condensation.
Hand out a cup to each group. Ask your students if the cup feels wet or dry. Place a couple of ice cubes in each plastic cup. Add enough cold water to each cup to cover the ice cubes. Allow the cups to sit for several minutes. When moisture begins to collect on the outside of the cups, ask your students to again feel the cups and answer the question, Do the cups feel wet or dry?
Here's what happened. The ice in the cups caused the air around them to cool down. When the air cooled down, the water molecules in the air cooled down as well and moved closer together. The water molecules that touched the cool surface of the cup collected together to form water droplets. The changing of gas into a liquid is condensation.
2. Students will conduct the following experiment to observe condensation and evaporation.
Students may work individually or in groups depending on availability of supplies. Have your students soak their paper towels in the cold water. place the paper towel in the bottom of the yogurt cup. Add approximately 10 grains of salt into the bottle cap. Be careful not to use too much salt. Place the bottle cap onto the paper towel taking care that it is level and that no water from the paper towel gets into the bottle cap. Place the lid tightly onto the yogurt container. Allow the yogurt container to sit undisturbed for at least one half hour. Ask your students to make predictions as to what they think they will see when they remove the yogurt lid. At the prescribed time, have your students carefully remove the yogurt lid taking care not to tip the container. Carefully remove the bottle cap, again, taking care not to tip it sideways. Students should see a collection of water droplets in the bottle cap and the salt has disappeared.
Here's what happened. Water evaporated from the paper towel and created air that had high humidity in the yogurt cup. Water molecules in the air collected around each salt crystal until the salt dissolved. Now, all that is left are small drops of salty water in the bottle cap. This process is called condensation. This is the same thing that happens in clouds before it rains. (Refer to the cloud section of this curriculum).
Next, have your students set the bottle cap, with the water droplets, in direct sunlight. Ask your students what they think will happen. Allow the bottle caps to remain in the sunlight for approximately 30 minutes. After the prescribed amount of time, the sunlight should have evaporated the water and all that will remain in the bottle cap is the salt. The salt should look like a white powdery layer in the bottle cap. Ask your students to explain what happened.
Here's what happened. The heat of the sun transformed the liquid water into invisible water vapor (water that is in a gas state) which returned to the atmosphere. The salt that was suspended in the water droplets remained behind. When a liquid is transformed into a gas, it is called evaporation.
3. Students will conduct the following experiment to demonstrate transpiration and condensation.
Students may work individually or in groups. Take your class outside and locate plants that are located in the sun as well as some that are located in the shade. If no plants are located near your school, use inside plants placing one in direct sunlight, and one away from windows and other sources of heat. Place a plastic bag over the end of the plant, taking care not to damage the plant, so that several leaves are inside of the bag. Tie the string around the open end of the plastic bag to create a closed environment, again, taking care not to damage the plant. Allow the bags to sit for 30 minutes. After the prescribed amount of time, go back and observe the plastic bags. The bags that are placed in direct sunlight should show a significant amount of condensation whereas the bags located in the shade should show less. Ask your students to explain what happened.
Here's what happened. Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves. In drought situations, transpiration contributes to loss of moisture in topsoil, which can adversely affect vegetation. Because we placed a plastic bag around the leaves, we trapped the water vapor before it could reach the atmosphere and the water vapor condensed into visible water droplets. This is a very simple demonstration of the complex process known as transpiration. Some interesting facts to share with your students, an acre of corn gives off about 3,000-4,000 gallons of water each day, and a large oak tree can transpire 40,000 gallons per year.
4. Students will label a diagram of the water cycle and provide a brief explanation of each step.