To develop an understanding of the water cycle by building and evaluating two different physical models.
The water cycle is of such profound importance to life on earth that students should have many experiences that will in time contribute to their understanding of evaporation, condensation, and the conservation of matter. At this grade level, students can conduct investigations that go beyond the observations made in the earlier grades. This will allow them to learn the connection between the liquid and solid forms of water.
In this activity, students will build and reflect on the usefulness of models that demonstrate the water cycle in a closed system. They will see evidence of condensation and evaporation. Students may have a difficult time understanding the existence of water they cannot see with their own eyes. They may think that evaporated water ceases to exist, or that it can only change into a form they can see, such as fog. (Benchmarks for Science Literacy, p. 336.)
Write the word "cycle" on the board. Discuss these questions:
- What images are suggested by the word "cycle"?
- What shapes best represent cycles?
- Have you ever heard of the water cycle? What do you know about it?
Now have your students use their Models of the Water Cycle student esheet to go to The Water Cycle, an interactive animation that lets them control the water cycle as they learn. Have them simply click on the "Auto" button found in the oval in the lower left of the screen. This will take them through the animation of the water cycle, which serves as a good review.
After they have finished viewing the animation, ask these questions:
- When and why does it rain?
- What happens to the rain water once it reaches the ground?
- What happens to the water when the sun comes out?
- What happens to the vapor in the air when it gets cold?
After a review of the water cycle, have students use their student esheet to go to and critique the illustration at Thirstin's Water Cycle Adventure.
Ask students these questions:
- Does the illustration do a good job of showing the water cycle?
- Can you think of ways that it can be improved?
Tell students that they will build two models of the water cycle and decide which model is best. Instructions can be found on the student esheet.
Working in groups, have students build the water cycle model described in the side bar on the Water Cycle page of the Oceans Alive website. Each group will need two half gallon jars, a rock, masking tape, and food coloring. Have the groups watch and record what happens to their model as it warms on a sunny windowsill in their science journals. In their own words, they should describe how the model explains what happens in the water cycle.
Have the student groups build the model on Going Further: Building a Model of the Water Cycle. Each group will need a large Ziploc® plastic baggie, food coloring, masking tape, and a thermometer with a large range. In addition to building the model, they should reflect upon and explain what their models show. They should answer all the questions in the activity in their science journals. In their own words, they should describe how the model explains what happens in the water cycle.
Have students reflect on the differences in the models. They should list the advantages and disadvantages of each.
- How accurately do the models represent the process of the water cycle?
- How are the models alike? How are they different?
- Was one more simple than the other?
- Which one does a better job of explaining the water cycle? Why?
- Which did you find most useful in helping you understand the water cycle?
- Is there an aspect of the water cycle that is not well represented by either model? If so, how could you change the model to make it better?
To assess student understanding of the water cycle, have students revisit the illustration on Thirstin's Water Cycle Adventure. Have students revise their critique of the illustration based on what they learned about the water cycle from studying and comparing the two models that they built.
To assess understanding of the usefulness of models, ask students to rank the following according to how well they represent the idea that the earth's water is constantly recycled: the water cycle illustration; model #1; and model #2. Students should support their rankings by listing reasons for their choices.
Water in the City, from the Science Learning Network and the Franklin Institute, can be used to extend the ideas in this lesson.
The Case of the Disappearing Water, on the Environmental Protection Agency website, provides a mystery that students can read and activities that they can do to help solve the mystery. The goal is to explain evaporation in the context of the water cycle. The six-page PDF file, which can be downloaded, contains background information for teachers and reproducible student sheets.