Students will observe the amount of water in an open container over time, and they will observe the amount of water in a closed container over time. Students will compare and contrast the sets of observations over time.
This lesson is the second in a three-part series that addresses a concept that is central to the understanding of the water cycle—that water is able to take many forms but is still water. This series of lessons is designed to prepare students to understand that most substances may exist as solids, liquids, or gases depending on the temperature, pressure, and nature of that substance. This knowledge is critical to understanding that water in our world is constantly cycling as a solid, liquid, or gas.
In these lessons, students will observe, measure, and describe water as it changes state. It is important to note that students at this level "...should become familiar with the freezing of water and melting of ice (with no change in weight), the disappearance of wetness into the air, and the appearance of water on cold surfaces. Evaporation and condensation will mean nothing different from disappearance and appearance, perhaps for several years, until students begin to understand that the evaporated water is still present in the form of invisibly small molecules." (Benchmarks for Science Literacy, pp. 66-67.)
In Water 1: Water and Ice, students explored how water can change from solid to liquid and back again.
In this second lesson, students will focus on the concept that water can go back and forth from one form to another and the amount of water will remain the same. Even at the end of elementary school, many students do not understand that matter is conserved. They do not accept weight as an intrinsic property, and so they may think that as a substance changes from a solid to a liquid, its weight will change. Because of this notion, students believe that as a substance is broken down into infinitely smaller pieces it comes to a point where the material no longer has weight. They do not assign an intrinsic weight to substances so they believe that weight can change as the substance changes. (Benchmarks for Science Literacy, pp. 336-337.)
Water 3: Melting and Freezing allows students to investigate what happens to the amount of different substances as they change from a solid to a liquid or a liquid to a solid.
You could search the following sites for useful background information as you prepare to do this lesson:
Show students a natural sponge that is wet with water. Move around the room and let students touch and look at it closely.
- What will happen to this wet sponge if I let it sit out in the air for a long time?
- What will happen to the water?
Explain to students that they will be looking at water to try to see what happens to the water if it is left out over time.
- Have you ever played in puddles outside on the sidewalk? Have you ever gone back the next day to play again? Were the puddles there?
- If you get your clothes wet at the drinking fountain, what happens to those clothes over time?
- Can you think of any other things like this that have happened around you?
Show students a coffee can filled halfway with water.
Tell students: "We are going to take two coffee cans with water. We will cover one can and let the other sit open. We are going to watch the cans. We are going to try to find out what will happen to the water over time. Your job is to keep a journal that shows the results of what you see happening in each can. We will try to decide what happens at the end of our study." Divide the students into partners so that they may work in small groups.
- Mark each can with a colored dot (you may wish to use more than one set of cans to reduce the number of students working at a can). Show the students the wooden splints. Ask students how the splints could be used to test what happens to the water level over time.
- Fill each can with the same amount of water.
- Have students measure the water levels by dipping their splints into water at the side of the can until it touches the bottom. They should make a thin line on their splint at the water level. Mark the splints with the colored dot that matches the coffee can where the measurement was taken.
- Place the can in a safe area and cover one of the cans. Explain again to the students that one can will remain covered for the tests.
- Students should check the cans on a regular basis (daily or every other day). They should use markers to mark the water levels on their splints. Based on their sticks, the students should draw an illustration that shows what is happening to the water in the cans over time.
- Have each group glue its marked sticks to a piece of construction paper. After the sticks have been glued, students should label each of the lines for the days of measurements taken. The students should use this to help them answer and understand the following questions.
At the end of the study, ask students:
- What happened to the level of the water in the closed can?
- What happened to the level of the water in the open can?
- Is there a difference in what we saw happen between the two cans? What is the difference?
- What did we do that was different with the cans?
- What if we used jars instead of cans? Do you think our result would be different? Why or why not?
At a later time, have students repeat this study using sponges that are wet with 1/4 cup of water. Students can squeeze out the water into the measuring cup to check the water level.
- What happened to the sponge in the closed can?
- What happened to the sponge in the open can?
- Is there a difference between the sponges in the two cans? What is the difference?
- Why do you think we used a closed can?
- Why do you think we used an opened can?
- What might be different about an open can than a closed can?
Have students draw the results of what happened to the wet sponges in the second study. Tell them to be prepared to share and describe their drawings.
Have students share their diagrams and describe the results that they saw for each of the studies. In both cases of the open can, what happened to the water in it? What happened to the water in the closed can? If you had a glass of water that you wanted to save, should you leave it opened or closed? Why?
Ask students to write/share a brief response to the following scenario. "You are at a pool. You notice a puddle at the side of the pool and splash in it for a little while. Next you go and swim for a long time. When you go back to the puddle, what do you think will have happened to it? Draw a picture that illustrates what has happened."
See the final Science NetLinks lesson in this series, entitled Water 3: Melting and Freezing.
Read Puddles by Jonathan London (illustrations by G. Brian Karas) and ask students the following questions:
- Where did the puddles and baby rivers come from?
- What will happen to the puddles over time?
- Will it take longer for this to happen to the big puddles or the small puddles? Why do you think this?
- What will happen to the wet grass over time?
- What will happen to the mud?
- You go out into grass early in the morning and your shoes get wet. Later in the afternoon you notice that your shoes do not get wet. What happened to the water/dew that was on the grass?
- If the children in the story had left their coats out in the rainstorm, what would have happened to their coats if they were left outside over time? What if the coat was put into a plastic bag and shut? Would this still happen? Why or why not?