What makes the bathtub overflow when a person settles in for a bath? Is it weight or volume? It's the volume, but children often believe it is weight that "pushes the water out of the way." In this investigation, students confront this idea head on.

Today, students gather evidence to decide if weight or volume is the determining factor in the displacement of water.

By the end of this investigation students will understand that objects submerged in water will always change the water level, and that it is volume, not weight, that determines how much water will be moved aside.

Learning Goals

• Discover what property of a sunken object makes the water rise
• Understand that two objects cannot occupy the same space at the same time

Sequence of experiences
1. Ask the question	All Class	10 Mins
2. Explore water displacement	Small Groups	15 Mins
3. Share data	All Class	5 Mins
4. Make meaning	Discussion	15 Mins

Materials and Preparation

For the class:

• Post the investigation question in a place where all students can see it.
• Prepare two class data charts, one for weight, another for volume; examples are shown in the notebooks and in Step 3.

Have the following materials ready for a demonstration at the start of class:

• 2 150cc containers with a vertical strip of masking tape applied to the outside, each approximately half full of water
• 2 rocks or minerals, one visibly larger than the other (each must fit in 150cc container)
• 1 aluminum cube (44g)
• 1 copper cube (147g)
• 1 44g cube of plastic modeling clay (same weight as aluminum cube)
• 1 3oz cup of water
• 1 pipette
• 1 plastic fork
• 1 fine tip permanent marker
• 1 paper towel

For each tray:

• 3 150cc containers with a vertical strip of masking tape applied to the outside, each approximately half full of water
• 1 aluminum cube
• 1 copper cube
• 1 piece of plastic modeling clay weighing approximately 50g
• 1 pipette
• 1 3oz cup of water
• 1 fork
• 1 fine tip permanent marker
• 2 paper towels
• 1 digital scale

Show the students the two containers of water and the two rocks.

• What do you predict will happen when I add this larger rock to this container of water?

As students predict, ask them to explain their reasoning.

Before placing the rock in the water, show students how to draw a line to record the initial water level. Demonstrate lowering your eyes to the level of the water to accurately see its level and then draw a line on the masking tape. Next, use a fork to lower the rock into the container. Explain that you don’t want any water to splash out because that would change the amount of water in the container. When students have confirmed that the water level has risen:

What do you think caused the water to rise?

Students may say, "The rock pushed the water away," or "The rock is heavier than the water so the rock went down and the water went up." Some may suggest "size" or "volume" is responsible, or maybe "pressure" or "gravity." Accept all suggestions without comment for now.

Show students the smaller rock.

• What will happen when I put this smaller rock into the second container? Will the water level rise the same amount? More? Less? Why do you think so?

Before lowering the smaller rock into the second container, make sure the water level matches the starting level marked on the first container. Use the pipette to add or remove enough water to get them exactly equal, then mark the container. Use the fork to lower the smaller rock into the water without splashing.

• What do you notice?

Students should see that both rocks caused the water level to rise, and that the larger rock raised the water level more than the smaller rock. Let students know that the scientific term for this change in water level is water displacement. The water moves away because the rock has taken its place.

• Why do you think one rock displaced more water than the other?
• Is it the weight of the bigger rock or its volume? How can we find out?

Have students brainstorm among themselves, then ask them to share their thoughts with the class. Listen for a suggestion of a controlled comparison or "fair test", i.e., to test two objects with the same volume but different weights — or the same weights but different volumes — and observe the difference in the amount of water that is displaced.

Return to the investigation question:

What causes the water level to rise?

Explicitly point out that we are trying to determine if volume or weight is the cause.

Distribute a tray of materials to each group. Ask students to examine the two cubes to see how they compare. Do they agree that they have the same volume? Can they tell that the copper cube is much heavier than the aluminum cube?

Point out the plastic modeling clay and the scale. Let students know they can cut and form the plastic modeling clay into a third cube having the same weight as the aluminum cube.

Ask students to help you list the steps for a fair test that will answer the investigation question. An example is given below.

Have students open their science notebooks and record their predictions, then set the groups to work. Students should first compare the aluminum and copper cubes and then work on comparing the aluminum and plastic modeling clay cubes.

As you circulate among the groups, confirm that the volume of the plastic modeling clay cube is greater than the volume of the aluminum cube. Make sure students are recording their predictions and their data in their notebooks [How much will the water level rise?].

Let students know that the only quantitative data they will record is the weight of the cubes (in grams); the volume data and the water levels will be comparative data, e.g., " volume is greater than the other." or "the water rose higher."

Ask students to help you fill in the class data charts. The data will look something like this.

Comparing Aluminum and Copper Cubes

	Volume	Weight	Water level
Aluminum cube	Same as copper	44 grams	Same as copper
Copper cube	Same as aluminum	147 grams	Same as aluminum

Comparing Aluminum and Plastic Modeling Clay Cubes

	Volume	Weight	Water level
Aluminum cube	Smaller than plastic modeling clay	44 grams	Lower than plastic modeling clay cube
Plastic Modeling Clay cube	Larger than aluminum	44 grams	Higher than aluminum cube

Resolve any conflicts in the data before moving on to the discussion.

Purpose of the discussion

The purpose of the discussion is to construct an explanation together of what causes the water to rise. The resulting explanation should be consistent with observations and evidence. Begin the discussion by returning to the investigation question.

Engage students in the focus question

Weight or volume, which causes the water level to rise?

Engage students in the topic by asking them to talk a couple of minutes to talk with a partner about their claims and the evidence that supports their claims. Provide three 150cc containers with equal volumes of water and an aluminum, copper, and plastic modeling clay cube for students to refer to during their discussion, as well as the data they recorded in their notebooks.

Open the full-class discussion with the focus question.

Is it weight or volume that causes the water level to rise?

Supporting questions

• What do the data tell us? (Refer to the class data chart.)
• What claim can you make: can you claim it's the volume that causes the water to rise? Or do you claim it's weight? What is your evidence that supports your claim? And what is your reasoning?
• How do you explain the results?

Support the discussion with "moves" such as:

• Who wants to repeat (student A)'s explanation using different words?
• Do you agree or disagree? Does that reasoning make sense to you?

When you think the group is convinced that it is volume that determines how much water is displaced, ask students for an explanation of the displacement phenomenon.

• Does it make sense that it's volume and not the weight of an object that determines how high the water will rise? How do you explain this?

• What happens to the water when an object sinks in it? Where does it go?

• Water will go in whatever direction the shape of the container allows. In a straight-sided container it will move straight up. In a bowl with sloping sides, it will move up and sideways. If the container is full, the water will overflow.

• Have you seen water displacement happen anyplace else?

• In the bath tub, in a cooking pot, in a puddle when you step in it.

Summarize the discussion and recap the investigation

Here is what I think I heard you say:

Use the same language that students have used to summarize the discussion. Include student comments that address claims and evidence, but be sure to highlight students' reasoning and explanations of displacement.

Describe the initial observations using rocks, and the dilemma of not being able to determine whether it was weight or volume that was causing the water level to rise. The rock with the greater volume also weighed more.

The tests students did using the three cubes allowed us to separate weight from volume; in both tests, it became clear that volume was the property that determined how much the water rose, and weight did not make a difference.

A key concept is that two objects cannot occupy the same 3-D space at the same time.