How can we compare the volumes of liquids?
1. Ask the question
Meet for an all class discussion. Set out the containers of earth materials: water, mineral oil, gravel, sand, and organic soil.
Take a minute to revisit how the unit has unfolded so far: students first investigated solid materials (rocks and minerals), then they investigated granular materials (gravel and sand). Let students know they will now investigate two liquids — mineral oil and water — starting where they left off in the last investigation, with volume.
Show the containers of oil and water, and introduce the investigation question:
How can we compare the volume of liquids?
Listen for comments about seeing the "amount of 3-D space" the liquids take up, or "how high they go" in their containers. Agree that this strategy works if the containers are the same size and shape, but what if they are different?
Show the class the three containers holding the blue-tinted water. Point out the labels — A, B, and C — and explain that the liquid is water tinted with food coloring. Now issue a challenge:
Can you put these liquids in volume order using only your senses? No rulers, no scales, no measuring cups, no numbers — just your eyes and your hands and your brains?
Encourage students to model the volumes with their hands and to offer strategies for comparison. If they hazard guesses, ask them to justify their predictions. As students think out loud, listen for an understanding of the difficulties of the task— the containers have different shapes, we do not know the weights of the samples, there are no volume markers, and so on.
Talk with your hands! When talking about volume, it is often useful to "talk with your hands." In fact, hands can often describe 3-dimensional spaces better than words can. For practice, ask students to use their hands to indicate the volume of some common objects such as an orange, a watermelon, or a desktop computer. Then have them model the volumes of the water they see in the containers. Are they capturing all the dimensions: length, width, and height?
