Purpose of the discussion

The purpose of this discussion is for students to develop more refined explanations for the differences they have observed in the compressibility of air and water. The discussion also generates a need for thinking about air on the particle scale. The discussion focuses on the question: Are air and water compressible?

Prepare for all class discussion

Distribute two annotated drawings you selected from Investigation 14 for students to discuss in pairs. Have student pairs briefly review and discuss the two drawings (5 minutes).

Students should use the discussion questions on the Annotated Drawings poster that you have displayed (these are also found on the Annotated Drawings resource at the back of the Science Notebooks).

• What are these drawings trying to explain? (FYI: the reason why air is compressible and water is not)
• Do you see anything in the drawing that you don't understand or would like clarified?
• If you compare the two annotated drawings, how are the explanations the same? How are they different?

Begin the all-class discussion by noting that water is not compressible. Remind students that they used the Particle Magnifier to look at the arrangement of particles in water. You may want to show the class Dot Sheets 1 and 2.

Engage students in the focus question

Why do you think air could be compressed and water could not?

Why do you think the water could not be compressed?

• Water is visible so this means the particles are close together. (Dot Sheet 1 – liquid water)
• Water cannot be visibly compressed because the particles are already close together and can't get any closer.

Next, build on students' ideas about why water can't be compressed to think about why air is compressible. Keep the focus on particles in these explanations.

Why do you think air can be compressed?

• Air is invisible so this means the particles are more spread out than particles of water. (Dot Sheet 2 - air)
• There are wider spaces between the particles of air.
• Air is visibly compressible because there is a lot of space for the particles to be squeezed closer together. This is not true for water, or for any liquid or solid.

Summarize the discussion

Students may not reach a scientifically accurate explanation of compressibility. This presents the need for a model of what is happening at a scale too small to see. The scientist's model of the structure of air in the Particle Magnifier is introduced.