Purpose of discussion

The purpose of this discussion is to help students make sense of the results of today's experiences, which provide evidence that air has weight and takes up space. Students may feel that today's results are in conflict with other experiences they have had with air. Focus the discussion on the investigation question: Is air matter?

Engage students in the focus question?

Is air matter or not?

• Does air take up space? What is your evidence?
• Does air have weight? What is your evidence?

Claim or position: Air takes up space:

• When I take a really big breath of air, my chest expands.
• An inflated balloon takes up more space than an uninflated balloon.

Claim or position: Air doesn't take up space:

• I can walk right through air. (We can also walk through water, but we agree that water takes up space.)
• When a classroom is "filled" with air, how can there be room for students to come into it? (Unlike the syringes, the classroom is an open system; when students come in, they push some of the air out.)

Claim or position: Air has weight:

• When we put air in one set of balloons the balance went down on the side with the inflated balloon.

Claim or position: Air doesn't have weight:

• Scales don't register the weight of air.
• We can't feel weight even if there's a lot of it on top of us.

Summarize the discussion and recap the investigation

Summarize the arguments for each position. See if there is consensus for the argument that air takes up space and has weight, and therefore, is matter.

Thinking of air as matter, which puts it in the same category as the sand, gravel, and water in the mini-lakes, can require an adjustment in our thinking.

Remind students of the dissolved salt. It's easy to think of salt as matter, but even after the salt particles became too small and too spread apart to see, the salt maintained its weight and it still took up space: it maintained its classification as matter. This may be the strongest connection students can make between air and another substance they accept as matter.

Reiterate the concept that air has weight and takes up space, and thus is matter. The reason we can't see air is because the particles are tiny on a scale that's hard to imagine and are spread far apart. On a windy day, we have an easier time sensing the presence of air.