First, what is matter? Matter is anything that has mass and takes up space by definition. So therefore, everything on the earth that you can see and touch consists of matter. But what about air?
The experiment that we've done in class does show air consists of matter. When we blow air into the balloon the balloon expand, so it shows that air takes up space.
Then if you compare the mass of the balloon before and after it is filled with air, and it'll show that the balloon is more heavy when it has air in it. This shows that air has mass.
If air takes up space and has mass, then air must be a matter, because the experiment shows that.
http://wow.osu.edu/experiments/gases/airhasmass.html
http://wow.osu.edu/experiments/gases/balloon.html
