Matter is anything that takes up space and has weight. We can prove air is a matter using a balloon. So, pick a balloon and note down its weight using a simple balance before we fill in air into it. Now, blow the balloon using your mouth or a blower machine. Now the empty balloon expands and becomes large with a certain shape. It shows that it has air inside, occupying space, and expands the balloon to a certain size. Then, weigh the inflated balloon. You will see the inflated balloon weighs more than the empty one. From these two observations, we get to know that air takes up space and has weight. So, anything which has weight and occupies space is a matter, and hence the air is a matter.