The three properties of air are that it has weight, can be compressed and takes up space. The fact that it can be compressed makes it quite different from solids and liquids, which are unable to be compressed.
Air is a mix of gases that is composed out of about 1/5 oxygen and 4/5 nitrogen. Though air is made up of mostly nitrogen and oxygen, it also contains traces of water vapor, argon, carbon dioxide and other substances. The pressure of air depends on the location as well as the season. The air pressure can change in a matter of minutes due to many factors.