Air & its gases

Air is all around you, and yet you cannot see it. When you take a deep breath, watch a campfire, or blow up a balloon, you are always dealing with the same invisible mixture of substances. Time to take a closer look at what air is actually made of.

Air is a mixture of gases

Air is not a single substance but a mixture of several gases. The largest share is nitrogen at about 78%. Next comes oxygen at about 21%. Just under 1% is the noble gas argon, which reacts with almost no other substance, and only about 0.04% is carbon dioxide, CO₂ for short. These figures are for dry air. Real air also contains water vapour, whose share swings between almost zero and a few percent depending on the weather.

Oxygen: breathing and burning

Oxygen is the most important part of the air for us. Your cells need it to get energy from food. Every fire needs oxygen too: put a glass over a burning candle and the flame goes out after a few seconds, because the oxygen inside is used up. Nitrogen, in contrast, is unreactive, so it hardly reacts with other substances and mostly just makes up the bulk of the air. It is useful all the same, because it dilutes the oxygen: in pure oxygen a match would burn far more fiercely.

Small but mighty: CO₂

At 0.04%, CO₂ sounds like almost nothing, but this tiny share has a huge effect. Plants use it together with light to build their sugar, and as a greenhouse gas CO₂ traps heat on Earth. If its share rises, the planet gets warmer. By the way, air also has mass: near the ground one cubic metre of air weighs about 1.2 kg, you just don't notice because it surrounds you from all sides.