The One Percent That Runs Everything

Breathe in. Breathe out.

Both of those were air. If you catch the two samples and analyse them, here is what you find:

Two things there should stop you.

The first is that you did not use most of the oxygen. You took in 21 and gave back 16, so you kept about a quarter of what you breathed. That is not laziness — it is why mouth-to-mouth resuscitation works. A person can be kept alive on somebody else's used breath, which would be flatly impossible if lungs took all the oxygen out.

The second is stranger. The carbon dioxide went up a hundred times over — and it is still only 4%. Even in breath you have just exhaled, carbon dioxide is a rounding error.

Which raises the question this lesson is really about. Carbon dioxide is 0.04% of the air around you. Argon — a gas you have probably never thought about once — is 0.93%, twenty-three times as much. One of them sets the temperature of the entire planet. The other does nothing at all.

How much of a gas there is tells you nothing about how much it matters.

By the end of this lesson you'll be able to:

• say what air is made of, and why nitrogen being 78% is not the interesting part
• explain why oxygen supports burning, and why oxygen itself does not burn
• use lime water to test for carbon dioxide, and say what a milky result does and does not prove
• explain the greenhouse effect as a mechanism, and say why it is not a synonym for pollution