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Two jars of limewater, one still clear and one gone milky A lit candle on the left, with a dashed line running from above its flame across to the jar on the right, showing where the invisible gas goes. Two jam jars stand side by side, each with a lid rim and a narrow neck. The middle jar has only room air in it and its liquid is drawn as an empty band, still clear. The right jar has taken the candle's air and its liquid is a solid filled block, because it has turned milky.

The Gas You Cannot See Leaving

About 10 minutes

We are one ingredient short. The wax is carbon and hydrogen; the hydrogen has left as water; the carbon has not been accounted for.

It has not stayed behind. Look at a candle that has burned all the way down and there is no black heap where the carbon should be. It went up with everything else, joined to oxygen from the air, as a gas with no colour and no smell.

That gas is carbon dioxide, and the problem with it is obvious. You cannot see it. You cannot smell it. You could stand in a jar of it and know nothing about it until you tried to breathe.

So this part is about a trick — the one that makes it visible.

Cloudy water

Take a lump of ordinary lime — the powdery white kind used in garden soil and in cement, not the fruit — stir it into water, and let it stand overnight. Most of it sinks. What is left on top is clear, colourless and looks exactly like water.

It is called limewater, and it has one talent. Blow carbon dioxide through it and it turns milky white in seconds.

What forms is chalk. Real chalk — the same substance as a cliff, a stick of blackboard chalk and a seashell. The carbon dioxide joins with the lime and makes a solid that will not dissolve, so it hangs in the water as a cloud of tiny grains and the liquid goes opaque.

That is the whole of the test, and it is exact. Nothing else in ordinary air does it. Bubble plain air through limewater as long as you like and it stays clear — the nitrogen does nothing, the oxygen does nothing, and the tiny amount of carbon dioxide already in fresh air is far too little to show. Only a real supply of carbon dioxide will cloud it.

Two jars of limewater, one bubbled with room air and one with a candle's air Two identical jars stand side by side, each holding the same clear limewater and each poured from the same jug. Nothing is done to the left jar and its liquid stays clear, drawn as an empty band. The right jar is held over a candle and its liquid has gone milky, drawn as a solid filled band. Only the second jar has changed. left alone still clear air from the candle milky with chalk same liquid
Two identical jars of limewater, poured at the same moment. Nothing is done to the left one and it stays clear. The right one is held over a candle and chalk forms in it.

Doing it

You can run this yourself. It is the most convincing five minutes in the whole book, because you are catching something invisible in the act.

  1. Make the limewater. An adult stirs a heaped teaspoon of lime into a jug of water and leaves it to stand for a few hours, ideally overnight.
  2. Take the clear liquid off the top and pour it through a coffee filter into both jars. It should be clear enough to read through. Any cloudiness now will ruin the test, because you are looking for cloudiness later.
  3. Set the first jar aside and do nothing to it. That is your control. It holds the same liquid, poured at the same moment, and its whole job is to show you what "unchanged" looks like. Do not blow into it — not yet, and part 6 will tell you why that matters.
  4. Catch the candle's air. Hold the second jar upside down above a burning flame for twenty seconds, so the rising gas collects in it. Turn it quickly the right way up, put a lid or a hand over it, and swirl the limewater around inside.
  5. Compare them. The candle jar goes cloudy. The untouched jar does not, which is how you know it was the candle and not the jar, the water, or the weather.

If the second jar clouds and the first does not, you have just seen carbon leave a candle. It went in as a solid stick of wax, it came out as an invisible gas, and it is now a white powder you can look at.

Two jars hold the same limewater. One is left alone and stays clear. The other is held over a candle and turns milky. What is the white cloud?

So the account is finally complete. In go wax and oxygen. Out come water and carbon dioxide, plus a great deal of heat and a little light. Nothing else. That is a candle, all the way down.

Which leaves one last thing to do with it, and it is not about candles at all.

Take the straw and the leftover jar of clear limewater, and blow through it with your own breath.