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Water gathering on a cold glass held above a candle flame A lit candle on the right. Above and to the left, a drinking glass is held upside down, drawn as a tumbler that is wider at its closed top than at its open mouth. Drops of water have gathered on the inside of it, and two of them have run together and fallen, so they hang in the air below. Nothing was poured in.

A Candle Makes Water

About 11 minutes

A cold glass held over a flame fogs up in two seconds. That fog is water, and it is a genuinely strange thing to get out of a fire.

Fire is the opposite of water. You put one out with the other. And yet burning a candle makes water reliably, in quantity, every time — Faraday collected bottles of it in front of an audience and passed them round. Burn a litre of lamp oil properly and you will get back more than a litre of water. More water out than oil in.

Nothing is being conjured. To see where it comes from you have to stop thinking of water as one thing.

Water is two things joined

Water is not a basic ingredient of the world. It is a compound: two different substances stuck together, and both of them are gases when they are on their own.

One of them you have already met. It is oxygen — the fifth of the air the candle needs.

The other is called hydrogen. It is the lightest substance there is. A balloon of it floats away far faster than a helium one. On its own it is a gas, it has no colour and no smell, and it burns with a flame so pale you can barely see it.

Put those two together in the right numbers and you do not get a gas at all. You get a liquid you can drink. That is what a compound means: the thing that comes out has none of the properties of the things that went in, and the only way to know what is inside it is to take it apart.

The candle is carrying half the recipe

So where would a burning candle get hydrogen from?

From the wax. Candle wax is made of just two elements, and it is worth saying their names because the rest of the book is about them: carbon and hydrogen. That is the whole ingredient list. Wax is a hydrocarbon, which is a word that means exactly what it looks like.

Now the sum comes out.

The water leaves as a hot invisible vapour, straight up with everything else. You never see it, because the flame is far too hot for it to be anything but a gas. Put something cold in its path and it condenses — and that is the fog on the glass. It was always there. You just gave it somewhere to land.

Where each ingredient starts and what it leaves as A flame in the middle. On the left, two labelled blocks stand for what the wax brings: carbon and hydrogen. On the right, two more stand for what the air brings: oxygen and nitrogen. Arrows lead into the flame from both sides. Below the flame, two arrows lead out to the two things that leave: water and carbon dioxide. Nitrogen has its own arrow going straight past, because it takes no part. hydrogen carbon oxygen nitrogen from the wax from the air water carbon dioxide straight past

Choose a part to follow it from where it starts to where it ends up.

  • Hydrogen, from the wax — rides up the wick inside the wax, boils off into the flame, and joins oxygen. It leaves as water vapour.
  • Carbon, from the wax — the same journey, but it is heavier and slower to burn. In the middle of the flame it is what makes the glowing specks. It also leaves joined to oxygen, and part 5 is about what it becomes.
  • Oxygen, from the air — arrives at the outside of the flame, one fifth of every breath of air the candle draws in. It joins both of the others. Every single thing leaving the flame has oxygen in it.
  • Nitrogen, from the air — four fifths of what comes in, and it leaves exactly as it arrived. It is warmed, pushed about, and otherwise ignored.

Why you get more water than fuel

The litre-of-oil business now stops being a paradox and becomes arithmetic.

The water that leaves the candle is not made only of candle. It is made of candle plus air. The hydrogen came out of the wax, but the oxygen it is joined to came out of the room — and oxygen is by far the heavier of the two. For every gram of hydrogen the wax gives up, eight grams of oxygen come in from the air to join it, and nine grams of water leave.

So a candle is heavier when it has finished than when it started. Not the wax — the wax is smaller, obviously. But if you could weigh the candle and every scrap of what left it, the total would have gone up, by exactly the weight of the oxygen it pulled out of the air.

That is worth sitting with. Burning does not destroy anything. It rearranges it and adds something from the room.

Burn a litre of lamp oil carefully and you collect more than a litre of water. How can there be more water than there was oil?

There is one loose end, and it is a big one.

Wax is carbon and hydrogen. We have followed the hydrogen and watched it leave as water. So where has all the carbon gone? It cannot be in the water — there is no carbon in water. It is not left behind on the plate; a clean candle leaves nothing but a stub.

Something is going up from that flame that nobody has caught yet.