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A jar over a candle, and how little of the air the flame can use On the left a sealed jar stands over a lit candle, so no new air can reach the flame. On the right, a long bar stands for all the air in that jar. Four fifths of the bar is an empty outline, and that is the nitrogen, which does nothing. Only the last fifth is filled in, and that is the oxygen the flame needs.

Nothing Burns Without the Air

About 12 minutes

A flame burns at its surface, where the fuel meets the air. Take the air away and there is no surface, so there is no flame. Everyone knows how to put out a candle with a jar.

What almost nobody knows is what the jar actually proves — because the obvious explanation is wrong, and the real one is better.

The candle does not use up the air

Put a jar over a burning candle and watch. The flame carries on for a few seconds, then shrinks, then goes out. If you tip the jar afterwards, nothing pours out and nothing rushes in. The jar is still full.

Full of something, anyway.

Air is not one substance. It is a mixture, and by far the biggest part of it — about four fifths — is a gas called nitrogen, which does nothing at all here. You cannot burn in it, you cannot breathe it on its own, and a candle in pure nitrogen goes out instantly. Nitrogen is the part of the air that is simply in the way.

The part that matters is oxygen, and it is only about one fifth of the air. That fifth is the whole reason anything burns anywhere.

And it does not use up the oxygen either

Here is the part that catches people out, and it is worth getting right because a lot of books get it wrong.

A candle under a jar does not burn until the oxygen is gone. It goes out long before that — when the oxygen has fallen from about 21 out of every 100 parts of air to about 16. There is still plenty left. More than three quarters of the oxygen that was in that jar is still in there when the flame dies.

Why? Because burning is a meeting. Fuel has to find oxygen fast enough to keep the shell of flame going, and as the oxygen thins out, the meetings get rarer. Below about 16 parts in 100 the flame cannot make enough heat to keep boiling wax off the wick, and the whole machine stops. It is not starvation. It is a machine that needs a certain rate to keep itself running.

How much oxygen is left in a jar, and whether the flame can still use it On the left a candle stands under a sealed jar. On the right a tall column stands for the oxygen in that jar, and the column shortens as oxygen is taken out. A dashed line crosses the column near the top of its lower part, marking sixteen parts in a hundred. Below that line the flame goes out, even though most of the column is still there. 16 in 100 below 16 the flame cannot keep itself going 21 parts in 100 oxygen left in the jar

Drag the handle to take oxygen out of the jar, one part in a hundred at a time, and watch the flame.

Fresh air sits at 21. The flame does not wait until zero — it gives up around 16, with three quarters of its oxygen still in the jar and no way left to reach it fast enough.

Something is leaving as well as arriving

If you only watched the flame you would think the story was one-way: the candle takes oxygen out of the jar and stops when there is not enough. But a candle is not a hole that things fall into. Whatever it takes in has to go somewhere.

There is a very easy way to catch the first thing coming back out, and you do not need a jar for it.

  1. Chill the glass in the fridge for ten minutes and dry the outside if anything has settled on it.
  2. Light the candle and let the flame settle.
  3. Hold the glass upside down, high above the flame, for two seconds. Take it away and look inside straight away.
  4. Look for the fog. The inside of the glass has gone misty, and if you hold it a second time you will see the mist gather into real droplets.

That mist is not steam from the wax and it is not smoke. It is water, and there was no water in the candle when you started.

A cold glass held above a flame collects water on the inside A lit candle at the bottom, drawn as a solid block with a flame. A dashed column of rising hot air goes up from it. Above, a drinking glass is held upside down, with its closed base drawn as a thick bar across the top. Drops of water have gathered on the inside and two have run together and fallen back down. water, out of a fire none was in the wax and none in the air
A cold glass held above a flame fogs on the inside within seconds. The wax had no water in it and neither did the air. Something has made it.

A candle under a sealed jar goes out. How much of the jar's oxygen has it used?

So the candle needs one fifth of the air, gives up while most of that fifth is still around it, and hands back something nobody put in.

Water, out of a burning candle. That needs explaining.