Activity: The great candle mystery

This week's activity has been around for a while, but it is one of the most frequently misunderstood science demonstrations that I know.

You will need:

• Birthday candles
• Matches
• Plasticine or blu-tak
• A glass jar large enough to hold the candles.
• A bowl
• Water
• Masking tape or "Invisible" tape
• Pen
• Three coins. They should all be the same type. I find five cent coins work well.
• An adult

Caution: This activity involves using matches and fire. For any activity involving fire you must have an adult present, plus water, a fire extinguisher and any other safety equipment the adult or yourself feels is needed. Take care that you only use fire in a safe place.

What to do:

1. Put a strip of masking tape onto the side of the jar.
2. Stick a lump of plasticine in the middle of the bowl and stand three candles in it.
3. Pour a couple of centimetres of water into the bowl.
4. Light the candles and let them burn for a few seconds.
5. Carefully place the jar over the candles.
6. After a while, the candles should go out. After the candles have gone out, some water will be sucked into the jar.
7. Mark the water level on the tape.
8. Take the jar off and wave at the end of it, so you put some fresh air into the jar.
9. Repeat steps 3-6, but this time only light one candle. If the water reached the wick the first time, it may take a while to get burning again
10. Compare the water level this time with the previous time.

If the jar fits perfectly against the bowl, you may find the water can't get in. In this case, the jar will not fill with water, but will be held against the bowl like a suction cup. To stop this happening, place some small coins around the candles so the jar rests on them, with a small gap to allow the water through.

The water sucked up into the jar.	With only one candle burning, much less water is sucked up into the jar.

What's going on?

Oxygen makes up about one-fifth of the air. When the candles burn, they use oxygen from the air. When there is no more oxygen in the air, they go out. The reason the water is sucked up into the jar is a little more complicated.

One theory is that when the oxygen is used up, there is one-fifth less air in the jar, so the water is sucked up. This theory often appears on web sites or even in books, which explain how this effect works. However, there are some problems with this idea:

• When fire burns, it combines oxygen in the air with carbon and hydrogen in the candle, to make carbon dioxide and water vapour. This means that although there is less oxygen gas inside the jar, it has been replaced with other gases. The exact amounts of carbon dioxide and water are hard to predict, because candle wax is made of a mixture of different chemicals. In most of the chemicals in candles, there are roughly two hydrogen atoms for every carbon atom, which means roughly two-thirds of the oxygen joins with carbon to make carbon dioxide (two oxygen atoms and one carbon atom) and one-third joins with hydrogen to make water vapour (two hydrogen atoms and an oxygen atom). Even if the water vapour condenses into liquid water, this is not enough to account for all the water being sucked up into the jar.
• The fire uses up oxygen while it is burning, but the water is only sucked up after the candles go out. If the water was being sucked up because the oxygen was being used up, it should start moving the moment you put the jar over the candles and stop when the candles go out.
• One candle still uses up the same amount of oxygen in the jar, it just takes longer to do it. However, when you only use one candle, there isn't as much water sucked up into the jar.

So why is the water sucked up? Another possible explanation is how gases respond to a change of temperature. When air is heated, it tries to expand, so it takes up more space. When air cools down, it contracts, so it takes up less space.

When you put the jar down over the candles, the fire heated the air inside the jar. As it was heated, the air expanded. If you put the jar down quickly, you may have noticed some bubbles escaping the jar (if you put it down slowly, the expanding air has time to escape out the bottom of the jar before it hits the water). Once the candles go out, the air in the jar cools down and contracts. The water is then pushed in by the surrounding air as the gas pressure inside the jar is lower due to there being less gas molecules inside the jar than when you started the whole procedure.

The reason that three candles had a greater effect than one is because they heated the air in the jar to a higher temperature.

There is one other factor at work too. Carbon dioxide made by the fire can dissolve in the water, which would reduce the amount of air in the jar. However, this appears to only be a small effect, because the carbon dioxide is made all the time, but the water is not sucked up until the fire goes out. I have also found that if the jar seals perfectly against the bowl, the air inside will still try to contract, even if there is no water inside the jar.