We saw this fun experiment in so many books and had to try it out – it is the quite deceptive: “the-burning-candle-uses-up-the-oxygen-and sucks-in-water” experiment! Our results were ‘excellent’ until we looked into it online…
First, the standard experimental approach and ‘standard’ explanation for you: The burning candle in a pan of water is covered with a jar – wait some seconds and the candles goes out as water is forced up inside the jar, because …
a) ‘obviously’ the burning candle uses the oxygen, and so two things then follow:
c) the water level inside the inverted jar goes up because oxygen has been used up.
Yes, that’s what we initially though too! But it’s all pretty much incorrect!
First the chemistry:
So it turns out that the hot carbon dioxide created as the candle burns is initially lighter than the air in the jar and so rises to the top and then accumulates in the jar from the top down.
Once the carbon dioxide reaches down to the level of the flame it extinguishes it (just like a carbon dioxide fire extinguisher).
Note that carbon dioxide gas is heavier than air once at the same temperature – though under normal conditions they would diffuse together (mix) to fill a volume evenly.
That explains why the candle goes out, but why does the water level go up with all this new carbon dioxide gas building up inside?
Second, the physics:
Then, as the candle flame is extinguished the air quickly cools against the cold glass of the jar and contracts taking up less volume thus drawing up water into the jar.
This explains why when you try it at home you will see scarcely any change in the water level whilst the candle is burning but a big inrush of water at the end as the candle goes out.
Thanks to the following websites for the ‘updated’ information:
Science Myths Unmasked: http://misconceptions.science-book.net/wp-content/uploads/2011/09/Chap2-1.pdf
The Independent: http://www.independent.co.uk/news/pupils-pour-cold-water-on-candle-experiment-1156584.html