Candle and rising water

My son saw a tea light and asked if we could do the experiment where we extinguished a candle with a jam jar again.  It's six months since we last did it, so I was impressed he could remember, and agreed to do it when his brother was napping (as a toddler who can reach the table, a lit candle and a jam jar are not a good combination!).  We had a go when I was able to supervise properly, and I suggested we try a slightly different version with some water.

Lit tea light floating on coloured water

He was intrigued, and let me put some water, and a drop of food colouring to make it more visible (this isn't essential if you haven't got it), in a pyrex bowl.  You don't need a great depth of water - we had a couple of centimetres. I floated the tea light candle on the water and lit it.  My son watched it floating around, and I asked him what he thought would happen if he put the jar over the candle this time.

Moving away after he'd placed the jar over the candle

He correctly predicted that the flame would go out because the oxygen would run out and the fire needs oxygen to burn.  I asked him what would happen to the water, and he wasn't sure.  So we had a go, and he carefully placed the jam jar over the floating candle.

Condensation and the extinguished candle

The flame did indeed go out, but before that we saw a few bubbles escaping under the jar.  When the candle stopped burning, condensation appeared inside the jar.  My son was surprised to see the water level inside the jar rise very quickly!  We had a look from the side and he could clearly see the water level in the jar was almost twice the depth of the water level in the dish outside it.

Side view shows the water level rise

Why does this happen?  It's a combination of effects.  The first is that the escaping bubbles were caused by the gases inside the jar expanding due to heat from the candle burning, and when the flame goes out, the gases inside the jar cool and contract, creating a vacuum which pulls the water up inside the jar.  The chemistry of burning wax from the candle involves the conversion of oxygen plus the hydrocarbons in wax to produce carbon dioxide and water.  When it's above 100 degrees Celsius, the water is a gas (water vapour), and we saw this condense back to liquid water on the glass jar when the contents of the jar cooled - this removes some of the gas, and also contributes to the vacuum.  The other effect is the reaction chemistry results in half as many molecules of carbon dioxide as oxygen, reducing the overall gas volume.  For my son, I just explained that the air inside the jar was hot when the candle was burning, and that it cooled down - he knows that hot air expands and rises (e.g. in a hot air balloon) and we had seen the escaping bubbles, so this satisfied his curiosity for now!

We repeated it a couple of times as my son likes to see that the same thing happens each time when we try something new.  My husband was treated to a re-run at the dinner table as it was clearly the highlight of my son's day!  It was a nice quick experiment to set up and maybe he will ask to do this one again unexpectedly sometime too.