Experiment to show that air contains about 20% oxygen

I remember that in my primary classed I had an experiment in science to show air contains 20% oxygen. The experiment involved taking a trough of water, lighting a candle in the middle of the trough and inverting a glass jar over the candle so that air supply for the candle is cut of. When candle extinguishes, water level in the glass jar rises. I have seen this happen.

My question is why should the water raise in the jar, since burning of candle should produce an equivalent amount of carbon dioxide, so the molar amount of gas in jar is constant? Also the heat from the candle should expand the gasses in the jar, water level should go down I think.

Answer

The paraffin wax is an alkane ($\ce{C_nH_{2n+2}}$); its combustion reaction is (complete combustion):

$$\ce{2C_nH_{2n+2} + $(3n+1)$ O2 -> $2(n+1)$ H2O + $2n$ CO2}$$ or (incomplete combustion): $$\ce{2C_nH_{2n+2} + $(2n+1)$ O2 -> $2(n+1)$ H2O + $2n$ CO}$$ In the two cases the quantity (in moles) of $\ce{O2}$ is greater than the quantity of $\ce{CO2}$ or $\ce{CO}$.

Moreover the gaseous $\ce{H2O}$ becomes liquid.