The seemingly well-understood candle experiment demonstrates how young and inquisitive minds are able to demolish false explanations propagated through school physics over the decades.
The consumption of oxygen may well contribute to the rising water level to a certain extent, because a given mole volume of oxygen will burn the wax’s carbon into roughly the same mole volume of carbon dioxide and the hydrogen into two mole volumes of water vapour respectively.
The former will partly dissolve into water, the latter will almost completely condense into liquid water. This will certainly lead to a net decrease in gaseous volume.
However, this is a minor consideration: the important influence is the heat created by the burning candle.
By the time you cover the candle with an upturned glass, an increased number of candles will have increased the air temperature around themselves more than a single candle would.
As soon as the candles go out, the surrounding air contracts as it cools and the ratio of contraction is directly proportional to the initial average temperature of the air volume under the glass. So more candles lead to more heat, a higher temperature and a higher water level upon cooling down.
All this tells us that we should never believe science teachers without asking a few pertinent questions first.
Congratulations to the children who experimentally disproved the common textbook misconception about the candle, the upturned jam jar, the dish of water and the alleged removal of all oxygen from the jar.
By observing that four burning candles cause the water to rise significantly higher up the jar, they have shown that the principal cause of this effect is the heat from the candles, causing the air in the jar to expand.
They will also have noticed that the expanded air makes a glug glug sound as it escapes around the rim.
There is a short pause after the candles go out, and only then does the water level rise as the remaining air cools and contracts again.
A candle flame goes out after only a small percentage of the available oxygen has been used up. So it is wrong to claim that this experiment demonstrates the proportion of oxygen in the air in some quantitative way.
The effect is partly caused by the thickness of the three extra candles. You get the same effect using single candles of different thickness. The thicker the candle, the higher the water will rise.
The water drawn in is squashed into the space between the candles and the glass.
The narrower this space, the higher the water will rise.