Candle in a bell jar with water...?

Question

This question has been asked before, but there were no definitive answers.

For those of you who have not heard about the experiment, a candle is lit and placed in a water-filled dish. A glass jar is then place over the candle and left. After a few seconds the flame goes out and water is 'sucked' into the jar. The commonly heard explanation is that the candle burns the oxygen that makes up ~20% of the air we breathe creating a vacuum that sucks up the water.

However, this does not seem to be a good explanation for me for the following reasons.

1. The candle is burning oxygen at a fairly constant rate, but the water is only sucked up AFTER the candle is extinguished.

2. Carbon dioxide is a product of the reaction, and is also a gas. Surely it takes up the same volume as the 'burned' oxygen.

So can anyone offer me a good explanation for this phenomenon?

Answer 1

The candle is burning which is using oxygen from the air. Th eoxidation of fuel and air produces CO2 (Carbon Dioxide) which ocurrs when there is complete combustion and CO (Carbon Monoxide) which is produced during incomplete combustion (such as when there is low oxygen content)

Also the buring produces heat and heat causes expansion.

So the rapid effect you see when the candel goes out has to do with the cooling and contraction of air and the increase in CO vs CO2 production. I submitt that the expansion of air due to heat in the begining of the experiment is counter acting the partial pressure of oxygen reduction during the CO2 production phase. And that the sudden rise has to do with the sudden contraction of air as it cools when teh candle extinguishes. Also CO production increases at this time as well.

(The CO2 and the CO do not take up the same volume. They have a lower partial pressure than O2. So compared to the normal atmosphere outside at 14.7 psi , the reduced partial pressure from the reductio of free O2 converted into CO and CO2 at lower pressures causes a net reduction in total pressure within the jar causing the atmosphere to press on the water raising its level until the pressure equalizes)

You can repeat this experiment slowly using the rusting of steel wool to absorb oxygen and there by cause the water level to rise. Compare the water levels between the two menthods....