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What controls the rate at which candles burn?

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Introduction

What controls the rate at which candles burn? A candle burns by operating as a self- sustaining capillary-action wax pump. The wax itself will burn only as a vapour, not as a solid or a liquid. After the flame is established on the wick, the heat of the flame vaporises the liquid wax in the pores of the wick, as well as liquefying part of the solid wax of the candle below. The vaporised wax then acts as a protective barrier around the wick, with the vapours diffusing out through the flame while the oxygen diffuses in. This is why the wick does not burn up straight away. This also explains why candle flames have different colours in them. At the base they are clear or blue as the wax vapour burns to completion, while further inside the flame, where oxygen is scarce, the reaction does not go to completion, resulting in hot carbon atoms which glow orange like natural flames. ...read more.

Middle

For example, fire will not occur in an environment of 100 per cent methane, but a mixture of one part methane to one part oxygen will burn quite readily. When a candle is burning, it is not the solid wax that is burning, but the wax that is vaporising from the heat of the flame. These vapours are burnt off in the flame, so more heat is generated and more wax is vaporised. To answer the question, when there is too much wind the flame may be extinguished, but it may reignite itself depending on a couple of factors. The first is how hot the wick of the candle is, because this provides the ignition source. The second is how much vapour is present around the ignition source the fuel-to-oxygen ratio because when the flame goes out, parts of the candle are still vaporising. If the oxygen and the temperature of the ignition source are at the required levels, the candle will reignite, and because there is a higher concentration of wax vapour than in a normal burning candle, it will cause the "pop" you hear. ...read more.

Conclusion

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(s). By the time you cover the candle(s) 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 candle(s) go(es) 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. ...read more.

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