Most people think that when fuels like wood, coal or some oils burn, they disappear during the process. The same is the thought about a burning candle. The truth is that these substances do not disappear, they just change their form. Combustion or burning is a chemical reaction that gives off light and heat and takes place in the presence of oxygen. During this process, the matter is neither created nor destroyed but simply changes its form. This is known as the ‘law of indestructibility of matter’. When a candle burns, the wax simply turns into other substances. The moment we light the wick, the wax starts melting because of the heat. The molten wax rises to the wick on account of the phenomenon called ‘surface tension’. Surface tension is a property of the liquids. Due to this very property, a blotting paper soaks up the ink. The liquid wax then changes into gas. This gas undergoes combustion, in which heat and light are produced. Wax is a complex hydrocarbon that contains hydrogen and carbon. During the burning process, carbon combines with oxygen in the air and forms carbon dioxide gas. Hydrogen of the wax combines with oxygen of the air and forms water vapour. In these two reactions wax is consumed, and as a result, the size of the candle goes on decreasing. If we collect and weigh the water vapour and carbon dioxide formed during this process, their weight would be more than that of the original candle.
The increase in weight is due to the combination of oxygen with carbon and hydrogen. Thus, we see that wax is not destroyed when the candle burns, it only changes into carbon dioxide and water vapour.