chemistry question #3724



Tridib Das, a 16 year old male from Guwahati, Assam, India asks on November 30, 2006,

Q:

Just like 1 mole of water contains Avogadros number of molecules and has the same mass as that of molecular mass of water i.e.,18 g.m.m, then can fire also be measured in terms of mole and determine its molecular mass and number of molecules? Again why is it that water can extinguish fire but kerosene, petrol or diesel cannot? Does it have anything to do with the composition of fire?

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the answer

Barry Shell answered on November 30, 2006, A:

Fire is fundamentally different from water. Water is a relatively simple chemical compound consisting of two atoms of hydrogen and one of oxygen. Fire is not a "thing", not one single compound but the phenomenon we observe when oxygen reacts with burnable gasses. The gasses can be all sorts of things, but usually are methane, carbon monoxide hydrogen and other hydrocarbons. We have answered the question, What is fire? elsewhere at science.ca

Yes you can measure the products of combustion from fire by all sorts of techniques. This was done in the 1700s, and was one of the ways that early scientists discovered oxygen. Water can extinguish fire because it does not react with oxygen. The other liquids you mention can react violently with oxygen present in air and hence they burn. To answer your last question, fire is based on the reaction of oxygen with things. You cannot have fire without oxygen (with a few rare notable exceptions such as gunpowder and rocket fuels which have other oxydizers built in). Think of fire as the oxygen reaction. Fire is oxygen in air reacting with whatever is burning. The reaction is basically the hydrocarbon such as petrol or kerosene combining with oxygen in air to form water and carbon dioxide. There are other things formed, but those are the main two.

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