Alex Levens, of Green Island Primary School, asks :-

How does fire start?

David McMorran, a chemist at Otago University, responded.

For almost as long as people have enjoyed sitting beside the fire on a winter's evening, they have realised that, when wood burns, it changes. Eventually scientists came to recognise that this change is a chemical reaction, which they called combustion. Combustion is an example of an exothermic reaction, which just means a reaction that makes heat when it happens (so sitting beside the fire makes us warm). In a combustion reaction, the chemical entities in the wood combine with oxygen molecules in the air. But trees don't react with the oxygen in the air normally, so something must happen in order to start the combustion reaction going.

You may have heard of the fire triangle. This shows that three things are needed to start a fire: oxygen, fuel (the wood) and heat. Trees don't just burn normally because it is not hot enough. The heat provides what chemists call the activation energy for the combustion reaction. If you have ever tried to push start a car you will know that it takes a lot of energy to get the car moving but then not so much energy to keep it moving. In the same way, the heat provides the initial 'push' to get the combustion reaction going.

So where does the heat come from? Well there are two ways that you normally light a fire. You can use a lighter, in which case the heat comes from the spark. Sparks, like lightning, are very hot - as much as 54000 degrees Celsius. The heat from the spark causes the fuel in the lighter to burn and the heat from this can then light your paper or wood. Or you can use a match. Matches contain a very interesting chemical element called phosphorus. Its combustion reaction has a very low activation energy, so low in fact that normal temperatures are hot enough to start the reaction. So when you strike a match, the phosphorus immediately starts burning and this, in turn, provides the heat needed to start the wood burning. Matches have been around since about 1830 and have a fascinating history which you can read about in John Emsley's book, "The Shocking History of Phosphorus".