Tuesday, December 12

Why is Fire Hot?

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Two years ago I renovated an old farmhouse in France. One the first things we had fitted was a beautiful, cast iron wood burner in the lounge. Upon lighting the fire, I told my little boys to keep their distance for obvious reasons. My eldest son asked “Dad, why is fire hot?”, a question I didn’t know the answer to. So I decided to do my research (www.butwhydad.co.uk) …

Fire is a chemical process involving the rearrangement of molecules. You may or may not remember from your schooldays that molecules comprise several atoms bonded together. Whenever molecules rearrange their atoms during such a chemical change, energy is either released or absorbed. With fire, the release of energy takes the form of heat (and light), which is basically why fire is hot. But let’s look at things in a little more detail…

Three things are needed to trigger the aforementioned chemical process and start a fire – oxygen, fuel and heat/an ignition source (e.g. a match, focused sunlight, etc). The chemical process is called oxidation. To give a familiar example of oxidation, when iron rusts, oxygen in the air combines and reacts with iron atoms (without having the scope to go into too much detail, electrons basically pass from the iron to the oxygen). This chemical reaction between the oxygen and iron releases energy. In the case of corroding iron, the oxidation rate and release of energy is extremely slow and so the rusting area increases only very slightly in temperature.  But in the case of fire, the oxidation rate (which in the case of a wood fire for example occurs between oxygen, hydrogen and carbon atoms) and resulting release of energy is very fast and thus a lot more heat is produced.

Of course, a pile of wood won’t just catch fire because it is surrounded by oxygen! For the combustion (i.e. burning) to take place you need the third element mentioned above – heat – to make the fuel reach its ‘ignition temperature’. If heat cannot be released faster than it is created then combustion happens. So, for example, sliding a match fast enough across a coarse surface will generate sufficient heat (due to friction) to produce a temperature which is at least as high as the chemicals within the match head’s relatively low ignition temperature, and it will ignite. The resulting combustion provides even more heat and the match will often continue to burn until the fuel (stick) runs out.

So, once something is ignited, enough heat needs to be maintained/produced to keep the fuel and oxygen at or above its ignition temperature and keep the fire going. It is a chain reaction of sorts in that the fire must sustain its own heat and have an ongoing supply of oxygen and fuel to keep going.

Phew, that’s a lot to try to take in! But in summary, fire is a chemical process called oxidation and requires oxygen, fuel and a heat source. During this process, the chemical bonds in oxygen and fuel are rapidly broken and new bonds are formed. Although some energy is used up when chemical bonds are broken, more is released during the creation of the new ones. That extra energy is released as heat, which is why fire is hot.

If you’re interesting in seeing answers to other commonly asked questions please feel free to visit my little website: http://www.butwhydad.co.uk

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