What is Fire?

Now that you know a little bit about molecules and plasmas, you are ready to try to understand what fire is. You will remember that when oxygen and hydrogen are burned, they turn into water and release a lot of energy in the form of heat. That is a fire: these high-energy molecules are broken apart and their atoms are recombined to form different kinds of molecules that store less energy in their bonds, thus allowing the energy left over to escape as heat and light. Some of the light that comes off a fire is invisible, but you can feel that this light is hot if you get close enough to the fire, and indeed that is how you can feel the heat of the sun, because it too comes here mainly as invisible light (called infrared light). There is also some heat in visible light.
Whatever material you burn, the same thing happens: you convert that material into other kinds of stuff which carries less energy, and the energy left over is released as heat and light. Once you have burned it, the stuff you are left with (ash) will not be able to burn so easily again because it has little energy left in it and there is therefore not a lot left to release in another fire. So, ash is a low-energy material because it has already lost its energy by being burned, but it can be used to fertilise soil, its molecules then being taken into plants to help them grow. Plants capture energy from sunlight and use that energy to break down low-energy molecules and recombine their atoms into high-energy material such as wood, so now the ash has been converted back into a high-energy wood and can potentially be burned all over again.
Food is high-energy material, and we break it up into molecules in our stomachs and then "burn" it in our muscles and brain: it is our fuel. This burning happens without fire, but heat is still released in the process and low-energy waste products are produced by the burning process: these waste products are then disposed of in urine (pee) - don't confuse this with the other waste products which are made from the parts of food which are too low in energy to be worth trying to use as fuel in the first place (poo). When things burn, the energy can be released as light, heat and movement. An explosion generates a lot of movement, and when we burn fuel in our muscles we use the energy released to make us move. It is not clear at what point we should decide that a burning process should be called fire, but perhaps visible light has to be produced if it is to count as fire. Flames are not essential for something to count as a fire, but they are interesting too, so let's think about them for a moment. If you look at a lit candle, the heat turns solid wax into a liquid, and then it turns the liquid into a gas. No chemical change has happened to the wax yet, but when it gets even hotter, it becomes a plasma and the molecules fall apart into individual atoms. Energy is released by this as the bonds between atoms break. The atoms find their way out of the flame and instantly try to link up with other loose atoms around them, but their electrons are now carrying less energy and they have to be more fussy about which other atoms they will join up with, each looking to make low-energy bonds. The result is that they produce low-energy molecules such as water, carbon dioxide, carbon monoxide, and soot.
Interestingly, the sun is not on fire. It isn't burning either, though often we talk about it as if it is. Even so, it is doing a very similar thing, because lots of energy is released in the form of heat and light. With normal burning, you break apart molecules and recombine their atoms into new molecules which carry less energy in their bonds. What happens in the sun is different because it doesn't work with molecules: it simply works with atoms on their own, making helium atoms out of hydrogen atoms. Because helium atoms are able to hold themselves together using less energy than the four separate hydrogen atoms from which they are built, there is some energy left over which is released as heat. This is a nuclear process rather than normal burning. Some really heavy atoms (ones with masses of protons and neutrons inside them) can be split apart into smaller atoms which collectively store less energy than the original heavy atoms from which they were made, so again there is some energy left over which can be released as heat: this is how energy is produced in nuclear power stations. Again it is not normal burning, because the energy comes from inside atoms rather than from splitting up high-energy molecules and rearranging their atoms into low-energy molecules.

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