In the last section, we saw that fire is the result of a chemical reaction between two gases, typically oxygen and a fuel gas. The fuel gas is created by heat. In other words, with heat providing the necessary energy, atoms in one gaseous compound break their bonds with each other and recombine with available oxygen atoms in the air to form new compounds plus lots more heat.
Only some compounds will readily break apart and recombine in this way -- the various atoms have to be attracted to each other in the right manner. For example, when you boil water, it takes the gaseous form of steam, but this gas doesn't react with oxygen in the air. There isn't a strong enough attraction between the two hydrogen atoms and one oxygen atom in a water molecule and the two oxygen atoms in an oxygen molecule, so the water compound doesn't break apart and recombine.
The most flammable compounds contain carbon and hydrogen, which recombine with oxygen relatively easily to form carbon dioxide, water and other gases.
Different flammable fuels catch fire at different temperatures. It takes a certain amount of heat energy to change any particular material into a gas, and even more heat energy to trigger the reaction with oxygen. The necessary heat level varies depending on the nature of the molecules that make up the fuel. A fuel's piloted ignition temperature is the heat level required to form a gas that will ignite when exposed to a spark. At the unpiloted ignition temperature, which is much higher, the fuel ignites without a spark.
The fuel's size also affects how easily it will catch fire. A larger fuel, such as a thick tree, can absorb a lot of heat, so it takes a lot more energy to raise any particular piece to the ignition temperature. A toothpick catches fire more easily because it heats up very quickly.
A fuel's heat production depends on how much energy the gases release in the combustion reaction and how quickly the fuel burns. Both factors largely depend on the fuel's composition. Some compounds react with oxygen in such a way that there is a lot of "extra heat energy" left over. Others emit a smaller amount of energy. Similarly, the fuel's reaction with oxygen may happen very quickly, or it may happen more slowly.
The fuel's shape also affects burning speed. Thin pieces of fuel burn more quickly than larger pieces because a larger proportion of their mass is exposed to oxygen at any moment. For example, you could burn up a pile of wood splinters or paper much more quickly than you could a block of wood with the same mass, because splinters and paper have a much greater surface area.
In this way, fires from different fuels are like different species of animal -- they all behave a little differently. Experts can often figure out how a fire started by observing how it affected the surrounding areas. A fire from a fast-burning fuel that produces a lot of heat will inflict a different sort of damage than a slow-burning, low-heat fire.
For much more information on the science of fire, check out the links that follow.