Is fire a solid, liquid or a gas?

Asked by Daniel Price

It wouldn’t be correct to say fire is a gas, but it is mainly made of a mixture of bright hot gases. There’s obviously other stuff to be found there, such as soot. Let’s imagine igniting a pool of petrol. The flame is always above the surface. Some flammable gas is constantly evaporating from the liquid. An initial heat source such as a match provides the energy needed to start the reaction with the oxygen in the air. The heat generated will supply the energy to keep it going and evaporate even more petrol from the liquid. That is the job of the wick in a candle: as the wax melts, it is absorbed by the wick and turns into a gas with the high temperatures.

If we describe the way a flame behaves, we obviously have to say it does so as a gas. Liquids have fixed volume and because molecules in liquids stick together, they are always quite dense compared with the gases in the air. Simply speaking, liquids fall down to the ground as soon as the drops are big enough.

José Monteiro, Science Museum