Whether something is combustible depends on its chemical makeup and on two key temperatures. The ignition temperature is how hot the substance must get to start burning. The flash point is the lowest temperature at which enough vapor forms above the liquid to catch fire.
Fire safety codes use these temperatures to classify chemicals. Under the NFPA 30 code, a combustible liquid has a flash point at or above 100 degrees Fahrenheit. That equals 37.8 degrees Celsius.
This is different from a flammable liquid, which ignites at lower temperatures and is considered more dangerous to handle. Combustion itself is a rapid oxidation reaction. Fuel reacts with an oxidizer, most often oxygen from the air, and chemical bonds break and reform.
This releases energy as heat and light. The main products are usually carbon dioxide and water vapor, along with leftover heat. Combustion can also happen with other oxidizers besides oxygen, such as chlorine or fluorine.
Firefighters use these flash point classifications to decide how to store and label chemicals safely. A material’s combustibility can also depend on how finely it is divided. Fine dust burns far more easily than a solid block of the same substance. Grain dust and sawdust, for example, can explode if they ignite while suspended in air.