Combustible
Definition and meaning of Combustible in chemistry.
Combustible describes a substance that can ignite and burn in air (or another oxidizer), releasing heat and usually light through an exothermic combustion reaction.
In more detail
Combustibility depends on a material's chemical structure and its ignition temperature and flash point (the lowest temperature at which enough vapor forms to ignite). In fire safety codes such as NFPA 30, "combustible" liquids are those with a flash point at or above 100°F (37.8°C), distinguishing them from more hazardous "flammable" liquids that ignite at lower temperatures. Combustion itself is a rapid oxidation reaction: fuel reacts with an oxidizer, typically atmospheric oxygen, breaking and reforming chemical bonds to release energy, most often as carbon dioxide, water vapor, heat, and light.
Key facts
| Field | General Chemistry |
|---|---|
| Fire-code threshold | Flash point ≥ 100°F (37.8°C), per NFPA 30 |
| Reaction type | Exothermic oxidation (redox) |
| Typical products | CO2, H2O, heat, and light |
Wood is combustible: when heated to its ignition temperature in air, its cellulose undergoes rapid oxidation, producing carbon dioxide, water vapor, ash, and a substantial release of heat and light (flame).
Frequently asked questions
What is the difference between combustible and flammable?
Both burn, but flammable substances ignite easily at or near room temperature (flash point below 100°F/37.8°C by NFPA classification), while combustible substances need higher temperatures to produce ignitable vapor, making them somewhat less hazardous to handle.
Does combustion always need oxygen gas?
Most everyday combustion uses atmospheric oxygen as the oxidizer, but the same type of exothermic reaction can occur with other oxidizers, such as chlorine or fluorine, or with oxygen-donating compounds like nitrates.