Flammable
Definition and meaning of Flammable in chemistry.
Flammable describes a substance that ignites easily and burns readily in air at ordinary working temperatures, posing a fire hazard.
In more detail
Flammability is governed largely by flash point, the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air. Under U.S. standards such as NFPA 30 and Department of Transportation regulations, a liquid is classified as "flammable" if its flash point is below 37.8°C (100°F); liquids with higher flash points (up to about 93°C) are labeled "combustible" instead. The Globally Harmonized System (GHS), which OSHA's current Hazard Communication Standard follows, does not use a separate "combustible" category, it instead defines flammable liquids across four numbered categories spanning flash points up to 93°C (199°F). Gases and finely divided solids can also be flammable if they ignite readily and support rapid combustion once a spark or flame is present. This classification drives safety labeling, storage requirements, and shipping regulations.
Key facts
| Field | General Chemistry |
|---|---|
| NFPA/DOT flammable-liquid threshold | Flash point < 37.8 °C (100 °F) |
| GHS/OSHA flammable liquid categories | 4 categories, flash points up to 93 °C (199 °F) |
| GHS hazard pictogram | Flame symbol (GHS02) |
| UN transport hazard class | Class 3 (Flammable liquids) |
| Contrasting term (NFPA/DOT) | Combustible (flash point ≥ 37.8 °C, up to ~93 °C) |
Gasoline is flammable because its flash point is about -43°C, so it releases ignitable vapor even in cold weather, allowing a small spark to set it alight at room temperature.
Frequently asked questions
Is there a difference between 'flammable' and 'inflammable'?
No, both words mean the same thing: a substance that burns easily. 'Inflammable' comes from the Latin prefix in- meaning 'into' (as in 'inflame'), not the negating prefix 'in-'.
How does flammable differ from combustible?
In the U.S., NFPA 30 and DOT regulations classify liquids by flash point: 'flammable' liquids ignite below 37.8°C (100°F), while 'combustible' liquids have higher flash points, up to about 93°C (200°F). The GHS system that OSHA's Hazard Communication Standard follows takes a different approach: it groups all such liquids into four numbered categories of 'flammable liquids' (flash points up to 93°C) and does not use the term 'combustible' at all.