Flammable
Definition and meaning of Flammable in chemistry.
Flammable describes a substance that ignites easily and burns readily in air at normal working temperatures. This property makes the substance a fire hazard that requires special handling and storage. Both liquids and gases, as well as some finely divided solids, can be classified as flammable.
In more detail
Flammability in liquids is governed mainly by flash point, the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air. Under United States standards such as NFPA 30 and Department of Transportation rules, a liquid counts as flammable if its flash point is below 37.8 degrees Celsius, or 100 degrees Fahrenheit.
Liquids with higher flash points, up to about 93 degrees Celsius, are labeled combustible instead. The Globally Harmonized System, which OSHA's current Hazard Communication Standard follows, works a bit differently. It does not use a separate combustible category at all.
Instead, it sorts flammable liquids into four numbered categories, covering flash points up to 93 degrees Celsius. Gases and finely divided solids, like flour dust or metal powders, can also be flammable if they ignite readily and support fast combustion once a spark is present. This classification system drives safety labeling, storage rules, and shipping regulations around the world.
Key facts
| Field | General Chemistry |
|---|---|
| NFPA/DOT flammable-liquid threshold | Flash point below 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 |
| UN transport hazard class | Class 3 (Flammable liquids) |
| Contrasting term (NFPA/DOT) | Combustible (flash point 37.8°C up to about 93°C) |
Gasoline is flammable because its flash point is about -43 degrees Celsius. It releases ignitable vapor even in cold weather, so a small spark can set it alight at room temperature. This is why gas stations post no-smoking signs and gasoline containers carry flame warning labels. By comparison, vegetable oil has a flash point above 300 degrees Celsius, so it is far harder to ignite under normal conditions. Acetone, commonly found in nail polish remover, has a flash point around -20 degrees Celsius, making it flammable as well.
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 rules classify liquids by flash point. Flammable liquids ignite below 37.8°C, while combustible liquids have higher flash points, up to about 93°C. The GHS system that OSHA follows takes a different approach and groups these liquids into four numbered flammable categories instead.
Why does flash point matter for safety?
Flash point tells workers the lowest temperature at which a liquid can ignite from a spark or flame. That value determines how the liquid must be stored, labeled, and transported.