Combustible
Definition and meaning of Combustible in chemistry.
Combustible describes a substance that can catch fire and burn in air or another oxidizer. Burning releases heat and usually light. This process is called combustion, and it is a fast, energy releasing chemical reaction.
In more detail
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.
Key facts
| Field | General Chemistry |
|---|---|
| Fire code threshold | Flash point at or above 100 degrees Fahrenheit (37.8 degrees Celsius), per NFPA 30 |
| Reaction type | Exothermic oxidation reaction |
| Typical products | CO2, H2O, heat, and light |
| Compared to | Flammable substances, which ignite at lower temperatures |
Wood is combustible. When heated to its ignition temperature in air, the cellulose in the wood reacts quickly with oxygen. This produces carbon dioxide, water vapor, ash, and a strong release of heat and light that we see as flame.
Frequently asked questions
What is the difference between combustible and flammable?
Both can burn, but flammable substances ignite easily near room temperature, with a flash point below 100 degrees Fahrenheit. Combustible substances need higher temperatures to release enough vapor to catch fire, making them somewhat safer to handle.
Does combustion always need oxygen gas?
Most everyday combustion uses oxygen from the air. The same type of reaction can also happen with other oxidizers, like chlorine, fluorine, or compounds that release oxygen, such as nitrates.
What is the flash point of a substance?
The flash point is the lowest temperature at which a liquid gives off enough vapor to ignite when exposed to a flame or spark. It is a key number used to classify fire hazards.