Flash point

The flash point is the lowest temperature at which vapours of a volatile material will ignite, when given an ignition source.

The flash point may sometimes be confused with the autoignition temperature, which is the temperature at which the vapor ignites spontaneously without an ignition source. The fire point is the lowest temperature at which the vapor will keep burning after being ignited and the ignition source removed. The fire point is higher than the flash point, because at the flash point the vapor may be reliably expected to cease burning when the ignition source is removed. Neither flash point nor fire point depends directly on the ignition source temperature, but it may be understood that ignition source temperature will be considerably higher than either the flash or fire point.

The flash point is a descriptive characteristic that is used to distinguish between flammable liquids, such as petrol, and combustible liquids, such as diesel.

It is also used to characterize the fire hazards of liquids. Depending on the standard used, liquids which have a flash point less than either 37.8 or 60.5 °C (100.0 or 140.9 °F) are called flammable — whereas liquids having a flash point above that temperature are called combustible.

All liquids have a specific vapor pressure, which is a function of that liquid's temperature and is subject to Boyle's Law. As temperature increases, vapor pressure increases. As vapor pressure increases, the concentration of vapor of a flammable or combustible liquid in the air increases. Hence, temperature determines the concentration of vapor of the flammable liquid in the air. A certain concentration of a flammable or combustible vapor is necessary to sustain combustion in air, the lower flammable limit, and that concentration is different and is specific to each flammable or combustible liquid. The flash point is the lowest temperature at which there will be enough flammable vapor to induce ignition when an ignition source is applied.

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