What's the difference between a flash point and autoignition?

One is specific (ignition)and one is generic (flash point)to the subject’s nature.That’s my guess for the day-don’t sue me ;)

From Wikipedia :

“The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid’s flash point requires an ignition source. This is not to be confused with the autoignition temperature, which requires no ignition source.”

@Harp so am I missing a point of reaction or is autoignition when it has ignited by another force? e.g. Flash point > ignition? or flash point > autoignition? Or flash point > a moment when the vapor is volatile in limbo when something can ignite it > autoignition = when it reach a sort of critical mass & ignites all on it’s own?

@Axemusica A certain minimum amount of energy (the “activation energy”) is required to enable the chemical reaction between the substance being burned and the oxygen in the atmosphere. This is why alcohol won’t burn at room temperature without a source of ignition.

The flash point of ethyl alcohol is 55.4 degrees F. So at room temperature, a match held just above the alcohol would provide enough activation energy to cause the vapors to ignite. If the alcohol were straight out of the fridge, however, it wouldn’t give off enough vapor to be ignited even by the match.

Another way to supply the activation energy would be to just heat the ethyl alcohol until it caught fire, say in an oven without any flame or other source of intense heat. You’d have to bring it up to its autoignition temperature for this to happen. In the case of ethyl alcohol, that’s 685 degrees F.

Flash point is a specific test, such as described in the APA Methods manual of the temperaure at which a lquid fuel will give off vapors ignitable by an open flame, Autoignition is related only to heat and is related to spontaneous combustion (oily rags, etc.).