Flammability and vapour pressure
Flammability is a measure of how easy it would be for a substance to catch alight and burn. The flash point of a substance is the lowest temperature that is likely to form a gaseous mixture you could set alight. If a liquid has a low enough flash point it is considered flammable (able to be ignited easily) while those with higher flash points are considered nonflammable. A substance that is classified as nonflammable can still be forced to burn, but it will not ignite easily.
When a substance is in the liquid or solid state there will be some molecules in the gas state. These molecules have enough energy to overcome the intermolecular forces holding the majority of the substance in the liquid or solid phase. These gas molecules exert a pressure on the liquid or solid (and the container) and that pressure is the vapour pressure of that compound. The weaker the intermolecular forces within a substance the higher the vapour pressure will be. Compounds with higher vapour pressures have lower flash points and are therefore more flammable.
Definition: Vapour pressure
The pressure exerted (at a specific temperature) on a solid or liquid compound by molecules of that compound that are in the gas phase.
Main intermolecular forces
Vapour pressure (kPa at 20℃)
Flash point (℃)
Table: Relationship between intermolecular forces and the flammability of a substance.
As the intermolecular forces increase (from top to bottom in the table above) you can see a decrease in the vapour pressure. This corresponds with an increase in the flash point temperature and a decrease in the flammability of the substance. The figure below shows a few examples.