Which property measures the potential for ignition in flammable materials?

The property that measures the potential for ignition in flammable materials is the flash point. The flash point is defined as the lowest temperature at which the vapor of a combustible liquid can ignite when exposed to an open flame or spark. Understanding the flash point is critical for handling and storing flammable materials because it provides important information about the conditions under which a substance could ignite and cause a fire.

High flash point materials are generally safer to handle because they require higher temperatures to ignite, while low flash point materials pose a greater risk of ignition under normal working conditions. This property helps in assessing risks and ensuring proper safety measures are implemented in environments where flammable substances are present.

The other options relate to different properties of materials but do not measure the potential for ignition. Vapor density indicates whether a vapor will rise or fall in air, the boiling point defines the temperature at which a substance changes from liquid to gas, and the expansion ratio pertains to the volume increase of a substance when it changes state, typically from liquid to gas. However, none of these properties specifically indicate the ignition potential of flammable materials like the flash point does.