The basis for classifying a chemical's flammability is its _______.

The classification of a chemical's flammability primarily hinges on its flashpoint. The flashpoint is the lowest temperature at which the vapor of a volatile substance can ignite in air when an ignition source is present. This temperature indicates the point at which there are enough vapors to form a combustible mixture with air, which is critical for assessing the potential for a fire hazard.

Understanding the flashpoint is vital for safety in environments where chemicals are handled, stored, or used, as it helps determine appropriate safety measures such as storage conditions, handling procedures, and necessary firefighting equipment. Lower flashpoints indicate higher flammability risks and dictate more stringent safety protocols.

The other options, while related to combustion, do not serve as the primary basis for flammability classification. The explosion limit pertains to the concentration range of a vapor in air that can lead to an explosion, while boiling point and ignition temperature relate to different aspects of the thermal behavior of substances, but do not directly reflect their flammability in the same way that flashpoint does.