What two physical characteristics of a liquid are primarily used to determine its flammability classification?

The two physical characteristics of a liquid that are primarily used to determine its flammability classification are the flash point and boiling point.

The flash point is the lowest temperature at which a liquid can produce enough vapor to form a combustible mixture with air. This measurement is crucial as it indicates at which temperature a liquid has the potential to ignite when exposed to an open flame or spark. A lower flash point typically signifies a higher risk of fire hazard.

The boiling point, on the other hand, is the temperature at which a liquid turns to vapor. This parameter influences how quickly and easily a liquid can produce flammable vapors, further contributing to its overall flammability risk.

Together, these two properties provide significant information about a liquid's behavior when exposed to heat or open flames, allowing for appropriate classification and safety measures to be implemented in various workplace environments. The other characteristics listed, such as vapor density, viscosity, or color and smell, do not directly relate to the primary indicators of flammability in terms of fire risk assessment.