Which term indicates how easily a substance will evaporate into air?

The term that indicates how easily a substance will evaporate into air is vapor pressure. Vapor pressure is a measure of the tendency of a substance to become a gas at a given temperature. It reflects the amount of molecules that escape from the liquid phase into the vapor phase. The higher the vapor pressure of a substance, the more readily it will evaporate; this is crucial for understanding how flammable materials behave under different conditions.

Vapor density refers to the mass of a vapor compared to the mass of air at the same volume, which helps determine how heavy the vapor is in comparison to air. Specific gravity is a ratio that compares the density of a substance to the density of water and is not directly related to evaporation tendencies. The flash point is the lowest temperature at which a liquid can form an ignitable mixture in air, which relates more to the ignition potential of the substance rather than its evaporation rate. Thus, vapor pressure is the appropriate term connected to the ease with which a substance transitions into the vapor phase.