Which term is used to indicate how volatile a substance is relative to its temperature and pressure conditions, typically referred to as the pressure of the vapor?

• A. Molecular Weight
• B. Specific Gravity
• C. Boiling Point
• D. Vapor Pressure

Answer: (D)

Volatility describes how readily a substance forms vapor under a given temperature and pressure. The measurement that captures this tendency is vapor pressure—the pressure exerted by the vapor when it’s in equilibrium with the liquid at that temperature. A higher vapor pressure at a given temperature means the substance is more volatile because its molecules escape into the gas phase more easily. Vapor pressure increases with temperature, which is why substances with high vapor pressures pose greater vapor hazards in warmer conditions. This differs from molecular weight or specific gravity, which don’t directly quantify how easily a substance becomes vapor, and from boiling point, which is the temperature at which vapor pressure equals the surrounding pressure. Boiling point is related but does not itself express volatility at a fixed condition. The term that best fits the description is vapor pressure.