What is the measure of a chemical's volatility in its pure state?

• A. Vapor pressure
• B. Water solubility
• C. Boiling point
• D. Henry's constant

Answer: (A)

Volatility in the pure state is about how readily a substance forms vapor at a given temperature. The direct measure of that tendency is vapor pressure—the pressure of the vapor that forms when the substance is in equilibrium with its liquid (or solid) at that temperature. A higher vapor pressure means the molecules escape into the gas phase more easily, indicating higher volatility at that temperature.

Water solubility isn’t about how easily a substance becomes vapor; it describes how well it dissolves in water. Boiling point relates to volatility indirectly—substances with lower boiling points tend to be more volatile, but it’s a temperature threshold rather than a direct measure of vapor formation at a fixed temperature. Henry’s constant describes how a gas partitions between air and an aqueous solution, which concerns behavior in liquids rather than the pure substance’s tendency to vaporize.

So the measure of volatility for a pure substance is its vapor pressure.