What is the temperature at which a substance changes from a liquid to a gas?

The process of a substance changing from a liquid to a gas occurs at the boiling point. This is the temperature at which the vapor pressure of the liquid equals the surrounding atmospheric pressure, allowing bubbles of vapor to form within the liquid. At this temperature, the molecules gain enough energy to overcome the forces that hold them together in the liquid state, transitioning into the gaseous state.

In contrast, the freezing point is the temperature at which a substance changes from a liquid to a solid, while the melting point is where a substance transitions from solid to liquid. Sublimation refers to the process where a solid changes directly to a gas without passing through the liquid phase, which occurs at specific conditions but is not what occurs during the liquid-to-gas transition. Therefore, the boiling point is the correct answer, as it directly defines the temperature of the liquid-to-gas phase change.