The vapor pressure of a liquid increases as the temperature increases. The dependence of vapor pressure on temperature follows the Clausius-Clapeyron equation:

This relationship assumes that an ideal gas forms as the liquid evaporates, but yields good results with many substances.

For liquids there exists a critical temperature above which the liquid state is not stable. In other words, when a substance is heated to its critical temperature, it exists as a fluid -- the liquid and vapor states become indistinguishable.