Vapor pressure is the pressure applied by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The vapor pressure shows how fast a liquid evaporates. A substance with a high vapor pressure at normal temperatures is often called a volatile. As the temperature of a liquid increases, the kinetic energy of its molecules also increases. As the kinetic energy of the molecules increases, the number of molecules changing into a vapor also increases, so it increases the vapor pressure.