High Quality Content by WIKIPEDIA articles! Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form. Vice versa, all gases have a tendency to condense back to their liquid form, or deposit back to solid form, as long as the temperature is below their critical temperature or decomposition temperature. In plain terms, a liquid evaporates at all pressures below its vapor pressure, while remaining stable at pressure above the vapor pressure.