<P> The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor . </P> <P> The boiling point of a liquid varies depending upon the surrounding environmental pressure . A liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure . A liquid at high pressure has a higher boiling point than when that liquid is at atmospheric pressure . For example, water boils at 100 ° C (212 ° F) at sea level, but at 93.4 ° C (200.1 ° F) at 2,000 metres (6,600 ft) altitude . For a given pressure, different liquids will boil at different temperatures . </P> <P> The normal boiling point (also called the atmospheric boiling point or the atmospheric pressure boiling point) of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere . At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and allow bubbles of vapor to form inside the bulk of the liquid . The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar . </P> <P> The heat of vaporization is the energy required to transform a given quantity (a mol, kg, pound, etc .) of a substance from a liquid into a gas at a given pressure (often atmospheric pressure). </P>

The normal boiling point of any substance would occur at