<P> Steam is water in the gas phase, which is formed when water boils . Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses . At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 ° C (212 ° F) at standard pressure . If heated further it becomes superheated steam . </P> <P> The enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,700 times at standard temperature and pressure; this change in volume can be converted into mechanical work by steam engines such as reciprocating piston type engines and steam turbines, which are a sub-group of steam engines . Piston type steam engines played a central role to the Industrial Revolution and modern steam turbines are used to generate more than 80% of the world's electricity . If liquid water comes in contact with a very hot surface or depressurizes quickly below its vapor pressure, it can create a steam explosion . </P>

Steam is to water as liquid is to