Steam

Steam is water vapor, often mixed with air and/or an aerosol of liquid water droplets. This may occur by evaporation or by boiling, where heat is applied until water reaches the enthalpy of vaporization. Superheated or saturated steam is invisible; however, wet steam, a visible mist or aerosol of water droplets, is often referred to as "steam".

When liquid water becomes steam, 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. Piston-type steam engines played a central role in the Industrial Revolution, and steam-based generation today produces 80 percent of the world's electricity. If liquid water comes into contact with a very hot surface or depressurizes quickly below its vapor pressure, it can create a steam explosion.