Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. Water vapor cannot be seen, though in common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air. Strictly speaking, in terms of the chemistry and physics, true steam is invisible. 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 temperature and pressure. If heated further it becomes superheated steam.
The enthalpy of vaporization is the energy required to turn water into the gaseous form when it increases in volume by 1,600 times at standard temperature and pressure; this change in volume can be converted into mechanical work by steam engines and steam turbines. Steam engines played a central role to the Industrial Revolution and modern steam turbines are used to generate electricity. If liquid water comes in contact with a very hot substance (such as lava, or molten metal) it can create a steam explosion. Steam explosions have been responsible for many foundry accidents, and may also have been responsible for much of the damage to the plant in the Chernobyl accident.
A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine. Both fuel and water supplies are carried with the locomotive, either on the locomotive itself or in wagons pulled behind.
Steam locomotives were first developed in Great Britain during the early 19th century and dominated railway transportation until the middle of the 20th century. From the early 1900s they were gradually superseded by electric and diesel locomotives.