In physical chemistry and in engineering, steam refers to vaporized water, typically at a temperature well above its sea level-pressure boiling point of 100 ° Celsius. It is a pure, invisible gas (for mist see below), which at atmospheric pressure occupies about sixteen hundred times the volume of liquid water. A steam engine uses the expansion of steam to drive a piston or turbine and so to perform work. In other industrial applications steam is used as a repository of energy, which is introduced and extracted by heat transfer, usually through pipes. Steam is a capacious reservoir for energy because of water's high heat of vaporization.
A steam explosion (also called a littoral explosions) is a violent boiling or flashing of water into steam, typically occurring when water is superheated. The water changes from a liquid to a gas with extreme speed, increasing dramatically in volume (see above). A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions (if not otherwise confined, e.g. by the walls of a container), creating a danger of scalding and burning.
Steam Explosions are most often encountered where hot lava meets sea water. A dangerous steam explosion can be created when liquid water encounters hot, molten metal. As the water explodes into steam, it splashes the burning hot liquid metal along with it, causing an extreme risk of severe burns to anyone located nearby and creating a fire hazard.
In common speech,steam most often refers to the white mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. After gaseous steam has intermixed with air, it is no longer properly called steam and is instead referred to as water vapor.
