Steam is water that has been changed into gas and is at least as hot as 100 °C (212 °F.), the boiling point of water. It is colorless and transparent. The white vapor often visible over the spout of a teakettle, in geysers, and in the white gases coming from smokestacks is not actually steam. Instead, this wet steam consists of tiny droplets of liquid water. It forms as invisible steam cools. Steam at a temperature much higher than the boiling point is called superheated steam.
Steam is produced by boiling water. Although water remains at 100 °C until it all turns to steam, it absorbs a large amount of heat in undergoing this change. For example, 100 calories of heat must be absorbed to raise 1 gram (0.04 ounce) of water from the freezing point (0 °C or 32 °F) to the boiling point. But to change the same gram of boiling water into steam takes 540 calories of heat (see Calorie ). The amount of heat absorbed to change boiling water to steam is called the heat of vaporization. This heat is released when the steam cools and changes back to liquid water.
Steam is used as a means of transferring heat from a source, such as the burning of coal, wood, or natural gas, to a place where this energy is needed. For example, steam is used to drive turbines that extract heat from the steam and use some of this energy to turn generators for the production of electricity. Steam is also commonly used in heating homes, in chemical processing, and in sterilizing food.
