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Ång, or steam, is the gaseous phase of water. It forms when liquid water is heated to its boiling point or when water evaporates under suitable conditions. In technical contexts, a distinction is made between saturated steam, which is in equilibrium with liquid water at a given temperature and pressure, and dry or superheated steam, which has been heated beyond the saturation point and contains little or no liquid water.

Steam is far less dense than liquid water and expands greatly when it forms. It can transfer heat efficiently through the latent heat of vaporization, about 2,260 kilojoules per kilogram at 100 °C (1 atmosphere). When it cools, steam condenses back to liquid water, releasing that energy.

Steam is produced in boilers by heating water with fuel, electricity, or nuclear energy. It may be wet (carrying droplets) or dry. Superheated steam is obtained by further heating and is used in turbines to improve efficiency.

Steam has a wide range of applications. It is used to generate electricity in power plants, provide industrial heating, drive locomotives, sterilize equipment, and in certain cooking processes. Safety concerns include burns from contact with hot surfaces or jets, and risks associated with high-pressure systems such as leaks or explosions; proper equipment, insulation, and pressure relief are essential.

Historically, steam engines and steam power drove the Industrial Revolution, with key advances by figures such as James Watt in the 18th century, enabling mechanization, transportation, and large-scale manufacturing.