Driespunt

Driespunt, in thermodynamics, is the temperature at which a liquid is in equilibrium with its solid phase. It is the temperature where the liquid begins to solidify when cooled, and, conversely, where a solid melts to liquid when heated, at a given pressure. For a pure substance, the freezing point equals the melting point; for impure substances, the presence of solutes lowers the freezing point, a phenomenon known as freezing-point depression.

Standard reference pressure is one atmosphere (101.3 kPa). At this pressure, water freezes at 0 °C (273.15 K). Other substances have different values; ethanol freezes around −114 °C; saltwater solutions freeze at lower temperatures depending on salinity.

The exact driespunt depends on pressure and impurities; phase diagrams depict how it shifts with pressure. In everyday language, the term is commonly used to describe the temperature at which water or other liquids turn to ice.

Applications of knowledge about the driespunt include meteorology, food preservation, cryogenics, and automotive engineering, where antifreeze compounds are used to depress the freezing point and prevent freezing of liquids in engines and other systems.