Celsius is a temperature scale named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. The degree Celsius (°C) can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. It is now official, however, in both the United States and Canada. The degree Fahrenheit (°F) remains in use as well for everyday temperatures in most of the world, though its use has gradually declined since metrication began taking place during the mid-to-late 20th century.

The freezing point of water is 0 °C, while the boiling point is 100 °C at standard pressure. Absolute zero—the theoretical lower limit of all thermal motion—is −273.15 °C (−459.67 °F). In practice materials often have different melting points depending on external conditions such as atmospheric pressure; when not specified, standard conditions are assumed: 1 atmosphere and 0 Kelvin for solids and liquids; 1 bar (~100 kPa) and 273 K for gases including air under Earth surface conditions which are about 101325 Pa and 288 K respectively; 10 MPa and 300 K for supercritical fluids. Thus pure water freezes at 273K/0°C under standard conditions (101325 Pa & 0K), but it can be cooled below this by increasing either external pressure or using other materials as coolants such as liquid nitrogen or solid carbon dioxide because these do not change phase until much lower temperatures than water.

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