Siedpunkt

Boiling point, or Siedpunkt in German, is the temperature at which a liquid's vapor pressure equals the surrounding pressure, causing vapor bubbles to form throughout the liquid. For a pure substance at a fixed pressure, the boiling point is a characteristic property of that substance. At standard atmospheric pressure (1 atm, 101.3 kPa), this temperature is called the normal boiling point. Water has a normal boiling point of 100 °C, ethanol 78.37 °C, and nitrogen −196 °C.

Boiling point is highly dependent on pressure. Increasing the external pressure raises the boiling point; decreasing pressure lowers it. For example, water boils below 100 °C at higher altitudes where atmospheric pressure is lower. The relationship is described by vapor pressure curves and can be estimated with equations such as the Antoine equation.

Pure liquids have a single boiling point at a given pressure. Mixtures often boil over a range of temperatures; azeotropes can boil at a nearly constant temperature that behaves like a pure substance. The latent heat of vaporization measures the energy required to convert liquid to vapor at the boiling point.

Measurement and use: boiling points are determined by methods such as capillary tube measurements and distillation experiments. In chemical engineering and purification, boiling points guide distillation design and separation processes. The boiling point is also a key indicator of volatility and is used in safety classifications and material data sheets.