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The boiling point is the temperature of an element at which it converts from the liquid to the gaseous state. At normal atmospheric pressure water reaches this point at 100 °C or 373.16 K. In other words, the boiling point is the temperature at which the vapour pressure equals the environmental pressure surrounding the liquid.

The boiling point is therefore dependant on two factors. One is the element, the other the surrounding pressure. The higher the surrounding pressure, the higher the temperature of the boiling point.

In the kitchen this property is used in pressure cookers. Pressure cookers are airtight vessels that cook food at 1.2 to 1.8 bar and at temperatures between 104 and 120 °C. Heating increases the pressure within the cooker. The boiling point of water in the pressure cooker is therefore increased. It has to become hotter than 100 °C to boil. This increase in temperature decreases the cooking time of the food up to 80 % and may save up to 50 % of the energy used for cooking.

Where the surrounding temperature is lower than atmospheric pressure, the boiling point decreases, for example in high mountainous areas. As a rule of thumb can be said that the boiling point decreases by one degree Celsius for each 300 m increase in height. In high, mountainous areas this leads to two problems during cooking:

  • Cooking times are prolonged.
  • At great heights (i.e. 5000 m) it may become difficult to impossible, to kill germs in water or food by boiling it.

Where solids like salt or sugar are dissolved in water, the boiling point of the water is raised according to the amount of dissolved matter. The boiling point of salted water for pasta or rice for example is about 1 degree Celsius higher than pure water, that of heavy syrup lies several degrees above. For simple syrup it is 2 degrees above that of water.

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Boiling range

Pure elements have a boiling point, mixed homogenous matters have a boiling range. The boiling point is identical with the condensation point.

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