1 Boiling point

• The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the atmospheric (external) pressure on the liquid surface.
• At that point bubbles of vapour can form throughout the liquid and rise.

Consequences:

• Under normal (standard) atmospheric pressure the boiling point of a pure liquid is constant (100 °C for water), so it can be used as an indicator of purity: impure liquids boil at a different temperature, and over a range.
• The boiling point depends on the external pressure:
  • Increasing the pressure raises the boiling point. Example: a pressure cooker holds the steam so that the pressure reaches about 2 atm and water boils at about 120 °C, cooking food faster.
  • Decreasing the pressure lowers the boiling point. At high altitude the atmospheric pressure is lower, so water boils at less than 100 °C (about 90 °C at 3000 m, about 71 °C at the top of Mount Everest). Food takes longer to cook there.