Chemistry · Liquids and solids. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Molecules in a liquid have different kinetic energies. Those at the surface with enough energy escape as vapour. This is evaporation. It happens at all temperatures and it cools the liquid, because the fastest molecules leave.
In a closed vessel, molecules also return to the liquid. When the rates of evaporation and condensation are equal, the pressure of the vapour is the vapour pressure. It rises with temperature.
A liquid boils when its vapour pressure equals the external pressure. So the boiling point changes when the external pressure changes. Liquid crystals are a state between solid and liquid: they flow like liquids but keep some order like crystals.
Students think a liquid has one fixed boiling point. The value quoted is for 1 atm (760 torr); at any other external pressure the liquid boils at a different temperature.
Answer: The water boils at 25 °C, without any heating. This is the idea behind vacuum distillation.
Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.
← Intermolecular forces and hydrogen bondingCrystalline solids →Take a Chemistry test