Why does boiling point change with pressure?

Boiling point changes with pressure because the boiling process depends on the pressure exerted on the liquid's surface.

Boiling is the process where a liquid turns into a gas. This happens when the vapour pressure of the liquid equals the atmospheric pressure. The vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. When the atmospheric pressure is high, the vapour pressure needs to be high as well for a liquid to boil, which means the liquid needs to be at a higher temperature. Therefore, the boiling point is higher.

Conversely, when the atmospheric pressure is low, the vapour pressure doesn't need to be as high for the liquid to boil. This means the liquid can boil at a lower temperature, so the boiling point is lower. This is why water boils at a lower temperature at high altitudes, where the atmospheric pressure is lower.

The relationship between boiling point and pressure can be explained using the Clausius-Clapeyron equation, which describes how the vapour pressure of a liquid or solid changes with temperature. According to this equation, the boiling point increases with increasing pressure and decreases with decreasing pressure.

In practical terms, this means that if you're cooking at high altitudes, you may need to adjust your cooking times because water boils at a lower temperature. Conversely, pressure cookers work by increasing the pressure, which raises the boiling point of water and cooks food faster. Understanding the relationship between boiling point and pressure is also important in many industrial processes, such as distillation and sterilisation.