Dependence of Melting Point and Boiling Point on Pressure
Simple Explanation
Both melting point and boiling point depend on pressure. For most substances, increasing pressure slightly raises the melting point (though water is a notable exception, where increased pressure slightly LOWERS its melting point). Increasing pressure generally raises the boiling point significantly, and decreasing pressure lowers it.
Why Do We Need It?
This pressure dependence explains why water boils at a lower temperature on a mountain (lower atmospheric pressure) and why pressure cookers can cook food faster (higher pressure, higher boiling point, hotter cooking temperature).
Why Does This Work?
Boiling occurs when a liquid's vapour pressure equals the surrounding (atmospheric) pressure — at lower atmospheric pressure, a lower vapour pressure (and therefore a lower temperature) is enough to reach this balance, so the liquid boils at a lower temperature. At higher pressure, more thermal energy (a higher temperature) is needed to raise the vapour pressure enough to match the increased surrounding pressure.
Real-Life Example
Cooking at high altitude versus using a pressure cooker
Food takes longer to cook by boiling at high mountain altitudes, but cooks faster in a pressure cooker at sea level.
At high altitude, lower atmospheric pressure lowers water's boiling point (sometimes to around 90°C or below), so boiling water is cooler and cooks food more slowly. A pressure cooker does the opposite — it raises internal pressure, raising the boiling point of water inside well above 100°C, cooking food faster.
Practice
Why does a pressure cooker cook food faster than an ordinary pot?
MediumCommon mistake
Assuming melting point ALWAYS increases with pressure, like boiling point generally does — water is a notable exception, where increased pressure actually LOWERS its melting point slightly (related to ice being less dense than liquid water).
Quick Review
- Boiling point generally rises with increased pressure, falls with decreased pressure.
- Melting point usually rises slightly with pressure — but water is a notable exception (it falls).
- Example: high-altitude cooking (lower boiling point) vs. pressure cookers (higher boiling point).