Boiling Point

In-depth explanation of Boiling Point

The boiling point of a liquid is the temperature at which its vapour pressure equals the external pressure acting on it. Boiling occurs throughout the liquid, with bubbles of vapour forming within it rather than only at the surface. Because the boiling point depends on external pressure, water boils at a lower temperature at high altitude, where the atmospheric pressure is lower. Dissolved impurities raise the boiling point, which is why salt water boils at a slightly higher temperature than pure water. Cooking takes longer at altitude for this reason.

Going deeper: Boiling Point

Key ideas: Boiling point rises with external pressure and falls with altitude by about 1 C per 300 m. Use Clausius-Clapeyron for estimates. In a pressure cooker at about 2 atm, water boils near 120 C, speeding cooking. Pure substances boil at a constant temperature, but mixtures often boil over a range.

Common mistakes and connections: A common error is thinking heating continues to raise temperature during boiling; it stays constant while the heat goes into vaporization. Another is saying boiling point is a fixed property regardless of pressure. Dissolved nonvolatile solutes raise it by colligative effects. Links to vapour pressure, normal boiling point and heating curves.

Visualization of Boiling Point

A liquid boils when its vapour pressure equals the surrounding pressure, so lowering pressure (altitude) lowers the boiling point. The interactive lesson shows this as a live animation or chart.

Worked example and use case

Water boils at about 90°C at an altitude of 3000 m, where the atmospheric pressure is about 70 kPa.

Where Boiling Point fits in States of Matter & Solutions

Boiling Point is topic 37 of 109 in States of Matter & Solutions.

Related topics

• Liquid StateBeginner
• Vapor PressureBeginner
• EvaporationBeginner
• Normal Boiling PointBeginner
• Clausius-Clapeyron EquationAdvanced
• ViscosityIntermediate

Browse all 109 topics in States of Matter & Solutions