Boiling point

The boiling point is the constant temperature at which a liquid changes to a gas throughout its bulk. Malay: takat didih · Chinese: 沸点.

The phrase throughout its bulk is what makes boiling different from evaporation: when a liquid boils, bubbles of gas form all through the liquid, not only at the surface. As with melting, the temperature stays constant during boiling because the heat energy is used to overcome the forces of attraction between the particles so that they can escape as a gas, rather than to raise the temperature.

Example. Water boils at 100 °C at normal atmospheric pressure. On a heating curve, boiling appears as a second flat plateau, higher than the melting plateau, which lasts until all the liquid has turned to gas.

Confusion to avoid. Do not confuse boiling with evaporation. Evaporation happens at any temperature below the boiling point and only at the surface of the liquid, and it causes a gradual change; boiling happens at a fixed temperature and throughout the whole liquid, and it is rapid. Take care too not to mix up the boiling point with the melting point, which is the lower temperature where the solid first becomes a liquid.

The boiling point of a substance also depends on the surrounding pressure: at lower pressure a liquid boils at a lower temperature, which is why water boils below 100 °C high up a mountain. For the exam, though, the value you quote is the boiling point at normal atmospheric pressure. A pure liquid boils at a single fixed temperature, while a mixture boils over a range, so the boiling point is again a test of purity.

In the exam you may be asked to read the boiling point from a heating curve, to explain the constant temperature during boiling, or to state the difference between boiling and evaporation. Because both rest on how particles gain energy and overcome attraction, the boiling point is another place where the kinetic theory of the Matter and the Atomic Structure chapter is put to work.