Melting point

The melting point is the constant temperature at which a solid changes to a liquid. Malay: takat lebur · Chinese: 熔点.

The key word is constant. While a solid is melting, its temperature does not rise, even though it is still being heated. This is because the heat energy absorbed is used to overcome the forces of attraction holding the particles in their fixed positions, rather than to increase their kinetic energy. A pure substance has a sharp, fixed melting point, and impurities lower it and spread it over a range.

Example. Ice, which is solid water, melts at 0 °C. On a heating curve, a graph of temperature against time, melting shows up as a flat plateau, during which solid and liquid are present together and the temperature holds steady until all the solid has melted.

Confusion to avoid. Do not confuse the melting point with the boiling point: melting is the change from solid to liquid, while boiling is the change from liquid to gas at a higher temperature. Note also that the melting point and the freezing point of a substance are the same temperature, the difference is only the direction of the change, melting on heating and freezing on cooling.

On a cooling curve, the reverse of a heating curve, the same substance freezes at this very temperature, and a plateau appears there too, because heat is now released as the forces of attraction between the particles re-form. A pure substance can therefore be recognised by its sharp, fixed melting point, whereas an impure sample melts over a lower and wider range; this is one way the exam links melting point to purity.

In the exam you may be asked to identify the melting point from a heating or cooling curve, or to explain why the temperature stays constant while a solid melts. The safe explanation is that the energy goes into overcoming the forces between particles. This ties the melting point directly to the kinetic theory that runs through the Matter and the Atomic Structure chapter.