The melting point is the temperature at which a substance changes state from solid to liquid. This is a characteristic physical property of matter.
Each pure solid substance melts and becomes liquid at a specific temperature. This temperature is called the melting point. It is also at this temperature that a liquid becomes solid. It is then called the freezing point (or solidification point). Thus, for the same substance, the melting and freezing points are identical. For example, ice melts at 0 °C and water freezes at 0 °C.
Depending on the type of substance, the temperature at which melting takes place varies enormously. For example, the vast majority of metals have fairly high melting points, while gases have extremely low melting points.
The following concept sheet indicates the melting point of some pure substances, measured at normal pressure, and of certain molecules: Reference Sheet - Melting Point.
Knowledge of the melting point allows for the identification of pure substances, and is useful in various fields. For example, pieces of equipment that must withstand great heat must have a melting point higher than the maximum temperature they will have to withstand in order to avoid deformation or breakage. On the other hand, the lead filament found in an electric fuse takes advantage of the low melting point of this pure substance. When the circuit current is too high, the lead wire heats up and melts, which prevents overloading by preventing current from flowing.
A mixture of different pure substances has a melting point which depends on:
-
the substances that make up the mixture;
-
the proportions of the substances in the mixture.
Ice melting at room temperature (left), melting iron at high temperature (centre), and melting chocolate chips in the kitchen (right).
It is important not to confuse the terms fusion and nuclear fusion. Fusion is another word for melting, which is a phase change that takes place when a solid melts and becomes liquid. However, nuclear fusion is the combination of two low-mass atomic nuclei to form a new, heavier one.