When impurities are mixed with a pure substance, the melting point is affected. Applications of this concept include:
- In cold countries, water in pipe lines freezes in winter season. Freezing causes volumetric expansion and may burst the pipes. Antifreeze is added to lower the melting point to avoid this kind of damages.
- Adding salt to water can reduce its melting point to as low as -18 °C. Salt is put onto the roads in cold countries during the winter season.
Mostly substance increases their melting point when a pressure is applied in their solid state.
Normal solids such as iron, copper undergoes expansion when they melt. When pressure is applied on the surface of a normal solid, the expansion is suppressed and melting is delayed. Thus, the melting point of a normal solid is raised by the application of pressure.
Abnormal solids, like ice and bismuth, contract on melting into liquids. When pressure is applied on the surface of such a solid, the change into the liquid is assisted by the increase in pressure. Thus, the melting point of ice is lowered by the application of pressure.
Note: Freezing point of pure water is 0 °C at standard atmospheric pressure. (Melting point of a substance must be stated together with its purity and surrounding pressure.)