Melting point of a solid is defined as the temperature at which a solid changes into a liquid. Melting point of a solid indicates the strength of the force of attraction between the particles of the solid. On heating a solid, its molecules absorb energy in the form of heat and their kinetic energy increases.
As the kinetic energy increases, the temperature of the solid increases. As a result, the force of attraction between the molecules decreases and the molecules become more and more separated. This increases the potential energy of the molecule and the particles leave their fixed positions and start moving more freely.
At a particular temperature, the separation of the molecules increases by a large amount and the solid melts and converts into liquid. This particular temperature is the melting point of that solid. The melting point of a pure substance is always higher than the melting point of that substance when a small amount of an impurity is present. Pressure also affects the melting point of a substance. As the pressure on the substance increases, the melting point decreases.
The melting point of a compound helps in the identification of that compound. Measurement of the melting point of a solid can also provide information on the purity of the substance. Melting point of mixtures is lower than the melting point of a pure solid. Pure crystalline solids melt over a very narrow range of temperatures, whereas mixtures melt over a broad range of temperatures.
The temperature of the reverse change from liquid to solid is called the freezing point or crystallization point. For most substances the melting point and freezing points are equal.
For ice, the melting point is 0°C or 273K. The chemical element with the highest melting point is tungsten (3410°C), which is used for making filaments in light bulbs.
During the melting process, till all the ice melts, the temperature of the system does not increase until after the melting point is reached. The whole of the supplied heat energy is consumed in increasing the potential energy of the ice molecules. The kinetic energy of the molecule does not increase further, and the temperature does not increase, so long as the melting continues.
The quantity of heat required to completely change 1 kg of ice into water at atmospheric pressure at its melting point is called latent heat of fusion. For ice, it is 3.34 x 105 J/kg at 0°C.
Melting points of some common substances are given in the table.
Melting point (°C)