56. Melting and hardening of crystalline bodies

The melting of crystalline bodies, as you know from the first book, occurs at a strictly defined temperature. Thus, ice melts at normal atmospheric pressure at \(0^0C\), mercury (\(Hg\), Atomic number: 80) at \(-39^0C\), tin (\(Sn\), Atomic number: 50) at \(232^0C\).

In order to melt, there must be a constant supply of heat. If the inflow of heat is stopped by insulating the system a solid body-liquid, the system will be in equilibrium, the mass of the substance in the crystalline and liquid states will not change. If the system gives heat to colder surrounding bodies, the reverse melting process - crystallization - will begin. The temperature will also be constant and equal to the melting point.

As the crystalline body heats up, the average kinetic energy of its molecules increases. The potential energy of molecules also increases. Once the melting point is reached, all the energy supplied goes to increase the potential energy of the molecules, since the kinetic energy at a constant temperature remains unchanged. This increase in potential energy will be large, because during melting there is a significant increase in volume in almost all bodies and, therefore, a significant increase in the distance between the molecules.

The quantity of heat required to convert \(1 kg\) of crystalline substance into a liquid at the same temperature is called the specific heat of melting.

When the body crystallizes, this amount of heat, on the contrary, is released. That's why it usually gets warmer during a snowfall. Ice melting heat \(3.4 \times 10^5 ~J/kg ~= ~80 ~Cal/g \).

The melting point of most bodies increases with pressure, just as the boiling point increases with pressure. This can be explained as follows. Compression of the substance prevents an increase in the distance between the molecules and, consequently, an increase in the potential energy of interaction of molecules, which is necessary for transition to the liquid state.

Some bodies: ice, bismuth (\(Bi\), Atomic number: 83), cast iron (iron, \(Fe\), Atomic number: 26) - when melting reduce their volume and, therefore, increase their density (if it was not so, the ice would sink in the water, not swim). In these bodies, the melting temperature decreases as the pressure increases. This feature is associated with a special form of crystal structure, in which the ordered arrangement of molecules corresponds to a larger volume than the disorder. But even in these bodies the potential energy of molecules grows during melting.

When the freezing water expands in a closed container, huge forces arise that can tear the thick-walled cast iron ball. This experience is easy to accomplish with a bottle filled with water down the neck and placed out in the freezing cold. An ice cap forms on the surface of the water that plugs the bottle, and as the freezing water expands, the bottle burst.
Freezing the water in the cracks at night during frost causes gradual destruction of rocks.