In solid state, the constituent particles are closely packed and the voids between them are very small. The average distance between the particles of the order of 10-8 cm. Due to small voids, the particle motion is limited to vibratory or oscillatory motion only about their mean position. The magnitude of interparticle forces is quite large. Some of the general characteristics of solids are explained as follows:
- The definite shape and volumes of solids are attributed to the definite geometric pattern involved in the close packing of particles.
- The particles possess low thermal energy and therefore they cannot break free from mutual attraction consequently, solids cannot flow.
- The voids between the particles are extremely small which cannot be further decreased. Hence, solids are incompressible .
- When we heat the solid, thermal energy gradually increases which weaken the intermolecular forces and cause them to move apart. When the average distance between the particles increases beyond w-7 cm, solid melts into liquid as shown in Fig. 14.1.
The temperature at which the solid start melting into liquid state and the two states coexist in equilibrium at the standard atmospheric pressure of 100 kPa, is called melting point (T m) of the solid.
Fig. 14.1 Change of state from solid to liquid.
There are some solids, whose particles on being heated, by-pass the liquid state and change directly into the vapor phase. These solids are said. to sublime and the process is called sublimation. Examples are iodine, naphthalene, solid carbon dioxide (dry ice) and ammonium chloride.
It may be noted that even though heat energy is continuously supplied to a melting solid, no temperature rise is recorded whilst melting takes place. The reason for this is that the heat energy supplied is used to overcome the cohesive forces and does not increase the kinetic energy. Overcoming the cohesive forces however, increases the potential energy. This heat is referred to as the latent (hidden) heat of fusion.