Application of the Kinetic Theory of Matter

Change of State

Melting - when a solid substance gains heat, the kinetic energy of its particles increases and they vibrate more vigorously. As this goes on, a point is reached when the kinetic energies of some of its particles become greater than the prevailing average kinetic energy, as well as greater than the forces of attraction holding them together.

At this point, melting begins - the particles break away from the rest and become mobile (in the liquid phase).

With continuous gain of heat, all the particles are soon able to overcome the forces of attraction holding them, and the crystalline structure collapses into the liquid state. The temperature at the point of melting is definite for a particular solid, and is called the melting point of the solid.

Notice that during melting, both liquid and solid phases are in equilibrium (i.e. the substance would contain an equilibrium mixture of both solid and liquid).

Boiling - when a liquid substance gains heat, the kinetic energy of the particles (e.g. molecules) increases.

The proportion of its fast moving particles increases and they rise to the surface. A point is reached when their kinetic energies become higher than the average kinetic energy of the particles in the system, as well as their forces of attraction.

The particles then evaporate, leaving behind those particles of lower average kinetic energy, resulting in the drop in temperature. As this continues, the saturated vapor pressure of the liquid also increases as more vapor is formed (this vapor is in equilibrium with the liquid, and it exerts a pressure on the surface of the liquid).

A point is reached when the saturated vapor pressure becomes same as the prevailing atmospheric pressure, then the liquid will begin to boil – i.e. bubbles will be produced in different parts of the liquid, and these will rise to the surface of the liquid.

The temperature at which this occurs is the boiling point of the liquid. Notice that during boiling both liquid and gaseous phases are in equilibrium.

Note: A change of state begins when some particles (e.g. molecules) gain energy, which is higher than the prevailing average kinetic energy of the system, and thus are able to overcome the forces of attraction holding them to other particles. This is the reason there is a mixture of both phases in the system.

The boiling point of liquids depends on the pressure above the liquid (i.e. the external pressure or the atmospheric pressure). At higher prevailing pressures, e.g. at 760 mm Hg, pure water boils at 100^oC; at 526 mm Hg, it boils at 90 ^oC and at 1075 mm Hg, it boils at 110 ^oC.

The higher the external pressure, the higher the boiling point of a liquid and vice versa (the external pressure can therefore be used to control the boiling point of a liquid).