A physical process occurs when object undergo a change that does not change their chemical nature. A physical process involves a change in physical properties. Physical properties can be seen without changing the composition of matter. Examples of physical properties: texture, shape, size, color, volume, mass, weight, and density.
Many physical processes are involved in the rearrangement of atoms and some processes are reversible also.
Substance exists in three forms of states: solids, liquids and gases. The following different types of equilibriums exist in three states:
1. The conversion of Solid in liquid or liquid to solid.
2. The conversion of Liquid in gas or gas to liquid.
3. The conversion of solid in gas (vapour) or gas to solid.
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In physical equilibrium, some of the measurable properties of the system become constant.
1. Solid-Liquid Equilibrium
When a solid is heated it starts melting at a fixed temperature (melting point). At this stage even when the heating is continued, the temperature remain constant until the whole of solid is converted into liquid. The state when a solid and liquid phase of a substance is present together is known as solid-liquid equilibrium.
If there is no heat exchanged with the surroundings, then the temperature and the mass of the two phases remain constant.
2. Solid-Vapour Equilibrium
When the solid get converted into vapour without passing through the liquid phase then this process is known as sublimation. Sublimation thus involves solid – vapour physical equilibrium. On cooling the vapours, the solid phase is given back. This type of physical equilibrium is obtained in closed systems only.
3. Liquid-Gas (vapour) Equilibrium
The conversion of liquid in gaseous phase is known as vaporization. As evaporation continues the number of gaseous molecules in the vapour phase increases gradually. During the random movement of gaseous molecule, some of the molecules strike the surface of liquid and get condensed. At starting, the rate of condensation is less than the rate of evaporation but as evaporation continues the concentration of gaseous molecules in the vapour phase increases. As a result, the rate of condensation also increases. Finally, at equilibrium, the rate of evaporation becomes equal to the rate of condensation. At this stage, the system exhibits constant pressure.
The required conditions for liquid-gas equilibrium are:
The system must be a closed system
The temperature of the system must be constant.
The visible properties of the system should not change with time.