Liquefaction of Gases:
Gases can be liquefied by lowering the temperature and increasing the pressure. According to kinetic molecular model of gases, the forces of attraction between the molecules are negligible. When the temperature of the gas is lowered, the kinetic energy of the molecules decreases. As a result the slow moving molecules come nearer to one another. At a sufficiently low temperature, some of the slow moving molecules cannot resist the force of attraction and they come closer and closer and ultimately the gas changes into the liquid state.
The increase in pressure can also bring the gaseous molecules closer to one another (due to decrease in volume). Therefore, gases can also be liquefied by increasing the pressure. Thus, liquefaction of gases can be achieved by decrease of temperature or increase of pressure or by the combined effect of both.
Critical Temperature:
Thomas Andrews studied the pressure-volume-temperature relationship and concluded that there is a certain temperature above which a gas cannot be liquefied no matter how high a pressure may be applied. The gas can be liquefied only below this temperature. This characteristic temperature is called critical temperature. It is written as Tc. Therefore, “Critical temperature is the temperature above which a gas cannot be liquefied however high the pressure may be applied”.
For example, the critical temperature of carbon dioxide is found to be 31.1°C. This means that carbon dioxide cannot be liquefied above this temperature even at pressure of several hundred atmospheres. Similarly,
Critical pressure (Pc):
Critical pressure is the minimum pressure required to liquefy the gas at the critical temperature.
Critical volume (Vc):
Critical volume is the volume occupied by one mole of the gas at critical temperature and pressure.
Vapour Pressure:
The pressure exerted by the vapours on the surface of liquid at equilibrium at a given temperature is called vapour pressure or saturated vapour pressure.
The vapour pressure is a characteristic property of a liquid at a given temperature. It does not depend upon the amount of the liquid or vapour phase. The magnitude of vapour pressure depends upon the following three factors namely nature of the liquid, surface area and temperature.
Surface Tension:
Surface tension is defined as the energy required to increase the surface area of the liquid by one unit is called Surface tension. Surface tension decreases with increase in temperature because the kinetic energy of the molecules increases. The magnitude of the surface tension is a measure of intermolecular attractive forces. When the attractive forces between the molecules are large, the surface tension is large.
Viscosity:
Viscosity may be defined as the viscosity is the internal resistance to flow in liquids which arises due to the internal friction between layers of liquid as they slip past one another while liquid flows. Viscosity of a liquid decreases with the rise in temperature and increases with the increase of intermolecular forces. e.g. water is more viscose than methyl alcohol.