Hello,

1. Discussing about critical phenomenon -

The essential condition for the liquefaction of the gas is described by the study of critical temperature, critical pressure and critical volume and their inter relationships.

When a gaseous system is transformed to its liquid state, there is a tremendous decrease in the volume. This decrease in volume can be effectively brought about by lowering of temperature, or by increasing pressure (or) by both. In both these effects the gaseous molecules come closer to each other and experience an increase in force of attraction which results in liquefaction of gases. At any constant temperature when pressure is increased volume is decreased and vice versa. Such P-V curves at constant temperature are known as isotherms.

2. Talking about Andrew Isotherms -

Thomas Andrew was first to study the phenomenon of effect of pressure on the volume of fixed volume of carbon  dioxide at different temperature.

1), It was found that all real gases behave in the same manner as carbon dioxide.

2) A Gas can be liquefied  by increasing pressure and decreasing temperature.

3) Isotherm below is plotted by experimental measurements of volume of carbon dioxide at different pressure keeping the temperature constant each time.

4) At high temperature the curve is as that of ideal gas.

5) As temperature is lowered it shows deviation from ideal behavior .

6) When the temperature is lowered upto 30.98 ºC and at 73 atm pressure the gas gets liquefied .

7). 30.98 ºC is known as the critical temperature and 73 atm pressure is known as critical pressure and the volume of gas at this point is known as critical volume . Above critical temperature gas can not be liquefied it remains in the vapour state.

8)At 13.1 ºC , on compression  Carbon dioxide remains a gas upto certain volume after that the pressure becomes contant i.e even on increasing compression , pressure does not change and thus at this point the liquid and gaseous carbon dioxide coexist.

9)On further increasing compression the gas condenses upto the point at which whole gas is liquefied and after this liquid carbon dioxide exists for which a steep slope is observed in the isotherm.

Below  30.98ºC  every temperature shows simmilar behavior.

Best Wishes.