Explain isotherms of co2

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.

The importance of critical temperature of a gas was first discovered by Andrews in his experiments on pressure - volume isotherms of carbon dioxide gas at a series of temperature. The isotherm of carbondioxide determined by him at different temperatures

Critical temperature (Tc)  : It is defined as the characteristic temperature of a gas at which increase in pressure brings in liquefaction of gas above which no liquefaction occurs although the pressure may be increased many fold. For instance Tc of CO₂ is 31.1 ^o C. This means that it is not possible to liquefy CO₂ by applying pressure when its temperature is above 31.1 ^o C.