explain the isotherms of co2 at various temp.

At the lowest temperature used that is 13.1⁰ C, at low pressure, CO₂ exits as a gas (here point A). As the pressure is increased, the volume of the gas decreases along the curve AB. At point B, liquefaction of the gas starts. Hence the volume decreases rapidly along BC because liquid has much less volume than the gas. At point C, liquefaction is complete. Now the increase in pressure has very little effect upon volume because liquids are very less compressible. Hence the curve CD is very steep. As the temperature is increased further and the moment it reaches 30.98⁰ C (point E), the horizontal portion becomes very small, indicating that above 30.98⁰ C, the gas cannot be liquified at all however high pressure is applied. So, 30.98⁰ C is the critical temperature. The corresponding pressure to liquify the gas is critical temperature and the volume occupied by one mole of gas under these conditions is critical volume.