1. Andrew plotted isotherms of carbon dioxide at different temperatures. it is then proved that many real gases behave in a similar manner like CO2 .
2. At a temperature of 303.98 K, CO2 remains as a gas. Below this temperature, CO2 , turns into liquid CO2 at 73 atm. It is called the critical temperature of CO2 .
3. At 303.98 K and 73 atm pressure, CO3,, becomes a liquid but remains a gas at higher temperature.
4. Below the critical temperature, the behaviour of CO2 is different.
For example, consider an isotherm of CO2 at 294.5 K, it is a gas until the point B, is reached. At B, a liquid separates along the line BC, both the liquid and gas co-exist. At C, the gas is completely condensed.
5. If the pressure is higher than at C, only the liquid is compressed so, a steep rise in pressure is observed.
Thus, there exist a continuity of state.
6. A gas below the critical temperature can be liquefied by applying pressures.