Question

Explain liquification of gases with the help of isotherm of CO₂. Write the process of liquification of gases.

Answer 1

Liquification of Gases:

Gases can be liquified by increasing pressure and decreasing temperature. On decreasing the temperature, the kinetic energy decreases and intermolecular attraction increases. Gases can be liquified using two methods :

(i) By cooling.

(ii) By compressing.

Gases with high critical temperature can be liquified by applying appropriate pressure. For example: SO₂ ,NH₃, CO₂, Cl₂ etc.
First of all, Boerhaave in 1732 attempted to liquify gases. He liquified water vapour. Later on in 1799, Van Marum liquified ammonia, in 1800 Mange and Clouet liquified SO₂, in 1805, Narthmore liquified Cl₂, SO₂ and HCl in 1823, Faraday liquified H₂S, CO₂, N₂O, SO₂, NH₃, HBr and other gases.

Cooling of gases can be done using following methods:

(i) By using a freezing mixture.

(ii) By the evaporation of liquids under reduced pressure.

(iii) By the adiabatic expansion of cold compressed gas.

(iv) Cooling by Joule-Thomson effect.

(v) Cooling by adiabatic demagnetisation.

(vi) Cooling by desorption.

Andrew’s Experiment and Isotherm of CO₂:

Graph plotted between pressure and volume at constant temperature is known as isothermal curve. For ideal gas PV = nRT, if temperature is constant then the PV will be constant. Hence, for ideal gas, isothermal curve will be rectangular hyperbola.

In 1869, Andrews plotted isotherms of CO₂ at various temperatures. It was found that all real gases behave like CO₂. At high temperature, 48°C isotherm of CO₂ looks like that of an ideal gas and gas can not be liquified even at very high pressure. As the temperature is lowered, shape of the curve changes and shows considerable deviation from ideal behaviour. At 30.98°C, carbon dioxide remains as gas upto 73 atmospheric pressure (Point E). On increasing the pressure, gas starts to compress and small decrease in volume observed.

At 21.5°C, carbon dioxide remains as gas only upto point B but slowly it becomes liquid i.e., from point B to C.

It is concluded from above explanation that at 31.1°C, CO₂ remains as gas and behave like ideal gas but at 30.98°C (point E) CO₂ appears as liquid for first time. Below this temperature, liquid state is effective. This temperature is known as critical temperature (T_C).