Notes On Gaseous State: Liquefaction Of Gases - Uttar Pradesh board Class 11 Chemistry
A gas can be converted into a liquid through a process called liquefaction. Liquefaction of a gas can occur only when the intermolecular force of attraction between the gas molecules are increased. Methods for liquefaction: (i) Increase in pressure of the gas at room temperature: On increasing pressure the gas molecules come closer to each other, results in an increase in the intermolecular forces of attraction between the molecules, which leads to the liquefaction of gases. (ii) Decrease in the temperature of the gas: As the gas gets cooled the kinetic energy of the gas molecules decreases, results in the decrease of the speed of their random motion and in the increase of the force of attraction between them. Carbon dioxide, ammonia, sulphur dioxide and hydrogen chloride can be liquefied by increasing the pressure and decreasing the temperature. There are some gases, which cannot be liquefied at room temperature even at very high pressure. These gases are called as permanent gases. Ex: H2, He, O2. Irish scientist Thomas Andrews studied the pressure and temperature conditions of liquefaction of several gases and established that for every gas there is a particular temperature beyond which, even though high pressure is applied the gas cannot be liquefied. This temperature is known as critical temperature. Critical temperature (Tc) can be defined as the characteristic temperature of a gas above which any increase in pressure will not result in the liquefaction of the gas. The minimum pressure required to liquefy 1mole of a gas at critical temperature is called critical pressure (Pc). The volume occupied by1 mole of a gas at its critical pressure and at critical temperature is the critical volume (Vc) of the gas. Tc, Pc and Vc are collectively called the critical constants of the gas. A point where there is no distinction between the liquid and the vapor state of a gas is called the critical point and the gas at this point is said to be in the critical state. A gas can be liquefied below critical temperature by applying pressure and then it is called the vapor of the substance. Ex: Carbon dioxide gas below its critical temperature, at 30.98℃, is called carbon dioxide vapor.

#### Summary

A gas can be converted into a liquid through a process called liquefaction. Liquefaction of a gas can occur only when the intermolecular force of attraction between the gas molecules are increased. Methods for liquefaction: (i) Increase in pressure of the gas at room temperature: On increasing pressure the gas molecules come closer to each other, results in an increase in the intermolecular forces of attraction between the molecules, which leads to the liquefaction of gases. (ii) Decrease in the temperature of the gas: As the gas gets cooled the kinetic energy of the gas molecules decreases, results in the decrease of the speed of their random motion and in the increase of the force of attraction between them. Carbon dioxide, ammonia, sulphur dioxide and hydrogen chloride can be liquefied by increasing the pressure and decreasing the temperature. There are some gases, which cannot be liquefied at room temperature even at very high pressure. These gases are called as permanent gases. Ex: H2, He, O2. Irish scientist Thomas Andrews studied the pressure and temperature conditions of liquefaction of several gases and established that for every gas there is a particular temperature beyond which, even though high pressure is applied the gas cannot be liquefied. This temperature is known as critical temperature. Critical temperature (Tc) can be defined as the characteristic temperature of a gas above which any increase in pressure will not result in the liquefaction of the gas. The minimum pressure required to liquefy 1mole of a gas at critical temperature is called critical pressure (Pc). The volume occupied by1 mole of a gas at its critical pressure and at critical temperature is the critical volume (Vc) of the gas. Tc, Pc and Vc are collectively called the critical constants of the gas. A point where there is no distinction between the liquid and the vapor state of a gas is called the critical point and the gas at this point is said to be in the critical state. A gas can be liquefied below critical temperature by applying pressure and then it is called the vapor of the substance. Ex: Carbon dioxide gas below its critical temperature, at 30.98℃, is called carbon dioxide vapor.

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