Mechanical Engineering - Heat Transfer, Refrigeration and Air Conditioning - Discussion

The correct answer is: reduced to a value below its critical temperature.

To liquefy a gas, its temperature must be lowered to a value below its critical temperature (Tc). The critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied.

When a gas is cooled to a temperature below its critical temperature, it can be liquefied by applying pressure. This is because the intermolecular forces between the gas molecules become stronger at lower temperatures, allowing the molecules to come together and form a liquid.

Description of other options:
- Increasing the temperature above the critical temperature will make it more difficult to liquefy the gas.
- Equaling the temperature to the critical temperature will not allow liquefaction, as the gas will still be in its supercritical state.
- None of the above is incorrect, as reducing the temperature below the critical temperature is indeed the correct answer.


Please note that the critical temperature is a unique property of each gas, and it must be known to determine the appropriate temperature for liquefaction.

B is the right answer.

B is right answer.

B IS RIGHT ANSWER.

The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied. Every substance has a critical temperature.

Some examples are shown below.

Substance ----------- critical temperature (oC)
NH3 ----------- 132
O2 ----------- -119
CO2 ----------- 31.2
H2O ----------- 374

Tubes containing water at several temperatures. Note that at or above 374oC (the critical temperature for water), only water vapor exists in the tube.

Critical Pressure

The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature.

Some examples are shown below.
Substance ----------- critical pressure (atm)
NH3 ----------- 111.5
O2 ----------- 49.7
CO2 ----------- 73.0
H2O ----------- 217.7

B is the right option.

Critical pressure: The minimum pressure required to liquefy a gas at its critical temperature.

Critical temperature: The temperature at or above which no amount of pressure, however great, will cause a gas to liquefy.

@Bharat.

You are correct, option B is the appropriate answer.

Why gas can also be liquefied below critical temperature why not option B?