Question

Under which conditions, can gas be cooled and converted into liquid?
A. At its critical temperature by decreasing pressure on it.
B. Above its critical temperature by increasing pressure on it
C. Above its critical temperature by decreasing pressure on it.
D. Below its critical temperature by increasing pressure on it.

Answer 1

Hint: The conversion of gas into the liquid is done by the different effects of temperature and pressure. We have to study the critical temperature first and then about the effect of pressure on the gas.

Complete Step-by-Step Answer:
- In the given question, we have to determine the favourable conditions for the conversion of the gas into a liquid.
- Now, we have to study about the concept of critical temperature first.
- Critical temperature is the temperature above which the gas cannot be liquified due to the high temperature.
- At high temperature, the kinetic energy of the molecules increases due to which the intermolecular attractive force between the molecule weakens and it will exist as gas instead of liquid.
- So, when the temperature of the gas reaches the critical point the gas starts converting into the liquid state with increasing temperature with increase in pressure.
- Also, when the pressure is increased below the critical temperature, then the liquid state will exist.
- But above the critical temperature, the gas state will exist and not the liquid state.
- Moreover, the minimum pressure which is required to convert gas into the liquid state is known as critical pressure.

Therefore, option D is the correct answer.

Note: The critical temperature has a great significance in thermodynamics as it tells about the strength of the molecules in the gaseous state. Because less is the intermolecular force between them more it will be more difficult to liquefy them.