Question:

A gas can be liquefied :

Updated On: Jul 2, 2022
  • above its critical temperature
  • at its critical temperature
  • below its critical temperature
  • at any temperature
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The Correct Option is C

Solution and Explanation

According to kinetic theory of gases, the gases consists of tiny molecules widely separated from one another by large empty spaces (voids). Gases change to liquid when the voids between the gas molecules become less than $10^{-5} cm$. This can be achieved by decrease in temperature (as KE of molecules decrease) or by increase in pressure where in the molecules are brought close. However, it has been experimentally found that above a particular temperature known as critical temperature $\left(T_{c}\right)$, the liquification of a gas does not take place at all, howsoever high pressure may be applied. It is because above critical temperature of a gas, the kinetic energy of the gas molecules is sufficient enough to over-come attractive forces and keep the molecules apart. The pressure required to liquify the gas at critical temperature is called critical pressure $\left(P_{c}\right)$. The volume of one mole of a gas at $T_{c}$ and $P_{c}$ is called cirtical volume $\left(V_{c}\right)$. Thus a gas can be liquefied only below its critical temperature.
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Concepts Used:

States of Matter

The matter is made up of very tiny particles and these particles are so small that we cannot see them with naked eyes.

There are three States of Matter:

The three states of matter are as follows:

Solid State:

  • The solid-state is one of the fundamental states of matter.
  • Solids differ from liquids and gases by the characteristic of rigidity.
  • The molecules of solids are tightly packed because of strong intermolecular forces; they only oscillate about their mean positions.

Liquid State:

  • The molecules in a liquid are closely packed due to weak intermolecular forces.
  • These forces are weaker than solids but stronger than that of gases.
  • There is much space in between the molecules of liquids which makes their flowing ability easy.

Gaseous State:

  • In this state of matter, distances between the molecules are large (intermolecular distance is in the range of 10-7-10-5 cm.
  • The intermolecular forces experienced between them are negligible.
  • Thus, translatory, rotatory and vibratory motions are observed prominently in gases.