Question

At critical point in Andrew’s isotherm:
A.Liquid vapour equilibrium exists.
B.Substance may change from liquid to gas phase.
C.Substance may change from gas to liquid phase.
D.Both states become almost indistinguishable.

Answer 1

Hint: In Andrew’s isotherm, carbon dioxide has been studied. The behaviour of gas was studied by Andrew at different temperatures.

Complete step by step answer:
In Andrew’s isotherm, carbon dioxide has been studied. The behaviour of gas was studied by Andrew at different temperatures.
Various pressures were plotted against volumes keeping the temperature constant.
These pressure versus volume plots at constant temperatures are known as isotherms. As the pressure increases, more carbon dioxide condenses. Volume keeps on decreasing till all the carbon dioxide changes from gaseous state to liquid state.
Liquefaction of carbon dioxide gas does not occur along the isotherm but occurs only along the critical temperature of the gas.
At the critical point of carbon dioxide in Andrew’s isotherm, gaseous state starts condensing into liquid state and liquid state starts vapourizing to gaseous state.
At this point, an equilibrium exists between the liquid state and gaseous state and both the gaseous state and the liquid state become almost indistinguishable.
Thus, at a critical point in Andrew’s isotherm, liquid vapour equilibrium exists, substance may change from liquid to gas phase, substance may change from gas to liquid phase and both states become almost indistinguishable.
Thus, the correct options are option (A), option (B), option (C) and option (D).


Note:
The highest temperature at which gaseous state starts condensing into liquid state is known as the critical temperature. The minimum pressure at the critical temperature when liquefaction of gas begins is known as the critical pressure. The molar volume at critical temperature and critical pressure is known as the critical volume.