Question

Which of the following is not true about critical points of a gas?

• A. Above the critical temperature the gas cannot be liquified
• B. At the critical temperature $\frac{dP}{dV}=0$ when V=3b (b= Vander Waal’s constant)
• C. Above the critical temperature $\frac{dP}{dV}\ne 0$
• D. Below critical temperature $\frac{dP}{dV}=0$ only when $P=\frac{a}{27b^2}$

Answer 1

The correct option is D Below critical temperature $\frac{dP}{dV}=0$ only when $P=\frac{a}{27b^2}$

Below the critical temperature $\frac{dP}{dV}=0$ for all values of V and P when the liquid is in equilibrium with its saturated vapours i.e. $P_{sat}$ is constant at given temperature (an isothermal process).