Hint : To understand critical volume, we need to have a clear picture of the concepts of critical temperature and critical pressure. All these concepts are interconnected and complete each other. The term critical says a lot about the things we are going to cover.
Complete Step By Step Answer: When we talk about critical temperature, it is a term used to define the temperature of a substance above which the vapour of that substance cannot be liquefied irrespective of the amount of pressure you apply. It is very important to know that every substance has a critical temperature and you need to keep that in mind while experimenting with substances. With the knowledge of what critical temperature is, we can now define critical pressure. At the critical temperature, the vapour phase and the liquid phase of the substance coexist, now at the phase equilibrium curve, the pressure at that point is termed as the critical pressure. We can learn this from the pressure-temperature graph too. After understanding both critical temperature and critical pressure, we can now learn about critical volume. The molar volume of a gas at critical pressure and critical temperature is called the critical volume. The state at which this happens is called the critical state.
Note : To demonstrate all of the critical points, we have a phase diagram of substances that have a critical point. It is the point where the gas phase and the liquid phase of the substance have the same density and so cannot be distinguished.