Explain relation between Fluid State, Liquid - Gas State, Critical temperature and Vapour form of substances.

Explanation

Thus there is continuity between the gaseous and liquid state. The term fluid is used for either a liquid or a gas to recognise this continuity. Thus a liquid can be viewed as a very dense gas.
Thus (i) a liquid and gas can be distinguished only when the fluid is below its critical temperature and (ii) its pressure and volume lie under the dome. Since in that situation liquid and gas are in equilibrium and a surface separating the two phases is visible. In the absence of this surface there is no fundamental way of distinguishing between two states.
At critical temperature, liquid passes into gaseous state imperceptibly and continuously, the surface separating two phases disappears.
Substance of (carbon dioxide of) vapour : Gas below the critical temperature can be liquified by applying pressure, and is called .substance vapour. of the substance.
..Carbon dioxide gas below its critical temperature is called carbon vapour...