Explain the relationship between the fluid state,liquid-gas state,critical temperature,and the vapour form of substances.

(N/A) There is a continuity between the gaseous and liquid states. The term 'fluid' is used for either a liquid or a gas to recognize this continuity. Thus,a liquid can be viewed as a very dense gas.
$(i)$ $A$ 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.
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 the two states.
At the critical temperature,a liquid passes into the gaseous state imperceptibly and continuously; the surface separating the two phases disappears.
Substance Vapour: $A$ gas below its critical temperature can be liquefied by applying pressure and is called the 'vapour' of the substance. For example,carbon dioxide gas below its critical temperature is called carbon dioxide vapour.
Critical temperature and pressure of some substances:

Substance	$T_{C} / K$	$p_{C} / bar$	$V_{C} / dm^{3} mol^{-1}$
$H_{2}$	$33.2$	$12.97$	$0.0650$
$He$	$5.3$	$2.29$	$0.0577$
$N_{2}$	$126.0$	$33.9$	$0.0900$
$O_{2}$	$154.3$	$50.4$	$0.0744$
$CO_{2}$	$304.10$	$73.9$	$0.0956$
$H_{2}O$	$647.1$	$220.6$	$0.0450$
$NH_{3}$	$405.5$	$113.0$	$0.0723$