Gas and Vapour

The words "gas" and "vapour" are often used loosely as if interchangeable, but they have a precise physical distinction rooted in a substance's CRITICAL TEMPERATURE -- the temperature above which no amount of pressure, however large, can liquefy that substance; to liquefy it, it must first be cooled below this critical temperature.

A substance in its gaseous phase while ABOVE its critical temperature is called a gas -- it genuinely cannot be liquefied by pressure alone. A substance in its gaseous phase while BELOW its critical temperature is called a vapour -- it CAN be liquefied simply by increasing the pressure enough. Table 7.5 lists critical temperatures for some familiar gases: nitrogen's critical temperature is -147 °C, so nitrogen gas at room temperature genuinely cannot be squeezed into a liquid by pressure alone -- it must first be cooled below -147 °C. Water vapour, by contrast, has a very high critical temperature (374 °C), so at ordinary temperatures water vapour in the …