Critical and Pseudocritical Temperature

Image via Wikipedia Critical Temperature is the temperature above which, no matter how much pressure you apply, you cannot force a gas to become a liquid. Interestingly enough, though, if you apply sufficiently high pressures, you can form a solid. Essentially, distinct liquid and solid phases of a substance no longer exist.

If you measure the vapour pressure of a substance at the critical temperature, that pressure is called the critical pressure. Alternatively it could be defined as the pressure which is required to liquefy a vapour at its critical temperature.

A substance is a vapour when it is in equilibrium with the substance in another phase, and a gas when there is no liquid or solid present. Therefore, by definition, except at the extremely high pressures mentioned above, any substance above its critical temperature, is a gas. A liquid does not have to boil, nor a solid to sublime (change state directly from solid to vapour/gas-Ed.) to form a vapour. You can draw a serious of lines, plotted on a graph where the x-axis shows volume, and the y-axis shows pressure, which correspond to different temperatures and called isotherms, which demonstrate what will happen to a substance as you increase temperature with a given volume (or pressure). The one with most relevance of course is nitrous oxide...(see here).

Pseudo-critical temperature is the critical temperature of a mixture of gases. In anaesthesia it is commonly used to describe the temperature at which a 50:50 mixture of oxygen and nitrous oxide separates (laminates) forming liquid nitrous oxide and gaseous oxygen, which occurs at (depending on the pressure) temperatures in the range -7 to -5.5 degrees Celsius in cylinders, and lower temperatures in a pipeline (due to lower pressures) at around -20 degrees Celsius.