Critical point

An elementary understanding of chemistry teaches that matter is made up of several states and that different elements, compounds and molecules have several critical points and can transition between different states, including  liquid, solid and gas.

Between each stage of matter, conditions are created in which matter freely moves between one state and the other. This is known as the critical point of a material.

The vapour-liquid critical point is when a material is pressurized and heated to a point where it can no longer remain either a liquid or a gas and becomes a supercritical fluid.

Critical point graph

The vapour-liquid critical point is actually the point at which vaporization of the material can no longer occur.

The Mathematical Principle

In essence, what happens mathematically is that the every substance has a vapour-liquid critical point temperature. The vapour-liquid critical point is actually the point at which vaporization of the material can no longer occur (zero).

Because the pressure can be readily manipulated by technology, one can bring many materials to their vapour-liquid critical point. At this point, no more vapour molecules will be formed, but given the right conditions a solid can be formed using the pressure.

The elements involved in the mathematical equation used to calculate the vapour-liquid critical point of a material are the reduced temperature (Tr), the reduced pressure (Pr), the reduced volume (Vr), and the universal gas constant (R).

Principle of Corresponding States

This idea denotes that all substances at equal reduced pressures and temperatures have equal reduced volumes. When you reduce the pressure and the temperature at the same time in a variety of substances, they will drop in volume at the same rate. The principle is a useful tool for normal applications, but large values of reduced pressure seem to create inaccuracies.

The vapour-liquid critical point is sometimes confused with the boiling point (saturation temperature) of a material, but this is usually a much lower temperature.

The International Union of Pure and Applied Chemistry (IUPAC) can help you find out more information about the physical sciences and getting into a science career.

Image: wikipedia

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