Critical Temperature - Overview, Definition, Substances, FAQs

As the temperature of a gas rises, liquefaction becomes more difficult because higher and higher pressures are necessary to counteract the molecules' increasing kinetic energy. In fact, regardless of pressure, every substance has a temperature above which it can no longer be liquefied. The molecules have too much kinetic energy above the critical temperature for the intermolecular attraction interactions to keep them together in a separate liquid phase. Instead, the substance condenses into a single phase that totally fills the capacity of the container.

Regardless of the applied pressure, a substance cannot form a liquid above its critical temperature. The molecules have enough kinetic energy above the critical temperature to overcome the intermolecular attraction interactions. The critical pressure is the minimal pressure required to liquefy a substance at its critical temperature. A substance's critical point is the sum of its critical temperature and critical pressure. A material exists as a dense fluid called a supercritical fluid above the critical temperature and pressure, which resembles a gas in that it entirely fills its container but has the density of a liquid.

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What is the critical temperature?

A substance's critical temperature is defined as the maximum temperature at which it may exist as a liquid. The substance in question (in its vapour/gaseous condition) can no longer be liquefied at temperatures above the critical temperature, regardless of the amount of pressure applied to it.

What is critical pressure?

The pressure that corresponds to a substance's critical point is known as the critical pressure (or critical state). A material's critical point is the temperature and pressure scale point at which a liquid substance and its vapour may coexist. A substance cannot be liquefied with any amount of pressure at temperatures above its critical temperature. A substance's critical temperature can thus be described as the pressure that must be applied to a substance in order for it to liquefy at its critical temperature. The sign ‘PC' is frequently used to represent a substance's critical pressure.

Phase boundaries are recognised to separate the three most frequent forms of matter, namely solid, liquid, and gas. These phase boundaries are determined by the two fundamental factors that influence a substance's physical state — temperature and pressure. As a result, adjusting the temperature-pressure combinations can aid in determining a phase boundary. A substance's triple point, for example, is the point at which it can exist in all three states (solid, liquid, and gaseous). The temperature and pressure values at the triple points of various substances vary. The triple point of water, for example, corresponds to 0.01 degrees Celsius (or 273.16 Kelvin) and 4.58 mm of Hg.

Similarly, the phase boundary that separates a substance's liquid and vapour states can be calculated by finding a comparable temperature-pressure combination. This temperature-pressure combination is frequently referred to as the substance's critical point. The critical pressure of a substance is the pressure that corresponds to its critical point.

Critical Temperatures and Pressures of Some Substances

The critical temperatures and pressures of several compounds are listed in a tabular column below. It should be noted that the temperature value corresponding to a substance's critical point is denoted by Tc, while the accompanying pressure is marked by Pc.

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The table above shows that metals have relatively high Tc and Pc values in general. Helium, on the other hand, has one of the lowest critical temperatures (valued at 5.19K or -267.96 ˚C).

What is critical point?

Phase barriers vanish at the critical point, which is defined by a critical temperature Tc and a critical pressure pc. Other examples include mixes' liquid–liquid critical points.

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