Critical Temperature

What is Critical Temperature?

The critical temperature of a substance can be defined as the highest temperature at which the substance can exist as a liquid. At temperatures above the critical temperature, the substance in question (in its vapour/gaseous state) can no longer be liquified, regardless of the amount pressure applied to it.

A graph describing the triple point (the point at which a substance can exist in all three states of matter) and the critical point of a substance is provided below. It can be noted that the graph is plotted with pressure on the Y-axis and temperature on the X-axis. Therefore, the critical temperature can be obtained from the X-axis value of the critical point. The corresponding Y-axis value of the critical point, which is the pressure required to liquefy a substance at the critical temperature, is known as the critical pressure of that substance.

Critical Temperature

Critical Temperature

The critical point of a liquid was discovered by the French physicist Charles Cagniard de la Tour in the year 1822. He observed that carbon dioxide could be liquified at a temperature of 31oC when 73 atm of pressure was applied, but it could not be liquified at higher temperatures, even when pressures above 3000 atm were applied. This maximum temperature at which substances could exist in the liquid phase was later named “Critical Temperature” by Dmitri Mendeleev in the year 1860.

Critical Temperatures and Pressures of Some Substances

The critical temperatures and pressures of some substances are listed in a tabular column below. It can be noted that the temperature value corresponding to the critical point of a substance is denoted by Tc whereas the corresponding pressure is described by the symbol Pc.

Substance Critical Pressure (Pc) Critical Temperature (Tc)
Ammonia (NH3) 111.3 atm 405.5 K
Carbon Dioxide (CO2) 72.8 atm 304.19 K
Nitrogen (N2) 33.5 atm 126.2 K
Water (H2O) 217.7 atm 647.09 K
Helium (He) 2.24 atm 5.19 K
Chlorine (Cl) 76.0 atm 416.9 K
Lithium (Li) 652 atm 3220 K
Gold (Au) 5000 atm 7250 K

From the table provided above, it can be observed that metals generally have very high Tc and Pc values. On the other hand, helium has one of the lowest critical temperatures (valued at 5.19K).

Points to remember:

  • The critical temperature of water is 647 K
  • The critical temperature of steel is 1533.15 K
  • Boiling point and critical point are not the same. The boiling point can be defined as the temperature at which a liquid is converted to vapour whereas the critical point is achieved when temperature and pressure are very high and the material is gaseous as well as the liquid state.

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