[Chemistry Class Notes] on Critical Temperature Pdf for Exam

We realise the meaning of temperature when we look at boiling water or find ourselves boiling in hot fever! We are intrigued by the mercury going up and down in the doctor’s thermometer. However, have we ever wondered or heard of a concept called critical temperature?

The highest temperature of a substance at which it can be condensed and remain in a liquid state is known as the critical temperature of a substance. In other words, the temperature above or at which a substance cannot be liquified from its vapour or gaseous state irrespective of what amount of pressure is applied on it, then that temperature is known as the critical temperature. It is denoted as T_c.

 

A substance can be liquified only at a suitable temperature and it becomes more difficult to do so by increasing the temperature because as the temperature increases then the Kinetic Energy of the particles also increases which makes up the substance. Thus, a substance can only be converted to liquid state from gaseous state up to a certain temperature (critical temperature) and not above it.

What happens When We Heat above the Critical Temperature?

As we continue to raise the temperature of a substance, its molecules start moving and colliding with each other at a rapid speed. At this point, two things happen – the density of the liquefied state of the substance goes down and the density of the vapourized or gaseous state increases. At a particular temperature, the vapour pressure increases to such an extent that the density of the vapour becomes equal to the density of the liquid. Hence, the vapourized and liquified states of the substance become almost the same or appear to be indistinguishable. This is known as the critical temperature. 

At this critical temperature, the density and various other properties of the liquid and the vapour become the same. The molecular forces are so high at this point, that no matter what critical pressure is applied, it becomes impossible to condense the substance into a calmer, liquified form. 

Graphical Representation of Critical Temperature

 

The above graph shows the graphical representation of the critical temperature. The graph has been plotted with pressure on the Y-axis and temperature on the X-axis which signifies that the critical temperature from the graph can be obtained from the value of X-axis and on the other side the value of the Y-axis signifies the value of pressure that is required to obtain the liquid state of the substance at the critical point when the temperature on the substance is the critical temperature then this pressure obtained is also known as the critical pressure (denoted by Pc) of the substance. In other words, critical pressure is the pressure acting on the substance when the temperature of the substance is the critical temperature at the critical point.

 

The graph also shows the triple point which is the point when the temperature and pressure of the substance remain at a value that makes the substance exist in all the three states of matter, i.e. solid, liquid and gaseous state. 

Example (Critical Temperature and Critical Pressure):

 

 

The critical pressures (P_c) of a few substances are shown in table 1 and the critical temperature (T_c) of the same substances is shown in table 2. From the above-shown tables, we can also understand that a substance Ammonia(NH₃) can not be liquified beyond the temperature of 405.5K (critical temperature) obtained by applying a pressure of 111.3 atm (critical pressure). Generally, metals have very high values of critical temperature (T_c) and critical pressure (P_c). Helium (a noble gas)  has one of the lowest critical temperatures (5.19K) and the lowest critical pressure (2.24 atm).

Fun Facts

When water is heated beyond its critical temperature (647K) and critical pressure(218 atm) then it possesses an unusual behaviour. Above the critical temperature, the difference between the liquid and gaseous states of water disappears, and water becomes a supercritical fluid. The ability of water to act as a polar solvent (a dissolving medium) also changes when it is subjected to temperature and pressure beyond the critical point. When water is heated more, it is much more likely that the molecules seem to interact with nonpolar molecules. Supercritical water can be used as a combustion medium for destroying toxic wastes as it possesses the ability to dissolve nonpolar substances. When organic wastes are mixed with oxygen insufficiently dense supercritical water and combusted in the fluid than the flame actually burns underwater and oxidation in supercritical water can also be helpful in destroying a huge variety of hazardous organic substances considering the advantage that a supercritical-water reactor is a closed system, so there are no emissions released into the air.