What is Critical Temperature?
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 Tc.
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
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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 (Pc) of a few substances are shown in table 1 and the critical temperature (Tc) 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 (Tc) and critical pressure (Pc). 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.
FAQs on Critical Temperature
1. What is Critical Pressure and Critical Temperature?
Critical pressure can be defined as the pressure that is applied to a substance at the critical temperature of the same substance to convert it from the gaseous to the liquid state or to liquefy the substance. Whereas, the critical temperature can be defined as the temperature for a substance above which a substance cannot be liquified from its vapour or gaseous state no matter what amount of pressure is applied. For e.g. an atmospheric pressure of 111.3 atm is the required amount of pressure (critical pressure) for Ammonia(NH3) to be converted into the liquified form at the temperature of 405.5K (critical temperature).
2. What are the Critical Point and Triple Point?
The critical point of a substance is the point when the temperature of the substance is also the critical temperature (i.e. the highest temperature above which the substance cannot be liquified) and the pressure acting on the substance is also the critical pressure (i.e. the pressure at which a substance is liquified). The triple point is the point when a substance can exist in all three states of matter, i.e. solid, liquid, and gaseous. In other words, it is the temperature and pressure of a substance when it can exist in all three states (solid, liquid, and gaseous).
3. How can we graphically represent critical temperature?
On the graphical representation of critical temperature, pressure is plotted on the Y-axis and temperature is plotted on the X-axis. This means that the critical temperature can be understood from the value of the X-axis. The value of the Y-axis denotes the value of pressure that we need to obtain the liquified state of a substance. This happens at the point when a critical temperature is being applied to the substance. Hence the pressure obtained is the critical pressure. Thus we can say that when the temperature of a substance is critical, at that critical point the pressure acting on the substance is the critical pressure.
4. List the critical temperatures and critical pressures of some substances?
The critical temperatures for ammonia, carbon dioxide and nitrogen are 405.5 K, 304.19 K, and 126.2 K respectively. For water, the critical temperature is 647.09 K and for helium, it is 5.19 K. For Chlorine and Lithium, the critical temperatures are 416.9 K and 3220 K respectively.
Similarly, the critical pressure for ammonia, carbon dioxide and nitrogen is 111.3 atm, 72.8 atm and 33.5 atm respectively. The critical pressure for water is 217.7 atm and for helium, it is 2.24 atm. For chlorine and lithium, the critical pressure is 76.0 atm and 652 atm respectively.
5. What are some fun facts about critical temperature?
The critical temperature of the water is 647K and the critical pressure is 218 atm. When it is heated beyond this point, its liquid and gaseous states disappear and water becomes a supercritical fluid. At this point, it functions as a dissolving medium. At such high temperatures, the molecules of water are seen to be interacting with nonpolar molecules. Supercritical water has various uses. Because it possesses the ability to dissolve a nonpolar substance, it is used as a combustion medium to destroy toxic waste substances. A supercritical-water reactor as a closed system is thus used to burn hazardous organic waste without releasing toxic emissions into the air.
6. What is the meaning of critical temperature?
For any substance, the temperature beyond which it cannot be liquified further from its gaseous state, irrespective of the pressure that is applied to it, is called the critical temperature.
7. What do you mean by critical point and triple point?
When the temperature of a substance is the critical temperature and the pressure acting upon it is the critical pressure then it is said to be a critical point for the substance. The triple point is when a particular substance at a given temperature and pressure can exist in all three states, i.e. the solid, liquid, and vaporized states.
