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 vapor or gaseous state irrespective of what amount of pressure is applied on it then that temperature is known as 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.
When the temperature of the substance is raised, the molecules start a more rapid movement, then there are two outcomes: (1) The density of the liquid goes down, and (2) the vapor pressure goes up. Thus the density of the vapor goes up. The vapor pressure at some temperature becomes so high that the density of the vapor is equal to the density of the liquid, at this point the vapor and liquid both become indistinguishable. This is, of course, the critical temperature. This temperature (critical temperature) is the temperature at which the density (and all other properties) of the liquid and the vapor become the same. The molecular movement is so high that the attractive forces acting on it are insufficient to cause them to condense into a liquid form.
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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 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 which makes the substance exist in all the three states of matter, i.e. solid, liquid and gaseous state.
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 on applying a pressure of 111.3 atm (critical pressure). Generally, metals have very high values of critical temperature (Tc) and critical pressure (Pc). And, Helium (a noble gas) has one of the lowest critical temperatures (5.19K) and the lowest critical pressure (2.24 atm).
When water is heated beyond its critical temperature (647K) and critical pressure(218 atm) then it possesses an unusual behavior. 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.
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 vapor 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 is 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).