Critical Point and Supercritical Fluids (SCF)
If a liquid is heated in a sealed container, boiling doesn’t occur. The temperature and vapor pressure rise continuously many times the atmospheric pressure. Then, we observe the followings;
1) density of liquid decreases; that of vapor increases and eventually the two densities become equal,
2) surface tension of the liquid approaches zero. The meniscus between the liquid and vapor becomes less distinct and eventually disappears. The fluid is not neither a liquid nor a gas now.
The critical point is the point at which these conditions are reached and the liquid and vapor become indistinguishable. At this point, the temperature is the critical temperature (Tc) and the pressure is the critical pressure (Pc).