2. Effect of pressure on liquids and molecular fluids

The phase diagram (P, T ) of any stable substance shows three coexistence curves: solid-liquid, solid-gas and liquid-gas. The liquid-gas coexistence curve ends at a critical point whose pressure-temperature coordinates are characteristic of each substance. Beyond this point, the notion of gas and liquid no longer exists, and becomes confused with the definition of fluid. Figures 2 and 3 show how the critical point is represented in the (P, V, T) diagram. Remarkably, the representation f (V, T) shows the restricted limit of the liquid domain. It turns out that, in the Universe, the domain of existence of the liquid state appears as an anomaly, difficult to describe theoretically, compared to the enormous...