What is the state of matter beyond the critical point?

oxivixo

Starting from a gas state and reaching the liquid state by getting around the critical point in a p-T phase diagram, what happens (qualitatively) to the molecules (or atoms)? Does a fraction of them start to form bonds?

This concept of "indistinguishable" liquid and gas is new to me.

atyy

Goldenfeld gives the analogy that it's like two rooms, but the wall between them doesn't go all the way to the top. So whether you see "latent heat" associated with a phase transition is path dependent.

Typically, a phase boundary is defined by a discontinuity in some equilibrium thermodynamic quantity. It is by that sort of definition that whether one encounters a phase boundary between liquid and gas is path dependent.

There was however this interesting article in Physics Today about how dynamical quantities can provide an approximate boundary between liquid and gas above the critical point.

What separates a liquid from a gas?
Vadim V. Brazhkin and Kostya Trachenko