Why does the boundary between liquid phase and gaseous phase disappear on heating a liquid upto critical temperature in a closed vessel? In this situation what will be the state of the substance?

In a closed vessel, below the critical point, the surface ofseparation between the liquid and its vapour is clearly visible but at the critical point the densities of liquid and the vapours become equal and the boundaries of their separation which existed below the critical point now disappears at the critical point.

The fluid at this stage is called “Supercritical fluid”. These super critical fluids have the efficiency of dissolving the organic substances.