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

(N/A) Below the critical temperature in a closed vessel,the surface of separation (meniscus) between the liquid and its vapour is clearly visible.
As the temperature approaches the critical point,the density of the liquid decreases due to expansion,and the density of the vapour increases due to compression.
At the critical temperature,the densities of the liquid and the vapour become equal,and the surface of separation disappears. The liquid and gaseous states are no longer distinguishable.
The fluid,which is now a homogeneous mixture,is called a supercritical fluid. Any fluid above its critical temperature and pressure is referred to as a supercritical fluid.
Supercritical fluids are used for the extraction of organic substances. For example,$CO_{2}$ above $31.1^{\circ}C$ and above $73.8 \ bar$ pressure has a density of about $1 \ g/cm^{3}$. It is used to dissolve caffeine from coffee beans as a safer alternative to chlorofluorocarbons.