Subcritical to supercritical: Journey of vapor to gas

Except at the triple point where all three phases coexist, the commonly known phases solid, liquid, and vapor are separated by phase boundaries. However, the liquid-vapor boundary terminates in an endpoint at critical temperature and critical pressure. This is a critical point. Any substance is characterized by a critical point which is obtained at specific conditions of pressure and temperature. When a compound is subjected to a pressure and a temperature higher than its critical point, the fluid is said to be " supercritical ".

When liquid and vapor coexist separated by phase boundary, the compound exists in a subcritical state as vapor.

Vapor vs gas

Phase diagram of water

Credit: Google

Vapor is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature. The same substance becomes gas above the critical temperature. Above the critical temperature, vapor cannot be liquefied by increasing pressure. All differences between liquid and vapor disappear beyond the critical point.

Explanation

Let us take water as an example and explain what do we know [217.75 atm / 647.096 K] 

Credit: Google

At the critical point, at equal reduced pressures and temperatures, both phases have equal specific reduced volumes. [What is reduced properties? In thermodynamics, the reduced properties of a fluid are a set of state variables scaled by the fluid's state properties at its critical point. The reduced pressure is defined as its actual pressure divided by its critical pressure and the reduced temperature of a fluid is its actual temperature, divided by its critical temperature]