What changes at the critical point between a liquid and a gas?

At the critical point, the distinction between a liquid and its vapour phase is eliminated because the two become identical.12

Key changes and characteristics at this point include:

• Density Equalization: As a substance approaches the critical point, the density of the liquid decreases while the density of the vapour increases. At the critical point, these densities become equal, causing the physical boundary between the two phases to vanish.13
• Loss of Distinct Phases: Above the critical temperature and pressure, the substance exists as a single fluid state. Because the phases are no longer distinct, a substance can transition from a liquid-like state to a gas-like state through a smooth, continuous change in properties rather than an abrupt phase change.2
• Optical Changes: Near the critical point, the compressibility of the substance becomes extremely high, allowing for large local fluctuations in density. These fluctuations scatter light effectively, often causing a normally transparent liquid to appear opaque and dark brown.3
• Supercritical Fluid: Beyond these conditions, the substance is referred to as a supercritical fluid, which possesses properties intermediate between those of a gas and a liquid.4