What are critical properties and why are they important?

Critical properties (Tc, Pc, Vc) define the critical point where liquid and vapor become identical. Critical temperature (Tc): above this, no liquid exists regardless of pressure. Critical pressure (Pc): pressure at critical point. Critical volume (Vc): molar volume at critical point. Importance: gases cannot be liquefied above Tc by pressure alone, critical properties are used in equations of state (reduced properties), corresponding states principle relies on critical properties, and supercritical fluids (T>Tc, P>Pc) have unique properties useful for extraction.