What is supercritical CO₂ and how does it differ from liquid and gaseous CO₂? (asked in MU June 2025 KT)

Answer: CO₂ above its critical temperature (31.1 °C) and critical pressure (about 73.8 bar) is a supercritical fluid: it has liquid-like density and dissolving power with gas-like viscosity and diffusion, and no surface tension.

Above the critical point, liquid and gas cannot be distinguished; the substance is a single supercritical fluid (SCF). For CO₂, \(T_c=31.1\) °C and \(P_c\approx73.8\) bar (72.8 atm), easily reached, so scCO₂ is a common green solvent.

Differences from liquid and gaseous CO₂:

• Density (and so dissolving power) is close to that of a liquid, unlike the gas.
• Viscosity is low and diffusivity high, like a gas, so it penetrates porous solids quickly, unlike the liquid.
• No surface tension and no liquid–gas boundary.
• Its solvent power can be tuned by small changes of pressure and temperature; on releasing pressure it turns to gas and leaves no residue.

Applications: decaffeination of coffee and tea; extraction of flavours, essential oils and hop extracts; dry cleaning of clothes (replacing perchloroethylene); solvent for polymerisation and hydrogenation reactions; cleaning of electronic parts; dyeing of polyester without water.