What happens at the critical point, and how is a supercritical fluid different from a liquid or gas?

• A. The liquid and gas become completely separate.
• B. The substance becomes a solid.
• C. The pressure becomes infinite.
• D. The liquid and gas become indistinguishable; a supercritical fluid has properties of both and cannot be distinctly separated.

Answer: (D)

At the critical point the boundary between liquid and gas disappears, so the two phases become indistinguishable. Beyond this point, the substance exists as a single phase called a supercritical fluid, which blends properties of both liquids and gases. It flows and expands like a gas, but its density is more like a liquid, enabling it to dissolve substances effectively. There’s no distinct liquid-vapor boundary because the critical temperature and pressure have been reached, so you can’t separate a separate liquid and gas phase there. For example, carbon dioxide becomes supercritical above about 31°C and 73.8 atm, and supercritical CO2 is used as a solvent precisely because it combines gas-like diffusion with liquid-like dissolving power. The other options don’t describe what happens: the phases don’t become completely separate, the substance doesn’t turn into a solid, and the pressure doesn’t become infinite.