Temperature and pressure above which a substance cannot condense into a liquid.

• A. Phase Diagram
• B. Phase Boundaries
• C. Critical Point
• D. Triple Point

Answer: (C)

The key idea is the critical point on a temperature–pressure map. At the critical temperature, the distinction between liquid and gas disappears, and beyond this point, increasing pressure cannot turn the gas into a liquid. The substance becomes a supercritical fluid with properties between those of a liquid and a gas.

On a phase diagram, the boundary between liquid and gas ends at this critical point. Below that temperature, you can condense a gas back into a liquid by increasing pressure; above it, no amount of pressure will produce a separate liquid phase.

The other terms are handy for understanding the diagram: a phase diagram is the whole chart, phase boundaries are the lines that separate phases, and the triple point is where solid, liquid, and gas can coexist. But the scenario described—where condensation into a liquid is no longer possible regardless of pressure—is defined by the critical point.