Intermediate Topic 271 of 1102 States of Matter & Solutions
Critical Point
The end of the liquid-gas boundary beyond which a supercritical fluid exists.
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In-depth explanation of Critical Point
The critical point is the endpoint of the liquid-gas coexistence curve on a phase diagram, defined by a critical temperature and a critical pressure. Beyond this point the liquid and gas phases become indistinguishable and there is no longer a distinct boundary between them. At the critical point the surface tension of the liquid falls to zero and the meniscus disappears, because the densities of the liquid and vapour become equal. Above the critical point the substance is a supercritical fluid, which cannot be liquefied by pressure alone. The critical point of water is at 374°C and 218 atm.
Going deeper: Critical Point
Key ideas: At the critical point, Tc, Pc and Vc are specific to a substance: water 647 K and 218 atm, CO2 304 K and 73 atm, N2 126 K and 34 atm. Enthalpy of vaporization falls to zero, and the compressibility is infinite. Critical opalescence, a milky scattering of light, appears near this point.
Common mistakes and connections: A frequent error is describing the critical point as the highest boiling point. The liquid-gas line ends but the substance continues as a single fluid phase, and a substance can go from gas to liquid without a sharp transition by going around it. Related to critical temperature, supercritical fluids and van der Waals constants.
Visualization of Critical Point
The critical point ends the liquid-gas line, at 31 C and 73 atm for CO2; beyond it the liquid and gas become indistinguishable. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Water reaches its critical point at 374°C and 218 atm, above which liquid and gas can no longer be told apart.
Frequently asked questions about Critical Point
What is Critical Point?
The end of the liquid-gas boundary beyond which a supercritical fluid exists.
Is Critical Point a beginner, intermediate or advanced topic?
Critical Point is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Critical Point?
Study these earlier topics in States of Matter & Solutions first: Deposition, Phase Diagram, Triple Point.
What comes after Critical Point?
The next topics in States of Matter & Solutions are Supercritical Fluids and Phase Diagram of Water.
Can I practise Critical Point with quizzes?
Yes. The interactive lesson includes a self-check question on Critical Point, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Critical Point fits in States of Matter & Solutions
Critical Point is topic 52 of 109 in States of Matter & Solutions.