Intermediate Topic 250 of 1102 States of Matter & Solutions
Critical Temperature
The temperature above which a gas cannot be liquefied by pressure alone.
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In-depth explanation of Critical Temperature
The critical temperature is the temperature above which a gas cannot be liquefied by pressure alone, no matter how much it is compressed. Above this temperature the thermal energy of the molecules is too large for attractions to hold them together as a separate liquid phase. Each substance has its own critical temperature, which is higher for molecules with stronger intermolecular forces, such as polar molecules and those that hydrogen bond. Knowing the critical temperature is essential for gas storage and liquefaction, because the gas must be cooled below this value before compression can produce a liquid. Critical temperature is one of the critical constants that define the critical point.
Going deeper: Critical Temperature
Key ideas: Critical constants relate to van der Waals constants: Tc = 8a/(27Rb), Pc = a/(27b^2), Vc = 3b. Examples: CO2 has Tc = 31 C and Pc = 73 atm; N2 has Tc = -147 C. Gases with Tc above room temperature, such as propane and CO2, are liquefied by pressure alone at 25 C; those below need cooling first.
Common mistakes and connections: Mistakes include saying a gas above Tc turns into a solid, or that it can still be liquefied with enough pressure. Another is mixing up critical temperature with boiling point; Tc is always higher. Stronger intermolecular forces give higher Tc. Links to supercritical fluids, phase diagrams and liquefaction of gases.
Visualization of Critical Temperature
Above the critical temperature (304 K for CO2) no amount of compression produces a liquid; at Tc the isotherm shows a flat inflection. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The critical temperature of water is 374°C, while that of oxygen is about -118°C, so oxygen must be cooled far below room temperature before it can be liquefied.
Frequently asked questions about Critical Temperature
What is Critical Temperature?
The temperature above which a gas cannot be liquefied by pressure alone.
Is Critical Temperature a beginner, intermediate or advanced topic?
Critical Temperature 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 Temperature?
Study these earlier topics in States of Matter & Solutions first: Real Gases, Van der Waals Equation, Compressibility Factor.
What comes after Critical Temperature?
The next topics in States of Matter & Solutions are Joule-Thomson Effect and Liquefaction of Gases.
Can I practise Critical Temperature with quizzes?
Yes. The interactive lesson includes a self-check question on Critical Temperature, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Critical Temperature fits in States of Matter & Solutions
Critical Temperature is topic 31 of 109 in States of Matter & Solutions.