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Critical Temperature — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementCritical TemperatureFRCA Primary

A nitrous oxide cylinder is maintained at 40.0°C. The critical temperature of nitrous oxide is 36.5°C. As the pressure is increased while the temperature remains constant, which statement best describes the significance of the critical temperature?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: B — Above this temperature, nitrous oxide cannot be liquefied by pressure alone

Explanation lettering: C = shown as A · D = shown as C · A = shown as D

B is correct. The critical temperature is the highest temperature at which a substance can be converted into a distinct liquid phase by applying pressure. At 40.0°C, nitrous oxide is above its critical temperature of 36.5°C, so compression alone cannot liquefy it; sufficiently high pressure instead produces a supercritical fluid. Below the critical temperature, nitrous oxide may exist as liquid, vapour, or a liquid–vapour mixture depending on pressure, so C and E are false. It is not necessarily liquid above the critical temperature, excluding A. The normal boiling point is the temperature at which saturated vapour pressure equals atmospheric pressure and is not the critical temperature, excluding D. In a cylinder below 36.5°C, liquid and vapour can coexist, with pressure determined mainly by temperature until the liquid is exhausted.

Reference: Royal College of Anaesthetists. Guide to the FRCA Examination: The Primary, fourth edition, question 44 and answer commentary, 2013. https://www.rcoa.ac.uk/sites/default/files/documents/2025-08/RCoAPrimaryGuide2013%20FOR%20WEBSITE.pdf