Which of the following best defines the critical pressure in a phase diagram?

• A. The pressure at which a solid melts to become a liquid.
• B. The pressure at which all three phases coexist in equilibrium.
• C. The minimum pressure required to liquefy a gas at its critical temperature.
• D. The pressure at which a liquid boils at its normal boiling point.

Answer: (C)

Verified step by step guidance

1. Understand that the critical pressure is a specific term used in phase diagrams related to the behavior of substances near their critical point.
2. Recall that the critical point is the temperature and pressure at which the distinction between liquid and gas phases disappears, meaning the substance becomes a supercritical fluid.
3. Recognize that the critical pressure is defined as the minimum pressure required to liquefy a gas at its critical temperature, which is the highest temperature at which a gas can be converted to a liquid by pressure alone.
4. Differentiate this from other phase diagram terms: the melting point pressure (solid to liquid), the triple point pressure (where solid, liquid, and gas coexist), and the normal boiling point pressure (usually 1 atm).
5. Conclude that the critical pressure specifically refers to the pressure needed to liquefy a gas at the critical temperature, making it a key parameter in understanding phase transitions near the critical point.