On a typical phase diagram, what does point D most commonly represent?

• A. The melting point, where solid turns into liquid
• B. The boiling point, where liquid turns into gas at 1 atm
• C. The critical point, where the liquid and gas phases become indistinguishable
• D. The triple point, where solid, liquid, and gas phases coexist

Answer: (C)

Verified step by step guidance

1. Understand that a phase diagram maps the states of matter (solid, liquid, gas) of a substance as a function of temperature and pressure.
2. Identify key points on a typical phase diagram: the melting point (solid-liquid boundary), the boiling point (liquid-gas boundary at 1 atm), the triple point (where solid, liquid, and gas coexist), and the critical point (where liquid and gas phases become indistinguishable).
3. Recall that the melting point is the temperature at which solid turns into liquid at a given pressure, usually along the solid-liquid boundary line.
4. Recognize that the boiling point is the temperature at which liquid turns into gas at 1 atm pressure, located on the liquid-gas boundary line.
5. Understand that the critical point is the end point of the liquid-gas boundary line, beyond which the liquid and gas phases merge into a supercritical fluid and cannot be distinguished.