Which point on a phase diagram represents the temperature and pressure at which all three phases (solid, liquid, and gas) coexist in equilibrium?

• A. Critical point
• B. Boiling point
• C. Melting point
• D. Triple point

Answer: (D)

Verified step by step guidance

1. Understand that a phase diagram shows the states of matter (solid, liquid, gas) of a substance at different temperatures and pressures.
2. Identify key points on the phase diagram: the melting point (solid-liquid boundary), boiling point (liquid-gas boundary), and critical point (end of the liquid-gas boundary).
3. Recognize that the triple point is a unique condition where all three phases—solid, liquid, and gas—exist simultaneously in equilibrium.
4. Recall that the triple point is represented as a single point on the phase diagram where the boundaries of all three phases intersect.
5. Conclude that the triple point is the correct answer because it specifically denotes the temperature and pressure at which solid, liquid, and gas coexist.