Based on the phase diagram of substance X, which point represents the triple point where solid, liquid, and gas phases coexist in equilibrium?

• A. The point where only the liquid and gas phases are in equilibrium
• B. The point at the highest temperature and pressure on the diagram
• C. The point where the solid-liquid and liquid-gas boundaries intersect
• D. The point at the lowest pressure and temperature on the diagram

Answer: (C)

Verified step by step guidance

1. Understand that a phase diagram shows the conditions of temperature and pressure under which different phases (solid, liquid, gas) of a substance exist.
2. Identify the lines (boundaries) on the phase diagram that represent equilibrium between two phases: solid-liquid, liquid-gas, and solid-gas.
3. Recognize that the triple point is the unique condition where all three phases coexist in equilibrium simultaneously.
4. Locate the point on the phase diagram where the solid-liquid boundary line intersects with the liquid-gas boundary line, as this intersection represents the triple point.
5. Confirm that this point is not necessarily at the highest or lowest temperature and pressure, but specifically where the three phase boundaries meet.