Which of the following best describes a phase equilibrium in a phase diagram?

• A. All three phases (solid, liquid, gas) coexist at any point on the phase diagram.
• B. One phase completely converts into another until only a single phase remains.
• C. The temperature and pressure are changing rapidly, causing continuous shifts between phases.
• D. The rates of transition between two phases are equal, resulting in constant amounts of each phase.

Answer: (D)

Verified step by step guidance

1. Understand that a phase equilibrium occurs when two phases coexist without net change over time, meaning the system is stable at certain temperature and pressure conditions.
2. Recognize that at phase equilibrium, the rate at which one phase transforms into another is exactly balanced by the reverse process, so the amounts of each phase remain constant.
3. Recall that on a phase diagram, lines (called phase boundaries) represent conditions where two phases coexist in equilibrium, such as solid-liquid or liquid-gas boundaries.
4. Note that the triple point is a unique condition where all three phases (solid, liquid, and gas) coexist in equilibrium simultaneously, but this is a specific point, not any point on the diagram.
5. Conclude that phase equilibrium is best described by the balance of transition rates between phases, not by continuous change or complete conversion to a single phase.