On a phase diagram, which term describes the point where solid, liquid, and gas coexist in equilibrium?

On a phase diagram, which term describes the point where solid, liquid, and gas coexist in equilibrium?

• A. The critical point
• B. The boiling point
• C. The melting point
• D. The triple point

Answer: (D)

On a phase diagram, the point where solid, liquid, and gas coexist in equilibrium is the triple point. This is a unique temperature and pressure where all three phases are simultaneously stable, so you can have solid, liquid, and gas present at the same time and in balance. The boundaries on the diagram show conditions where two phases are in equilibrium (solid-liquid, liquid-gas, or solid-gas), and the triple point is where all three boundaries meet. Understanding the other terms helps place this in context: the boiling point is where liquid turns into gas at a given pressure, marking the boundary between liquid and gas; the melting point is where solid becomes liquid at a given pressure, marking the boundary between solid and liquid; the critical point is the end of the liquid-gas boundary, beyond which liquid and gas are no longer distinct and a supercritical fluid forms. For water, the triple point occurs at a precise low temperature and pressure, illustrating that all three phases can coexist only under specific conditions.

On a phase diagram, the point where solid, liquid, and gas coexist in equilibrium is the triple point. This is a unique temperature and pressure where all three phases are simultaneously stable, so you can have solid, liquid, and gas present at the same time and in balance. The boundaries on the diagram show conditions where two phases are in equilibrium (solid-liquid, liquid-gas, or solid-gas), and the triple point is where all three boundaries meet.

Understanding the other terms helps place this in context: the boiling point is where liquid turns into gas at a given pressure, marking the boundary between liquid and gas; the melting point is where solid becomes liquid at a given pressure, marking the boundary between solid and liquid; the critical point is the end of the liquid-gas boundary, beyond which liquid and gas are no longer distinct and a supercritical fluid forms. For water, the triple point occurs at a precise low temperature and pressure, illustrating that all three phases can coexist only under specific conditions.