In the context of phase diagrams, what is the triple point?

• A. The pressure at which a liquid and its vapor are in equilibrium at the boiling point.
• B. The temperature and pressure at which all three phases (solid, liquid, and gas) of a substance coexist in equilibrium.
• C. The point on a phase diagram where the solid and liquid phases are indistinguishable.
• D. The temperature at which a substance transitions directly from solid to gas without passing through the liquid phase.

Answer: (B)

Verified step by step guidance

1. Understand that a phase diagram is a graphical representation showing the conditions of temperature and pressure under which distinct phases (solid, liquid, gas) of a substance exist.
2. Recognize that the triple point is a unique set of conditions where all three phases—solid, liquid, and gas—coexist in thermodynamic equilibrium.
3. Note that at the triple point, the substance can simultaneously exist as a solid, liquid, and gas, and any small change in temperature or pressure will shift the balance toward one or two phases.
4. Distinguish the triple point from other important points on the phase diagram, such as the boiling point (liquid-vapor equilibrium) and the critical point (where liquid and gas phases become indistinguishable).
5. Summarize that the triple point is defined by a specific temperature and pressure unique to each substance, marking the only condition where all three phases coexist stably.