The triple point of water is the temperature and pressure at which water coexists as which phases?

• A. Ice point
• B. Solid, Liquid, and Gas
• C. Boiling point
• D. Freezing point

Answer: (B)

The main idea is that the triple point is the special condition where all three fundamental phases—solid, liquid, and gas—are in equilibrium with each other. For water, this happens at about 0.01°C and 611.7 pascals, where ice, liquid water, and water vapor can coexist in balance. This precise point is useful for calibrating thermometers and defining temperature scales because it fixes a unique combination of temperature and pressure.

Other terms describe equilibria between only two phases. The ice point refers to solid and liquid in balance (around 0°C at 1 atmosphere), the boiling point is liquid and gas in balance at a given pressure, and the freezing (melting) point is when solid and liquid are in balance. They don’t involve all three phases simultaneously.