What is the triple point of water?

• A. The pressure and temperature where water coexists at all 3 states, solid, liquid, gas.
• B. The boiling point of water at 1 atm
• C. The melting point of ice
• D. The point where water becomes steam

Answer: (A)

The triple point is the unique pressure–temperature combination at which solid, liquid, and gaseous water are all in thermodynamic equilibrium. For water, this occurs at 0.01°C (273.16 K) and about 611.657 Pa (roughly 0.00604 atm). At this precise point, all three phases coexist; move away from it and you’ll have only two phases in equilibrium—two-phase regions on either side of the triple point.

To put it in context, the melting point is where solid and liquid coexist (ice melting to water) at a given pressure, typically 0°C at 1 atm. The boiling point is where liquid and gas coexist (water boiling to steam) at a given pressure, typically 100°C at 1 atm. The triple point is not those two-phase equilibria; it’s the special condition where all three phases meet.

This concept helps explain why the triple point is a precise reference point used for calibrating thermometers and temperature scales.