Point on a phase diagram where all three phases are in equilibrium.

• A. Phase Diagram
• B. Triple Point
• C. Critical Point
• D. Phase

Answer: (B)

The triple point is the point on a phase diagram where solid, liquid, and gas coexist in equilibrium. At this unique temperature and pressure, all three phases are simultaneously present and can transform into one another without a net change in amount. On the diagram, the lines separating the solid–liquid, liquid–gas, and solid–gas areas converge at this point, showing that any small shift away from it favors two phases instead of three. For water, this occurs at about 0.01 °C and 611.657 Pa, a standard reference used to calibrate thermometers. The critical point, by contrast, is where the distinction between liquid and gas disappears, so only two phases coexist beyond that location.