What is the term for the equilibrium point where sublimation, melting, and the vapor pressure lines intersect?

• A. Triple point
• B. Critical point
• C. Boiling point
• D. Melting point

Answer: (A)

The term is the triple point. It is the unique temperature and pressure at which solid, liquid, and gas are all in equilibrium, so all three phases can coexist. On a phase diagram, the solid–liquid boundary (melting), the solid–gas boundary (sublimation), and the liquid–gas boundary (vaporization) meet at this point. For water, the triple point occurs at about 0.01°C and 611 pascals. The other options refer to other concepts: the critical point is where liquid and gas become indistinguishable, the boiling point is where vapor pressure equals the surrounding pressure, and the melting point is where solid and liquid are in equilibrium at a given pressure.