triple point
Summary
The triple point is the single temperature and pressure at which a pure substance's solid, liquid and gas phases coexist in equilibrium. On a phase diagram it is where the three boundary lines meet. For water it is 273.16 K (0.01 °C) at about 0.006 atm.
Detail
Meaning: The triple point is the one temperature and pressure at which a pure substance's solid, liquid and gas phases coexist in equilibrium. On a phase diagram it is where the solid-liquid, liquid-gas and solid-gas lines meet. Because it is a single point, changing either T or P moves the system off it, and at least one phase disappears. Below the triple point pressure, no liquid can exist, so heating the solid makes it sublime.
On the MCAT: A passage gives a phase diagram with pressure on the y-axis and temperature on the x-axis. You name the phase at a point or predict what happens when T or P changes. Compare 1 atm with the triple point pressure. For water (273.16 K, about 0.006 atm) the triple point pressure is below 1 atm, so heating ice at 1 atm melts it. For CO₂ (about 5.1 atm) it is above 1 atm, so dry ice sublimes at 1 atm.
Common confusion: Do not confuse it with the normal melting point or the critical point. The normal melting point is the solid-liquid temperature at 1 atm (0 °C for water, slightly different from 0.01 °C). The critical point is where the liquid-gas line ends. The triple point is where three lines meet.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.