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Under which conditions can solid, liquid, and gas phases coexist at a triple point?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

The triple point is the unique condition of temperature and pressure under which the solid, liquid, and gaseous phases of a pure substance can coexist in thermodynamic equilibrium.123

At this specific point, the system has zero degrees of freedom, meaning both the temperature and pressure are fixed for that substance. On a phase diagram, the triple point is represented by the junction where the three two-phase equilibrium curves—the melting curve, the sublimation curve, and the vapour-pressure curve—intersect.3245

Because these values are specific to each substance, the conditions vary widely. For example, the triple point of hydrogen occurs at 14 K, while other substances have triple-point pressures as low as $10^{- 7}$ bar. If a substance's triple-point pressure is higher than atmospheric pressure, such as with carbon dioxide, it will sublime at atmospheric pressure rather than melting into a liquid.54