Most substances can exist in a number of phases – solid, liquid and gas, for instance – and exhibit a special condition known as a triple point. As expressed in the Gold Book, published by the International Union of Pure and Applied Chemistry (IUPAC), the triple point occurs when the temperature and pressure of three phases of a substance in a one-component system are in equilibrium. This means that, at a particular combination of temperature and pressure, all three phases can coexist and can be transformed from one to another with the slightest of changes in either temperature or pressure.
Although such behaviour may seem bizarre or exotic, it is in fact very useful. The scientific unit of temperature, the kelvin, is derived – by definition – from the solid-liquid-vapour triple point of water.
This triple point occurs at 273.16 K (0.01 °C), with a partial vapour pressure of 611.73 pascals – about 1/170th of normal atmospheric pressure. Under these conditions molecules of water can evaporate directly from ice into water vapour, a process known as sublimation, with a slight temperature increase. At a slightly higher pressure, the molecule will pass through the liquid phase before evaporating. At the triple point vapour pressure, the boiling point of the liquid is the same temperature as the sublimation point of the solid. Water does have other triple points where the phases can include different types of ice.
Because the necessary conditions are so finely balanced, the fact that the water is actually at its triple point means the temperature can be known exactly, which is what makes it possible for it to be used as a definition for the kelvin.
Triple point conditions can be measured using a triple point cell, a vessel containing a pure substance, and this is used to calibrate thermometers to a high degree of accuracy. As different substances display triple points at different temperatures, a range of calibration points can be defined and this forms the basis of the ITS-90 international standard for thermometer calibration.