The triple point is that in which the solid state, the liquid state and the gaseous state of a substance coexist in equilibrium. It is defined with a temperature and a vapor pressure.

The triple point of water, for example, is at 273.16 K (0.01 °C) and at a pressure of 611.73 Pa ITS90. This temperature, because it is a constant value, is used to calibrate the Kelvin and Celsius scales of the most accurate thermometers.

Triple point of water

The only combination of pressure and temperature at which water, ice, and water vapor can coexist in stable equilibrium occurs at exactly 273.1598 K (0.0098 °C) and at a partial pressure of water vapor 611.73 pascals (6.1173 millibars, 0.0060373057 atm). At that time, it is possible to change the state of the entire substance to ice, water, or steam arbitrarily by making small changes in pressure and temperature. It should be noted that even if the total pressure of a system is well above 611.73 pascals (i.e. a system with normal atmospheric pressure), if the partial pressure of water vapor is 611.73 pascals, then the system may still be at the triple point of water. Strictly speaking, the surfaces separating the various phases must also be perfectly flat, to avoid the effects of surface stresses.

Water has an unusual and complex phase diagram (although this does not affect the general considerations made about the triple point). At high temperatures, by increasing the pressure, liquid water is obtained first, followed by solid water. Above about 109 Pa, a crystalline form of ice is obtained which is denser than liquid water. At lower temperatures by virtue of compression, the liquid state ceases to appear, and the water passes directly from solid to gas.

At constant pressures above the triple point, heating ice causes it to go from a solid to a liquid and from there to a gas, or vapor. At pressures below the triple point, such as those found in outer space, where the pressure is close to zero, liquid water cannot exist. In a process known as sublimation, ice skips the liquid phase and turns directly into vapor when heated.

The triple point pressure of water was used during the Mariner 9 mission to Mars as a reference point to define "sea level". More recent missions make use of laser altimetry and gravimetry instead of atmospheric pressure to measure elevation on Mars.

Triple point table

The triple points of some common substances are included in this table. These data are based on those provided by the National Bureau of Standards (now NIST) of the United States of America.

Substance T (K) P (kPa)
Acetylene 192.4 120
Ammonia 195.40 6.076
Argon 83.81 68.9
Graphite 3900 10100
Carbon dioxide 216.55 517
Carbon monoxide 68.10 15.37
Deuterium 18.63 17.1
Ethane 89.89 8 × 10−4
Ethylene 104.0 0.12
Helium -4 2.19 5.1
Hydrogen 13.84 7.04
Hydrogen chloride 158.96 13.9
Mercury 234.2 1.65 × 10−7
Methane 90.68 11.7
Neon 24.57 43.2
Nitric oxide 109.50 21.92
Nitrogen 63.18 12.6
Nitrous oxide 182.34 87.85
Oxygen 54.36 0.152
Palladium 1825 3.5 × 10−3
Platinum 2045 2.0 × 10−4
Sulfur dioxide 197.69 1.67
Titanium 1941 5.3 × 10−3
Uranium hexafluoride 337.17 151.7
Water 273.16 0.61
Xenon 161.3 81.5
Zinc 692.65 0.065