# Triple Point Definition – Triple Point of Water

In chemistry and physics, the triple point of a pure substance is the combination of temperature and pressure where three phases exist in thermodynamic equilibrium. Usually, the triple point refers to the point where a substance’s solid, liquid, and vapor phase coexist in equilibrium. However, the term also applies to the temperature and pressure at which any three phases coexist. So, if a substance has multiple solid phases (crystal structures), it has multiple triple point values.

### Triple Point of Water

People often refer to the triple point temperature of water, but the triple point is really a combination of both temperature and pressure. The triple point of water is at 273.16 kelvin (0.01 °C or 32.02 °F) and a pressure of 611.7 pascals (6.117 millibars, 0.0060373057 atm). (Some sources list a pressure of 611.73 Pa, while others cite a pressure of 611.657 Pa.) At this point, ice, water, and water vapor coexist in stable form. The tiniest change in temperature or pressure causes water to change forms between ice, liquid water, and water vapor.

Note that the total pressure of the system can be well above the pressure of the triple point, providing the partial pressure of water is at 611 Pa. Also note that the triple point corresponds to the lowest pressure under which liquid water exists. Below this pressure, ice converts directly into vapor and vapor undergoes sublimation into ice.

Usually, the triple point is the minimum temperature at which the liquid form of a substance can exist. Water is unusual in this respect because of the way its melting point decreases as a function of pressure (at least for hexagonal ice).

### Table of Triple Point Values for Common Substances

This table lists triple point values for several pure substances. Most of the data comes from the NIST (U.S. National Institute of Standards and Technology), with helium values from Donner et al. and Hoffer et al. Triple point values usually come from examination of samples with “six nines” purity. In other words, samples are 99.9999% pure.

### Is There Only One Triple Point?

Most substances have more than one triple point because their solid assumes different forms or allotropes. If there are p phases of matter, the number of triple points is p!/(p-3)!3!

For example, if solid sulfur has two solid phases (rhombic and monoclinic), a liquid phase, and a vapor phase, its number of triple points is 4!/(4-3)!3!.

number of triple points = 24 / (1)(6) = 4

Water has 15 known phases of ice and hundreds of triple points! At least 10 of these values are known. The usual single value you see is for hexagonal ice. The other forms of ice occur at very low temperature or very high pressures.

### Triple Point vs Critical Point

The triple point and critical point of a substance often get confused, but the two terms have different meanings. The triple point is the temperature and pressure at which three states of matter exist in equilibrium. The critical point is also a combination of temperature and pressure, but it is the end point of a phase equilibrium curve where a liquid and its vapor can coexist. The critical point indicates the highest temperature where the substance becomes a supercritical fluid and cannot be condensed into a liquid using pressure alone. At the critical point, the physical properties of the liquid and vapor phases become similar to each other and sometimes very different from the normal behavior of the liquid.

The critical point for water is 647.096 K (373.946 °C; 705.103 °F) and 22.064 megapascals (3,200.1 psi; 217.75 atm). This is a higher temperature and pressure than water’s triple point. Near the critical point, liquid water becomes compressive, a poor dielectric, and a poor solvent for electrolytes. These properties are the opposite of water’s normal properties.

### References

• Cengel, Yunus A.; Turner, Robert H. (2004). Fundamentals of Thermal-Fluid Sciences. Boston: McGraw-Hill. ISBN 0-07-297675-6.
• Donnelly, Russell J.; Barenghi, Carlo F. (1998). “The Observed Properties of Liquid Helium at the Saturated Vapor Pressure”. Journal of Physical and Chemical Reference Data. 27 (6): 1217–1274. doi:10.1063/1.556028
• Hoffer, J. K.; Gardner, W. R.; Waterfield, C. G.; Phillips, N. E. (April 1976). “Thermodynamic properties of 4He. II. The bcc phase and the P-T and VT phase diagrams below 2 K”. Journal of Low Temperature Physics. 23 (1): 63–102. doi:10.1007/BF00117245
• IUPAC (1997). “Triple point.” Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific. doi:10.1351/goldbook.T06502
• Murphy, D. M. (2005). “Review of the vapour pressures of ice and supercooled water for atmospheric applications”. Quarterly Journal of the Royal Meteorological Society. 131 (608): 1539–1565. doi:10.1256/qj.04.94
• Papon, P.; Leblond, J.; Meijer, P. H. E. (2002). The Physics of Phase Transition: Concepts and Applications. Berlin: Springer. ISBN 978-3-540-43236-4.