AccessScience from McGraw-Hill Education Page 1 of 2 www.accessscience.com Triple point Contributed by: Robert L. Scott Publication year: 2014 A particular temperature and pressure at which three different phases of one substance can coexist in equilibrium. In common usage these three phases are normally solid, liquid, and gas, although triple points can also occur with two solid phases and one liquid phase, with two solid phases and one gas phase, or with three solid phases. According to the Gibbs phase rule, a three-phase situation in a one-component system has no degrees of freedom (that is, it is invariant). Consequently, a triple point occurs at a unique temperature and pressure, because any change in either variable will result in the disappearance of at least one of the three phases. See also: PHASE EQUILIBRIUM . Triple points are shown in the illustration of part of the phase diagram for water. Point A is the well-known ◦ triple point for Ice I (the ordinary low-pressure solid form) + liquid + water + water vapor at 0.01 C (273.16 K) and a pressure of 0.00603 atm (4.58 mmHg or 611 pascals). In 1954 the thermodynamic temperature scale (the absolute or Kelvin scale) was redefined by setting this triple-point temperature for water equal to exactly 273.16 K. Thus, the kelvin (K), the unit of thermodynamic temperature, is defined to be 1 ∕ 273.16 of the thermodynamic temperature of this triple point. ◦ Point B , at 251.1 K ( − 7.6 F) and 2047 atm (207.4 megapascals) pressure, is the triple point for liquid water + Ice ◦ I + Ice III; and point C , at 238.4 K ( − 31 F) and 2100 atm (212.8 MPa) pressure, is the triple point for Ice I + Ice II + Ice III. At least four other triple points are known at higher pressures, involving other crystalline forms of ice. For most substances the solid-liquid-vapor triple point has a pressure less than 1 atm (about 100 kilopascals); such substances then have a liquid-vapor transition at 1 atm (normal boiling point). However, if this triple point has a pressure above 1 atm, the substance passes directly from solid to vapor at 1 atm. See also: SUBLIMATION . For a two-component system, the invariant point in a phase diagram is a quadruple point at which four phases coexist. The three-phase situation is then represented by a line in the three-dimensional pressure-temperature-composition diagram. See also: BOILING POINT ; ICE POINT ; MELTING POINT ; TRANSITION POINT ; VAPOR PRESSURE ; WATER . Robert L. Scott AccessScience from McGraw-Hill Education Page 2 of 2 www.accessscience.com UnlabelledPhase diagram image for water, showing gas, liquid, and several solid (ice) phases; triple points at A, B , and C. The pressur e scale changes at 1 atm from logarithmic scale at low pressure to linear at high pressure. 1 atm = 100 ◦ kPa; F = (K × 1.8) − 459.67. Additional Readings J. K. Fink, Physical Chemistry in Depth , Springer-Verlag, Berlin, Germany, 2009 M. J. Moran et al., Fundamentals of Engineering Thermodynamics , 7th ed., John Wiley & Sons, Hoboken, NJ, 2011 D. L. Reger, S. R. Goode, and D. W. Ball, Chemistry, Principles and Practice , 3d ed., Brooks ∕ Cole, Belmont, CA, 2010 .