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Definition:
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The temperature at which ice and water are in thermodynamic equilibrium at a given pressure. The pressure-melting temperature is 273.15 K when the pressure is 101 325 Pa, changing, when the water is saturated with air, at 9.8 108 K Pa-1 or, in ice of density 900 kg m-3, about 0.86 103K m-1. This means, for example, that beneath 4000 m of such ice the pressure-melting temperature is 269.75 K3 K m-1. For pure water and ice the corresponding rates are 7.4 108 K Pa-1 and about 0.65 10Factors other than pressure can alter the melting point; see temperate ice. The pair (273.15 K, 101 325 Pa) is known in thermodynamics as the ice point. The specified pressure is the sea-level pressure of the standard atmosphere defined by the International Civil Aeronautical Organization (1993). See also triple point.
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