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Upper critical solution temperature
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<p>The upper critical solution temperature (UCST) or upper consolute temperature is the <a href="/facts/Critical_temperature/pkM7RLJK">critical temperature</a> above which the components of a mixture are <a href="/facts/Miscible/HwQaolbQ">miscible</a> in all proportions. The word <i>upper</i> indicates that the UCST is an <a href="/facts/Upper_bound/YJje9oC2">upper bound</a> to a temperature range of partial miscibility, or miscibility for certain compositions only. For example, <a href="/facts/Hexane/JxPecNPt">hexane</a>-<a href="/facts/Nitrobenzene/JCEYpjAA">nitrobenzene</a> mixtures have a UCST of 19 °C (66 °F), so that these two substances are miscible in all proportions above 19 °C (66 °F) but not at lower temperatures.: 185 Examples at higher temperatures are the <a href="/facts/Aniline/eFXck1Rj">aniline</a>-water system at 168 °C (334 °F) (at pressures high enough for liquid water to exist at that temperature),: 230 and the <a href="/facts/Lead/EYljasJN">lead</a>-<a href="/facts/Zinc/80b1qgk3">zinc</a> system at 798 °C (1,468 °F) (a temperature where both metals are liquid).: 232 </p><p>A solid state example is the palladium-hydrogen system which has a solid solution phase (H2 in Pd) in equilibrium with a hydride phase (PdHn) below the UCST at 300 °C. Above this temperature there is a single solid solution phase.: 186 </p><p>In the phase diagram of the mixture components, the UCST is the shared maximum of the concave down <a href="/facts/Spinodal/vIlJQL8C">spinodal</a> and <a href="/facts/Binodal/NLxKuxV4">binodal</a> (or coexistence) curves. The UCST is in general dependent on pressure. </p><p>The <a href="/facts/Phase_separation/9AQPClRu">phase separation</a> at the UCST is in general driven by unfavorable energetics; in particular, interactions between components favor a partially demixed state. </p>