The upper critical solution temperature (UCST) or upper consolute temperature is the critical temperature above which the components of a mixture are miscible in all proportions. The word upper indicates that the UCST is an upper bound to a temperature range of partial miscibility, or miscibility for certain compositions only. For example, hexane-nitrobenzene 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. Examples at higher temperatures are the aniline-water system at 168 °C (334 °F) (at pressures high enough for liquid water to exist at that temperature), and the lead-zinc system at 798 °C (1,468 °F) (a temperature where both metals are liquid).
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.
The phase separation at the UCST is in general driven by unfavorable energetics; in particular, interactions between components favor a partially demixed state.
Some polymer solutions also have a lower critical solution temperature (LCST) or lower bound to a temperature range of partial miscibility. As shown in the diagram, for polymer solutions the LCST is higher than the UCST, so that there is a temperature interval of complete miscibility, with partial miscibility at both higher and lower temperatures.
By adding soluble impurities the upper critical solution temperature increases and lower critical solution temperature decreases.
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