Solute seperates as a solid when a saturated solution is cooled. Why ?

1 Answer
anor277
Mar 23, 2018

Because solubility is a FUNCTION of temperature, as well as of the intrinsic solubility of the solute in the solvent..

Explanation:

#"Saturation"# defines an equilibrium condition....i.e.

#"Undissolved solute"+Deltastackrel("solvent")rightleftharpoons" dissolved solute"#

And a temperature is usually specified given that a hot solvent can generally dissolve more solute than a cold solvent...and this is intuitively reasonable given that the act of dissolution involves the breaking of bonds between the SOLUTE particles.

And on cooling, the formation of solute-solute bonds, i.e. crystallization is necessarily favoured. In other words, upon cooling, upon withdrawal of heat, the equilibrium moves to the left as written as we face the page....

And of course, this is the principle behind recrystallization, and it also underlies the principle of #"supersaturation"#, where a solvent holds an amout of SOLUTE GREATER than that which would be in equilibrium with undissolved solute.

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