After a solution undergoes crystallization, is the remaining solution saturated, or unsaturated?

1 Answer
Apr 5, 2017

After crystallization has been completed the solution is #"SATURATED......"#


Saturation describes an equilibrium condition, viz.:

#"Solid solute "rightleftharpoons" Dissolved solute"#

And thus, in a saturated solution, we might typically see some UNDISSOLVED solute, i.e. a crystalline residue, on the bottom or the sides of the flask.

For a given solvent (usually water), a temperature is specified because a hot solution can normally dissolve more solute than a cold solution.

And now attend, because this definition seems to cause a lot of confusion. If the solution contains a GREATER amount of SOLUTE than would be in equilibrium with UNDISSOLVED solute (i.e. a greater amount of solute than does the saturated solution), the solution is said to be #"supersaturated"#.

How could we make a #"supersaturated solution"#? Well, one way might be to take a saturated solution (with a mass of undissolved solute), give it a good heat blast to bring ALL of the SOLUTE into solution, and cool the flask carefully, so that the crystals do not precipitate out of solution.

#"Supersaturation"# is a metastable condition, and sometimes, the solution can be brought back to equilibrium, to #"saturation"#, by scratching the sides of the flask, or introducing a seed crystal, and a mass of solute precipitates; sometimes it does so and generates a lot of heat, which represents the heat required initially to bring the solute to supersaturation. After the mass of crystals deposit, the solution is #"NOW SATURATED."#

#"Unsaturation"# describes a solution that contains a #"LESSER amount of SOLUTE than would be in equilibrium"# #"with UNDISSOLVED solute"#; such a solution is #"unsaturated"# with respect to the #"solute."#

For more of the same, see here, and enclosed links.

Note that a definition of #"saturation"#, such as the #"solvent holds all the solute that it can"#, is #"MANIFESTLY INADEQUATE and INCORRECT"#. If you are unclear as to what I have said, ask again, and someone will help you.