Question

A saturated solution of potassium chloride at `60` `""^@C`, is cooled to `20` `""^@C`. What happens, and how do we describe the solutions?

Answer 1

Saturation defines an equilibrium condition. The solution at `60` `""^@C` is SATURATED.

Explanation:

Now you have saturated `KCl(aq)` at `60` `""^@C`; this means that at this temperature there are 50 g or so of salt in solution. Now a hot solution can normally hold more solute than a colder one, so as this solution cools, the salt precipitates out IF EQUILIBRIUM (and thus SATURATION) are maintained.

Of course, the solute might not precipitate out. In this case the solvent contains a greater amount of solute than would be in equilibrium with undissolved solute. This describes a SUPERSATURATED SOLUTION.

The CONCLUSION: a saturated solution holds an amount of solute equal to that amount that would be in equilibrium with undissolved solute. Normally a temperature is specified because a hot solution can hold more solute than a cold one. As temperature decreases, if solute precipitates (i.e. becomes undissolved), the solution maintains saturation as per the definition.

The one fatal error you can make is to say that `"a saturated solution holds as much solute as it can"`. Say this, and your chemistry teacher will cry.