Question

Why must a solution be saturated for it to be at equilibrium?

Answer 1

Well, `"saturation of A SOLUTE IN A GIVEN solution"` does specify an equilibrium condition....

Explanation:

And as I have said before here, a saturated solution contains an amount of solute that is equal to that amount of solute that would be in equilibrium with UNDISSOLVED solute....

And thus `"saturation"` is represented by the following equilibrium:

`"dissolved solute "rightleftharpoons" undissolved solute"`

See here and links.

And a temperature is specified because a hot solution can generally hold more solute than a cold one.

And in the case of `"supersaturation"`, the solvent contains an amount of solute GREATER than that which would be in equilibrium with undissolved solute. This is recognized as a problem area in A level and undergraduate chemistry, and a little care in learning definitions could win easy marks.