What is a saturated solution?

A saturated solution is a solution in which the concentration of the SOLUTE is equal to that concentration that would be in equilibrium with UNDISSOLVED solute. As for any equilibrium we would normally specify a temperature (because at higher temperature, the solvent could probably dissolve more solute).

So for a water soluble salt, `M^(+)X^-`, we could write the following equilibrium reaction:

`M^(+)X^(-)(s) stackrel(H_2O)rightleftharpoons M^(+)(aq) + X^(-)(aq)`.

We would probably also specify a temperature, and we could write the equilibrium condition as : `K_(sp) = [M^(+)(aq)][X^(-)(aq)]` (`sp` stands for `"solubility product"`, and has been measured for many salts). If `K_(sp)` were small, what could you say with respect to the solubility of the salt?

When the ion product (i.e. `[M^(+)(aq)][X^(-)(aq)]` `=` `K_(sp)`, the solution is said to be `"saturated"` with respect to `MX`; when the ion product `>` `K_(sp)`, the solution is said to be `"SUPERSATURATED"`, and when the ion product `<` `K_(sp)` the solution is `"UNSATURATED"`.

These definitions are very poorly understood at undergraduate level, and I urge you to consider them and test your understanding. I reiterate that saturation defines an equilibrium condition, that has been measured for many salts at various temperatures.