# What are some common mistakes students make with points of saturation?

Oct 21, 2015

Saturation is an equilibrium concept; IT DOES NOT MEAN that the solvent holds all the solute that it can.

#### Explanation:

When we say a solvent is saturated with solute, we usually specify a temperature, because a hot solution can hold more solute than a cold one. When a common solute is dissolved to saturation point, we mean that the solvent holds the same amount of solute that would be equilibrium with undissolved solute. So for $N {a}_{2} S {O}_{4}$ dissolved in water, we could write the equation:

$N {a}_{2} S {O}_{4} \left(s\right) r i g h t \le f t h a r p \infty n s 2 N {a}^{+} + S {O}_{4}^{2 -}$

As for any equilibrium we could write the equation,

${K}_{s p} = {\left[N {a}^{+}\right]}^{2} \left[S {O}_{4}^{2 -}\right]$. ${K}_{s p}$ is simply a number, which may be large or small, and has been extensively measured for a range of sparingly soluble inorganic salts in water. At this saturation point, addition of any further sodium or sulfate ion would result in the precipitation of sodium sulfate.

The $s p$ subscript stands for "solubility product". If I have gone off the track, would you please respecify your question?