All of the examples of chemical equilibrium we have looked at thus far in this unit are homogeneous. There is a single phase in which forward and reverse reactions take place simultaneously.

There is an important example of heterogeneous equilibrium which we have mentioned in passing and a more detailed discussion of it is now possible with the foundation we have established.

When ions in aqueous solution experience a greater attraction for one another than the separate attractions they may have for water molecules, a precipitate forms. The solution becomes saturated and equilibrium is established between the undissolved (or precipitated) solute and the dissolved solute that stays in solution as ions. The rates of precipitation and dissolution are equal as long as the temperature remains constant and no water is added or removed.