Solubility rules determine which atoms remain in solution.
Solubility rules determine which atoms remain in solution.

Magnesium nitrate and potassium hydroxide exist as ions in aqueous solutions. Magnesium ions combine with hydroxide ions to form solid magnesium hydroxide. The potassium and nitrate ions remain in a solution. The balanced chemical formula for this reaction is Mg(NO3)2 (aq) + 2KOH (aq) → Mg(OH)2 (s) + 2KNO3 (aq).

Balancing the Reaction

Initially, you could write the reaction as Mg(NO3)2 (aq) + KOH (aq) → Mg(OH)2 (s) + KNO3 (aq). However, there are some inconsistencies in this equation: the left side has two nitrate ions and one hydroxide ion, while the right side has two hydroxide ions and one nitrate. Therefore, you must add coefficients to the potassium hydroxide and potassium nitrate to balance the number of atoms on both sides.

Solubility Rules

To understand what happens during this reaction, you must be familiar with solubility rules. All nitrates are soluble, so magnesium nitrate exists as magnesium and nitrate ions in solution. Potassium hydroxide is one of the soluble hydroxides, so it exists as potassium and hydroxide ions in solution. Magnesium hydroxide is insoluble, so it will form as a precipitate in this reaction. The remaining nitrate and potassium ions will not form a precipitate.