Solubility Trends
Group 2 hydroxides
- Going down the group, the solutions formed from the reaction of Group 2 oxides with water become more alkaline
- When the oxides are dissolved in water, the following ionic reaction takes place:
O2- (aq) + H2O(l) → 2OH- (aq)
- The higher the concentration of OH- ions formed, the more alkaline the solution
- The alkalinity of the formed solution can therefore be explained by the solubility of the Group 2 hydroxides
Solubility of the Group 2 Hydroxides Table
- The hydroxides dissolve in water as follows:
X(OH)2 (aq) → X(aq) + 2OH- (aq)
Where X is the Group 2 element
- When the metal oxides react with water, a Group 2 hydroxide is formed
- Going down the group, the solubility of these hydroxides increases
- This means that the concentration of OH- ions increases, increasing the pH of the solution
- As a result, going down the group, the alkalinity of the solution formed increases when Group 2 oxides react with water
Going down the group, the solubility of the hydroxides increases which means that the solutions formed from the reactions of the Group 2 metal oxides and water become more alkaline going down the group
Group 2 sulfates
- The solubility of the Group 2 sulfates decreasing going down the group
Going down the group, the solubility of the sulfates decreases
Exam Tip
You may be wondering why there are no trends here for the solubility of Group 1 hydroxides and sulfates. You should recall from previous studies that Group 1 compounds are all soluble in water. They will therefore not produce any precipitates when testing for cations, so to identify them you need to use flame tests.
Group 1 hydroxides will be more soluble than Group 2 hydroxides. Even though we say the solubility of the Group 2 hydroxides increases down the group barium hydroxide is less soluble than a Group 1 hydroxide such as potassium hydroxide.
At 25 °C the solubility of Ba(OH)2 is 4.68 g / 100 cm3
At 25 °C the solubility of KOH is 121 g / 100 cm3
