Trends in Solubility of Group 2 Hydroxides & Sulfates (Cambridge International AS Chemistry)
Exam board Cambridge International · ASUpdated 2026-10-19
Trends in Solubility in Group 2 Hydroxides & Sulfates
Trends in Solubility of Group 2 Hydroxides & Sulfates
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
| Group 2 hydroxide | Solubility at 298 K (mol / 100 g of water) |
|---|---|
| Mg(OH)2 | 2.0 x 10–5 (sparingly soluble) |
| Ca(OH)2 | 1.5 x 10–3 |
| Sr(OH)2 | 3.4 x 10–3 |
| Ba(OH)2 | 1.5 x 10–2 (soluble) |
- The hydroxides dissolve in water as follows:
X(OH)2(s) → X2+(aq) + 2OH–(aq)
- Where X is the Group 2 element
- When the metal oxides react with water, a Group 2 hydroxide is formed:
XO (s) + H2O (l) → X(OH)2(aq)
- 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
The solubility of the Group 2 hydroxides
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
| Group 2 sulfate | Solubility at 298 K (mol / 100 g of water) |
|---|---|
| MgSO4 | Highly soluble |
| CaSO4 | 1.2 x 10–3 |
| SrSO4 | 7.6 x 10–4 |
| BaSO4 | 1.0 x 10–4 (insoluble) |
- The sulfates dissolve in water as follows:
XSO4(s) → X2+(aq) + SO42-(aq)
- Where X is the Group 2 element
- When the metal oxides react with sulfuric acid, a Group 2 sulfate is formed:
XO (s) + H2SO4(aq) → XSO4+ H2O (l)
- Going down the group, the solubility of these sulfates decreases
The solubility of the Group 2 sulfates
Going down the group, the solubility of the sulfates decreases
Exam Hint You only need to be able to state the variation in the solubilities of the hydroxides and sulfates for this topic.
Enthalpy of Hydration: Ionic Charge & Radius in the A level course gives more detail on the solubility of Group 2 sulfates.
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