10.1.5—Variation in the solubilities
- Syllabus
- 9701–2028–2029
- Objective
- 10.1.5
- Level
- AS
Hydroxide and sulfate solubility trends are different because dissolution balances different energies
Group 2 hydroxide solubility generally increases down the group, while sulfate solubility generally decreases. Each trend reflects a balance between lattice energy and hydration energy.
A dissolving solid must pay lattice energy and gain hydration energy. The relative change down the group is different for OH⁻ and SO₄²⁻, so do not assume one trend applies to both.
Ba(OH)₂ is more soluble than Mg(OH)₂, but BaSO₄ is far less soluble than MgSO₄. This contrast is useful in qualitative analysis.
“Larger ion means always more soluble” is false. Solubility is a free-energy balance, not a single-size rule.
ConceptA-Level CAIE Chemistry AS