Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.15 Solubility trends-Study Notes - New Syllabus

8.15 Trends in Solubility of Group 2 Hydroxides and Sulfates

The solubility of Group 2 compounds shows opposite trends for hydroxides and sulfates as you go down the group (Mg → Ca → Sr → Ba).

(A) Hydroxides: \( \mathrm{M(OH)_2} \)

Trend

• Solubility increases down the group.

Examples

• \( \mathrm{Mg(OH)_2} \) → very sparingly soluble
• \( \mathrm{Ca(OH)_2} \) → slightly soluble (limewater)
• \( \mathrm{Ba(OH)_2} \) → relatively soluble

Explanation

• Lattice enthalpy decreases down the group (larger ions).
• Hydration enthalpy also decreases.
• Overall, lattice enthalpy decreases more → easier to dissolve.

Result

• Solutions become more alkaline down the group.

(B) Sulfates: \( \mathrm{MSO_4} \)

Trend

• Solubility decreases down the group.

Examples

• \( \mathrm{MgSO_4} \) → soluble
• \( \mathrm{CaSO_4} \) → slightly soluble
• \( \mathrm{BaSO_4} \) → insoluble (white precipitate)

Explanation

• Lattice enthalpy decreases down the group.
• Hydration enthalpy decreases more significantly.
• Less energy released on hydration → less soluble.

Key Comparison

• Hydroxides: solubility ↑ down group
• Sulfates: solubility ↓ down group

Summary Table

• \( \mathrm{Mg(OH)_2} \) (low solubility) → \( \mathrm{Ba(OH)_2} \) (higher solubility)
• \( \mathrm{MgSO_4} \) (soluble) → \( \mathrm{BaSO_4} \) (insoluble)

Therefore, understanding these trends helps predict solubility and related chemical behaviour.