Solubility of Hydroxides & Sulfates (Oxford AQA International A Level Chemistry)

Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Stewart Hird

Solubility of Hydroxides & Sulfates

Group 2 hydroxides

  • The hydroxides dissolve in water as follows:

X(OH)2 (aq) → X (aq) + 2OH (aq)

  • When the metal oxides react with water, a Group 2 hydroxide is formed

  • Going down the group, the solubility of these hydroxides increases

    • Mg(OH)2 is sparingly soluble

    • Ba(OH)2 is soluble

  • 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

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

Ca(OH)2

1.5 x 10–3

Sr(OH)2

3.4 x 10–3

Ba(OH)2

1.5 x 10–2

The solubility of the Group 2 hydroxides 

Group 2 - Trends Group 2 Hydroxide,
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 decreases going down the group

  • Barium sulfate is virtually insoluble

The solubility of the Group 2 sulfates

A diagram to show how the solubility of the sulfates decreases down Group 2
The solubility of the sulfates decreases down Group 2

Examiner Tips and Tricks

The insolubility of barium sulfate is used to test for sulfate ions.

Addition of HCl (aq) followed by BaCl2 (aq) OR HNO3 (aq) followed by BaNO3 (aq) will form a white precipitate of barium sulfate if sulfate ions are present.

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Philippa Platt

Author: Philippa Platt

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.