The majority of compounds formed by group II elements are ionic. The balance between the attraction of oppositely charged ions to one another and the attraction of separate ions to water dictates the solubility of ionic compounds.

• If the ions are more attracted to each other than the water then the compound will be insoluble. The solid will not dissolve and a precipitation reaction will occur instead.
• If the ions are more attracted to water than each other then the compound will be soluble. The solid will dissolve and no precipitation reaction will occur.

Trend in the solubility of sulphates

As you descend the group there is a decrease in the solubility of the sulphate

If you add sodium sulphate or sulphuric acid to an aqueous solution containing calcium, strontium or barium ions a white precipitate forms:

• Ba²⁺ + SO₄^2-(aq) ? BaSO₄(s) (thick white precipitate forms)
• Sr²⁺ + SO₄^2-(aq) ? SrSO₄(s) (thick white precipitate forms)
• Ca²⁺ + SO₄^2-(aq) ? CaSO₄(s) (faint white precipitate forms)

There is no reaction or precipitate when dilute sodium sulphate or sulphuric acid is added to a solution containing Mg²⁺ or Be²+ ions.

You can test for sulphate ions by using the thick white precipitate that forms when barium ions are added to sulphate ions. Add 1cm³ of aqueous barium chloride and 1cm³ of dilute hydrochloric acid to 1cm³ of the unknown solution. If a thick white precipitate forms which does not dissolve when dilute HCl is added to it then there are either ions of sulphate or hydrogensulphate present. The acid is used to remove any additional ions, like hydroxide or carbonate ions, which could affect the test results.

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Trend in solubility of hydroxides

As you descend group II hydroxide solubility increases.

If you add dilute sodium hydroxide to a solution of Mg²⁺ or Ca²⁺ ions a precipitate forms:

• Mg²⁺(aq) + 2OH^–(aq) ? Mg(OH)₂(s) (white precipitate forms immediately)
• Ca²⁺(aq) + 2OH^–(aq) ? Ca(OH)₂(s) (faint white precipitate forms)

There is no reaction or precipitate when dilute sodium hydroxide is added to a solution of Sr²⁺ or Ba²⁺ ions.

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Uses of sulphate and hydroxides

• Magnesium hydroxide: this is the most insoluble and can be brought as a suspension in water. Known as milk of magnesia it is used to alleviate constipation.
• Calcium hydroxide: as a solid it is known as slaked lime and helps neutralise acidic soil.
• Barium sulphate: it is ingested as part of a ‘barium meal’ in order to take an outline of the gut due to its ability to absorb X-rays. Barium ions are very toxic but they are also completely insoluble which makes this technique safe.