Group 2 compounds and their usesAQA A-Level Chemistry: Revision notes

The solubility of the Group 2 hydroxides increases down the group.

• Mg(OH)₂ is sparingly soluble; a suspension of it in water is only mildly alkaline.
• Ca(OH)₂ is slightly more soluble. A saturated solution is limewater.
• Sr(OH)₂ and Ba(OH)₂ are more soluble and give more alkaline solutions.

This is why adding NaOH(aq) to Mg²⁺ gives a white precipitate: Mg²⁺(aq) + 2OH⁻(aq) → Mg(OH)₂(s), while Ba²⁺ gives little or no precipitate.

• Mg(OH)₂ in medicine: it is an antacid in indigestion remedies. It neutralises excess stomach acid, Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. Because it is sparingly soluble, it is only mildly alkaline and safe to swallow.
• Ca(OH)₂ in agriculture: farmers add it to neutralise acidic soil and raise its pH.
• CaO or CaCO₃ in flue gas desulfurisation: the flue gases of power stations contain SO₂, an acidic oxide that causes acid rain. A basic calcium compound reacts with it:
  • CaO + SO₂ → CaSO₃
  • CaCO₃ + SO₂ → CaSO₃ + CO₂

The solubility of the Group 2 sulfates decreases down the group.

• MgSO₄ is soluble.
• CaSO₄ is slightly soluble, and SrSO₄ is less so.
• BaSO₄ is insoluble.

The trend for hydroxides is the opposite way round to the trend for sulfates.

Add dilute hydrochloric acid and then barium chloride solution. A white precipitate of barium sulfate shows that sulfate ions are present:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

• Barium chloride is used because barium sulfate is insoluble, so a precipitate forms only when sulfate ions are present.
• The solution is acidified to remove carbonate ions (and sulfite ions), which would also form white precipitates with Ba²⁺. In acid, carbonate ions react to form CO₂ and water, so any precipitate left is barium sulfate.
• Do not use sulfuric acid to acidify, as it would add sulfate ions.

Barium ions are toxic, but BaSO₄ is insoluble so it is not absorbed. Barium is dense and absorbs X-rays strongly, so a 'barium meal' of BaSO₄ shows the digestive system on an X-ray image.

BaCO₃ cannot be used as it would react with stomach acid, BaCO₃ + 2HCl → BaCl₂ + CO₂ + H₂O, releasing toxic Ba²⁺ ions.

Worked example: 0.0100 mol of sulfate ions forms 0.0100 mol of BaSO₄ (M = 233.4 g mol⁻¹), a mass of 2.33 g.