4 Solubility of hydroxides and sulfates

Group II hydroxides become more soluble down the group. All Group II hydroxides when not soluble appear as white precipitates. Calcium hydroxide is reasonably soluble in water. It is used in agriculture to neutralise acidic soils. An aqueous solution of calcium hydroxide is called lime water and can be used a test for carbon dioxide. The limewater turns cloudy as white calcium carbonate is produced. Ca(OH)2 (aq) + CO2 (g)  CaCO3 (s) + H2O(l) Barium hydroxide would easily dissolve in water. The hydroxide ions present would make the solution strongly alkaline. Ba(OH)2 (S) + aq  Ba2+ (aq) + 2OH-(aq) Magnesium hydroxide is classed as insoluble in water. Simplest Ionic Equation for formation of Mg(OH)2 (s) Mg2+ (aq) + 2OH-(aq)  Mg(OH)2 (s). A suspension of magnesium hydroxide in water will appear slightly alkaline (pH 9) so some hydroxide ions must therefore have been produced by a very slight dissolving. Magnesium hydroxide is used in medicine (in suspension as milk of magnesia) to neutralise excess acid in the stomach and to treat constipation. Mg(OH)2 + 2HCl  MgCl2 + 2H2O It is safe to use because it is so weakly alkaline. It is preferable to using calcium carbonate as it will not produce carbon dioxide gas.Group II sulphates become less soluble down the group. BaSO4 is the least soluble. Testing for Presence of a Sulphate ion BaCl2 solution acidified with hydrochloric acid is used as a reagent to test for sulphate ions. If acidified Barium Chloride is added to a solution that contains sulphate ions a white precipitate of Barium Sulphate forms. Simplest ionic equation Ba2+ (aq) + SO4 2-(aq)  BaSO4 (s). BaSO4 is used in medicine as a ‘Barium meal’ given to patients who need x-rays of their intestines. The Barium absorbs the x-rays and so the gut shows up on the x-ray image. Even though Barium compounds are toxic it is safe to use here because of its low solubility. 2HCl + Na2CO3  2NaCl + H2O + CO2 Fizzing due to CO2 would be observed if a carbonate was present. Other anions should give a negative result which is no precipitate forming. If Barium metal is reacted with sulphuric acid it will only react slowly as the insoluble Barium sulphate produced will cover the surface of the metal and act as a barrier to further attack. Ba + H2SO4  BaSO4 + H2 The same effect will happen to a lesser extent with metals going up the group as the solubility increases. The same effect does not happen with other acids like hydrochloric or nitric as they form soluble group 2 salts. The hydrochloric acid is needed to react with carbonate impurities that are often found in salts which would form a white Barium carbonate precipitate and so give a false result. You could not used sulphuric acid because it contains sulphate ions and so would give a false positive result. N Goalby chemrevise.org An equation for the formation of the precipitate can be written as a full equation or simplest ionic equation Full equation : SrCl2 (aq) + Na2SO4 (aq)  2NaCl (aq) + SrSO4 (s) Ionic equation: Sr2+ (aq) + SO4 2-(aq)  SrSO4