Separate aqueous solutions of three metal salts are each tested by adding a few drops of aqueous sodium hydroxide. In every tube a precipitate forms, as sho...
Question 1 Report
Separate aqueous solutions of three metal salts are each tested by adding a few drops of aqueous sodium hydroxide. In every tube a precipitate forms, as shown.
(a) State the colour of the precipitate formed when aqueous sodium hydroxide is added to a solution containing copper(II) ions, iron(II) ions and iron(III) ions. [3]
(b) Write an ionic equation, including state symbols, for the formation of the iron(III) hydroxide precipitate. [2]
(c) A solution of a zinc salt gives a white precipitate with a little aqueous sodium hydroxide. State what is observed when an excess of aqueous sodium hydroxide is then added. [1]
(d) A fourth solution is warmed with aqueous sodium hydroxide. State what would be observed if the solution contains ammonium ions, and how the gas produced is identified. [2]
(e) Write an ionic equation, including state symbols, for the reaction of copper(II) ions with hydroxide ions. [2]
Answer Details
Adding aqueous sodium hydroxide to metal-salt solutions gives coloured metal-hydroxide precipitates whose colours identify the metal ion. This question also needs balanced ionic equations with state symbols.
(a) The precipitate colours are: copper(II) ions give a light blue precipitate [1]; iron(II) ions give a green (dirty green) precipitate [1]; iron(III) ions give a red-brown precipitate [1]. [3]
(b) Iron(III) hydroxide forms as:
\[ \text{Fe}^{3+}(aq) + 3\text{OH}^{-}(aq) \rightarrow \text{Fe(OH)}_3(s) \]
Correct formulae and charges [1]; balanced with correct state symbols [1]. [2]
(c) Zinc hydroxide is amphoteric, so with excess sodium hydroxide the white precipitate dissolves to give a colourless solution [1]. [1]
(d) If ammonium ions are present, warming with sodium hydroxide releases a pungent/choking gas, ammonia [1], which turns damp red litmus paper blue [1]. [2]
(e) Copper(II) hydroxide forms as:
\[ \text{Cu}^{2+}(aq) + 2\text{OH}^{-}(aq) \rightarrow \text{Cu(OH)}_2(s) \]
Correct formulae and charges [1]; balanced with correct state symbols [1]. [2]
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