A trainee analyst is asked to identify the ions in a range of salts. The photograph shows a flame test being carried out on one of the solids. (a) Describe ...
Question 1 Report
A trainee analyst is asked to identify the ions in a range of salts. The photograph shows a flame test being carried out on one of the solids.
(a) Describe how to carry out a flame test on a solid, including how the wire is prepared. [3]
(b) Four solids give the flame colours below. Name the metal ion present in each: red, yellow, lilac, orange-red. [4]
(c) A solid Z is warmed with aqueous sodium hydroxide and a pungent gas is released. Identify the ion responsible, name the gas, and state the result seen with damp red litmus paper. [3]
(d) Solid Z also gives a white precipitate when acidified aqueous barium nitrate is added to its solution. Name this anion, write an ionic equation (with state symbols) for the precipitate, and hence name compound Z. [4]
(e) A further solution gives a cream precipitate with acidified aqueous silver nitrate. Name the anion present and name the precipitate formed. [2]
(f) Describe the test, and the positive result, for carbon dioxide and for oxygen. [4]
(g) Aqueous ammonia is added to a solution of iron(III) ions. Write an ionic equation, with state symbols, for the reaction and state the colour of the precipitate. [2]
Answer Details
Labelled answer diagram:
This is a full qualitative-analysis question: flame tests for cations, gas tests, and precipitation tests for anions, with balanced ionic equations.
(a) Dip a clean nichrome (or platinum) wire into concentrated hydrochloric acid and heat it in the flame to clean it [1]; dip the clean wire into the solid so some sticks to it [1]; hold the wire in the edge of a hot (blue, roaring) Bunsen flame and observe the colour [1]. [3]
(b) Red: lithium, Li+ [1]; yellow: sodium, Na+ [1]; lilac: potassium, K+ [1]; orange-red: calcium, Ca2+ [1]. [4]
(c) A pungent gas released on warming with sodium hydroxide identifies the ammonium ion, NH4+ [1]; the gas is ammonia [1]; it turns damp red litmus paper blue [1]. [3]
(d) A white precipitate with acidified aqueous barium nitrate identifies the sulfate ion, SO42- [1]:
\[ \text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s) \]
correct species [1]; balanced with state symbols [1]. Since Z contains both ammonium and sulfate ions, compound Z is ammonium sulfate, (NH4)2SO4 [1]. [4]
(e) A cream precipitate with acidified aqueous silver nitrate identifies the bromide ion, Br- [1]; the precipitate is silver bromide (\(\text{AgBr}\)) [1]. (Chloride gives white, iodide gives yellow; cream is bromide.) [2]
(f) Carbon dioxide: bubble the gas through limewater [1]; the limewater turns milky [1]. Oxygen: hold a glowing splint in the gas [1]; the glowing splint relights [1]. [4]
(g) Aqueous ammonia with iron(III) ions:
\[ \text{Fe}^{3+}(aq) + 3\text{OH}^{-}(aq) \rightarrow \text{Fe(OH)}_3(s) \]
balanced with state symbols [1]; the precipitate is red-brown [1] (aqueous ammonia supplies the hydroxide ions). [2]
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