Using aqueous sodium hydroxide, aqueous ammonia and potassium iodide to identify Pb²⁺, Al³⁺, Ca²⁺, Zn²⁺, Fe³⁺, Fe²⁺, Cu²⁺ and NH₄⁺, with ionic equations and a systematic method.
5 narrated scenes. Press play, listen and read along — the points appear as they are explained. Use ‹ Prev / Next › or the scene bar to jump to any scene.
A single drop of sodium hydroxide can tell you which metal ion is in a solution: you only need to watch the colour and what happens in excess.
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Pale blue ppt; with NH₃ excess gives deep blue solution.
Dirty green ppt; insoluble in excess; goes brown.
Reddish-brown ppt; insoluble in excess.
White ppt with NaOH; none with NH₃.
White ppt; dissolves in excess NaOH; insoluble in NH₃.
White ppt; dissolves in excess of both.
White ppt; dissolves in excess NaOH; KI gives yellow ppt.
Warm with NaOH: NH₃ turns damp red litmus blue.
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8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better. Work each item on paper first.
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Question 1 [16 marks]
Solutions of four unlabelled salts, W, X, Y and Z, each contain one metal cation: Al³⁺, Zn²⁺, Cu²⁺ or Fe²⁺. Tests were carried out with aqueous sodium hydroxide and aqueous ammonia.
(a) (a) State the observation expected for Cu²⁺ with a few drops of NaOH and then with excess aqueous ammonia. [3]
(b) (b) State the observation for Fe²⁺ with aqueous NaOH and what happens on standing. [2]
(c) (c) Describe how aluminium ions can be distinguished from zinc ions. [3]
(d) (d) Write the ionic equation for the reaction of Cu²⁺ with hydroxide ions. [2]
(e) (e) A solution gives a white precipitate with both NaOH and ammonia that dissolves in excess NaOH but not in excess ammonia. A yellow precipitate forms with potassium iodide. Identify the ion and write the equation for the yellow precipitate. [3]
(f) (f) Describe a test for ammonium ions. [3]
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