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Tests for cations in solution. Cations are positive ions. The cations you need to be able to identify are: copper iron(III) silver iron(II) magnesium.

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Presentation on theme: "Tests for cations in solution. Cations are positive ions. The cations you need to be able to identify are: copper iron(III) silver iron(II) magnesium."— Presentation transcript:

1 Tests for cations in solution

2 Cations are positive ions. The cations you need to be able to identify are: copper iron(III) silver iron(II) magnesium zinc aluminium lead barium sodium

3 Copper Solutions containing Cu 2+ (aq) will be blue/green in colour. Pour a little of the test solution into a clean test tube.

4 Add a few drops of dilute sodium hydroxide solution. A pale blue precipitate indicates the presence of copper ions. Cu 2+ (aq) + 2OH – (aq) → Cu(OH) 2 (s)

5 To confirm the presence of Cu 2+ (aq), add about 5 mL of ammonia solution. The pale blue precipitate redissolves to form a clear, royal blue solution. Cu(OH) 2 (s) + 4NH 3 (aq) → [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH – (aq)

6 Iron(III) Solutions containing Fe 3+ (aq) will be orange in colour (or yellow if very dilute). Pour a little of the test solution into a clean test tube.

7 Add a few drops of dilute sodium hydroxide solution. An orange or dark brown precipitate forms. Fe 3+ (aq) + 3OH – (aq) → Fe(OH) 3 (s)

8 Silver Silver ions also react with sodium hydroxide solution to form a brown precipitate Iron(III) solutions are usually coloured and the precipitate is dark brown or orange. Silver solutions are colourless and the precipitate is mud-brown or the colour of milky coffee. 2Ag + (aq) + 2OH – (aq) → Ag 2 O(s) + H 2 O(l)

9 To confirm Fe 3+ Potassium thiocyanate, KSCN, can be used to detect the presence of Fe 3+. This test can be used on very dilute solutions or in the presence of other metal ions (especially Fe 2+ ).

10 To a fresh sample of the test solution, add a few drops of potassium thiocyanate solution. If Fe 3+ is present the solution will turn blood-red. Fe 3+ (aq) + SCN – (aq) → [FeSCN] 2+ (aq)

11 Iron(II) Solutions of Fe 2+ are colourless or pale green. Pour a little of the test solution into a clean test tube.

12 Add a few drops of sodium hydroxide solution. If Fe 2+ is present you will see an olive-green precipitate or gel form. Fe 2+ (aq) + 2OH – (aq) → Fe(OH) 2 (s)

13 Magnesium, zinc, aluminium and lead These four cations all form white precipitates with sodium hydroxide solution. Three form soluble complexes with excess hydroxide, one also forms a soluble complex with ammonia solution. It is very important when adding sodium hydroxide to colourless solutions that you start with one or two drops only – otherwise you might miss the formation of the precipitate when it redissolves in excess hydroxide.

14 Magnesium Pour a little of the test solution into a clean test tube. Add one or two drops only of sodium hydroxide solution. If Mg 2+ is present a white precipitate forms. Mg 2+ (aq) + 2OH – (aq) → Mg(OH) 2 (s)

15 When excess sodium hydroxide is added, more precipitate is formed: the precipitate does NOT redissolve.

16 Zinc Pour a little of the test solution into a clean test tube. Add one or two drops only of sodium hydroxide solution. If Zn 2+ is present a white precipitate forms. Zn 2+ (aq) + 2OH – (aq) → Zn(OH) 2 (s)

17 Add excess sodium hydroxide solution (about 5 mL). The white precipitate slowly redissolves to form a clear, colourless solution. Zn(OH) 2 (s) + 2OH – (aq) → [Zn(OH) 4 ] 2– (aq) Al 3+ and Pb 2+ also form complexes with OH –, but only Zn 2+ also forms a complex with ammonia.

18 To a fresh sample of the test solution add a few drops of sodium hydroxide to form a white precipitate as before.

19 Add excess ammonia solution (about 5 mL). If Zn 2+ is present the white precipitate will slowly redissolve to form a clear, colourless solution. Zn(OH) 2 (s) + 4NH 3 (aq) → [Zn(NH 3 ) 4 ] 2+ (aq) + 2OH – (aq)

20 Aluminium Pour a little of the test solution into a clean test tube. Add one or two drops only of sodium hydroxide solution. If Al 3+ is present a white precipitate forms. Al 3+ (aq) + 3OH – (aq) → Al(OH) 3 (s)

21 Add excess hydroxide (about 5 mL). The precipitate redissolves to form a clear, colourless solution. Al(OH) 3 (s) + OH – (aq) → [Al(OH) 4 ] – (aq)

22 To a fresh sample of the test solution, add a few drops of sodium hydroxide solution to form a white precipitate as before.

23 Add excess (5 mL) ammonia solution. The white precipitate does NOT redissolve.

24 Both Al 3+ and Pb 2+ form white precipitates with OH – which redissolve in excess OH – but not in excess NH 3. However, aluminium sulfate is soluble in water, while lead sulfate is not. To a fresh sample of the test solution add a few drops of sulfuric acid. If Al 3+ is present there will be NO precipitate.

25 Lead Pour a little of the test solution into a clean test tube. Add one or two drops only of sodium hydroxide solution. A white precipitate forms. Pb 2+ (aq) + 2OH – (aq) → Pb(OH) 2 (s)

26 Add excess (5 mL) sodium hydroxide solution. If Pb 2+ is present the white precipitate redissolves to form a clear, colourless solution. Pb(OH) 2 (s) + 2OH – (aq) → [Pb(OH) 4 ] – (aq)

27 To a fresh sample of the test solution add a few drops of sodium hydroxide solution as before.

28 Add excess (5 mL) ammonia solution. If Pb 2+ is present the white precipitate does NOT redissolve.

29 To a fresh sample of the test solution add a few drops of dilute sulfuric acid. If Pb 2+ is present a white precipitate will form. Pb 2+ (aq) + SO 4 2– (aq) → PbSO 4 (s)

30 Barium and sodium Solutions containing Ba 2+ or Na + will be colourless. Barium hydroxide is moderately soluble, while sodium hydroxide is very soluble.

31 Add a few drops of sodium hydroxide. No precipitate suggests barium or sodium. Add more hydroxide (5 mL). If barium is present there may be a slight cloudiness after excess hydroxide is added. If sodium is present the solution will remain clear and colourless. Ba 2+ (aq) + 2OH – (aq) → Ba(OH) 2 (s)

32 Barium salts form a white precipitate with sulfuric acid. Sodium salts do not form a precipitate with sulfuric acid. Ba 2+ (aq) + SO 4 2– (aq) → BaSO 4 (s)


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