8.5 Flame tests and metal hydroxides
6 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Which metal ion gives a lilac flame?
Choose one answer.
Correct Not quite: the answer is Potassium
Mark scheme
C
Model answer
C: potassium ions give a lilac flame.
A solution gives a brown precipitate with sodium hydroxide solution. Identify the metal ion.
Mark scheme
- iron(III) / \mathrm{Fe^{3+}} (1)
Model answer
Iron(III) ions.
A white precipitate forms when sodium hydroxide solution is added to a solution. Describe how you could tell whether it contains aluminium ions.
Mark scheme
- add excess sodium hydroxide (1)
- the precipitate dissolves if aluminium ions are present (1)
Model answer
Add more sodium hydroxide until it is in excess: aluminium hydroxide dissolves, but calcium and magnesium hydroxides do not.
Explain why the nichrome loop must be cleaned before a flame test.
Mark scheme
- to remove other metal ions that could colour the flame / contaminate the result (1)
Model answer
Any traces of other metal compounds on the loop would colour the flame and give a false result.
Higher tier.
Write the ionic equation, with state symbols, for the formation of iron(II) hydroxide.
Mark scheme
- \mathrm{Fe^{2+}} + 2\mathrm{OH^-} → Fe(OH)2 (1)
- state symbols (aq) + (aq) → (s) (1)
Model answer
\mathrm{Fe^{2+}}(aq) + 2\mathrm{OH^-}(aq) → Fe(OH)2(s)
Explain why a flame test cannot always identify the metal ions in a mixture.
Mark scheme
- the colour of one ion can hide / mask the colour of another (1)
Model answer
The flame colour of one metal ion, such as sodium's yellow, can mask the colours of the others.
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