Flame test colours and the precipitates formed with sodium hydroxide, including the aluminium result that dissolves in excess. Chemistry only content.
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1.How is a flame test carried out?
A clean wire loop is dipped in the sample and held in a hot blue Bunsen flame, and the colour of the flame is observed.
2.Why must the wire loop be clean?
Any residue from a previous sample would give its own colour and mask or confuse the result.
3.What flame colour does lithium give?
Crimson.
4.What flame colour does sodium give?
Yellow.
5.What flame colour does potassium give?
Lilac.
6.What flame colour does calcium give?
Orange-red.
7.What flame colour does copper give?
Green.
8.Why is a blue Bunsen flame used rather than a yellow one?
A yellow flame would mask the colour produced by the sample.
9.Give a limitation of flame tests.
If the sample contains a mixture of metal ions, one strong colour such as sodium can mask the others.
10.What is added to test for metal ions in solution?
Sodium hydroxide solution, added dropwise until in excess.
11.What is formed if a metal hydroxide is insoluble?
A precipitate, whose colour identifies the metal ion.
12.What precipitate do calcium ions give?
A white precipitate.
13.What precipitate do magnesium ions give?
A white precipitate.
14.What precipitate do aluminium ions give?
A white precipitate that then dissolves when excess sodium hydroxide is added.
15.How do you tell aluminium from calcium and magnesium?
Only the aluminium precipitate redissolves in excess sodium hydroxide. The others stay.
16.What precipitate do copper(II) ions give?
A blue precipitate.
17.What precipitate do iron(II) ions give?
A green precipitate.
18.What precipitate do iron(III) ions give?
A brown precipitate.
19.Why is the sodium hydroxide added slowly at first?
So you can see whether a precipitate forms and then whether it redissolves as more is added.
20.Why can a flame test not be used on a solution containing sodium hydroxide?
The sodium in the reagent would give a yellow flame regardless of what metal ion the sample contains.
21.A white precipitate forms and does not dissolve in excess. Which ions are possible?
Calcium or magnesium.
22.How could you then distinguish calcium from magnesium?
A flame test. Calcium gives an orange-red flame, magnesium does not give a distinctive colour.
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