GCE 'O' 2011 Science (Chemistry) Paper 5
You are provided
with a sample of solid salt, Q. Solid salt Q contains three diferent ions.
Carry out the
following experiments and test any gases evolved. Carefully record your
observations.
The volumes given
below are approximate and should be estimated rather than measured.
test
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observations
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(a)
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Use a spatula to
transfer approximately one third of solid Q into a dry hard glass
test-tube.
Heat the sample gently for 30 seconds and then very strongly until no further
changes are seen.
Test any gases
evolved using red and blue litmus paper, together with any other tests you
think are appropriate.
*Potassium dichromate(VI) can’t be use!
Condemned by MOE already. |
On heating gently, a colourless, odourless
gas that turned anhydrous blue cobalt chloride paper pink was evolved. Water
vapour was evolved.
Upon strong heating, a colourless, choking
gas that turned blue litmus red was evolved.
[4]
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You must retain solution Q for use in parts (b), (c) and (d).
(b)
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Place 3cm3 of solution Q in a clean test-tube.
To this solution, add
3 or 4 pieces of magnesium ribbon.
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Solution Q is green in colour.
The magnesium ribbon becomes smaller as grey
solids are formed.
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Gently warm the
mixture.
Leave this test-tube to stand in the test-tube rack for a few minutes.
While you are waiting, continue with part (c).
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The green solution turns colourless.
[4]
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(c)
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Place 2 cm3 of solution Q
in a clean test-tube.
To this solution, add 2cm3 of
dilute sodium hydroxide.
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A dirty green precipitate was obtained.
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Gently warm the mixture.
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On warming the mixture, a colourless, pungent gas that turned red litmus
blue was evolved.
Ammonia gas was evolved.
The green precipitate turns red brown.
[3]
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(d)
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Place 2 cm3 of solution Q
in a clean test-tube.
To this solution, add 1 cm3 of
dilute hydrochloric acid, followed by 1 cm3 of barium chloride
solution.
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A white precipitate is formed.
[1]
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(e) Consider the results of your experiments.
Name the three ions
present in salt Q. Give evidence to support each of your choices.
ion present
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evidence
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Iron (II)
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Dirty green precipitate of Fe(OH)2 formed in part (c).
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Ammonium
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Ammonia gas evolved in part (c) when solution Q with dilute sodium
hydroxide is warmed.
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Sulfate
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White precipitate of BaSO4 formed in part (d).
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[3]
