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Thursday, October 15, 2015
GCE 'O' 2015 Science (Chemistry) Paper 5
You
are provided with three solutions, X,
Y and Z. Between them these three solutions contain two
different sodium salts. You are also provided with a solution P.
Carry
out the following tests. You should test any gases evolved. Carefully record
your observations.
The
volumes given below are approximate and should be estimated rather than
measured unless instructed otherwise.
Test
|
Observations with solution X
|
|
(a)(i)
|
Label one test-tube X and add about 1cm3
of solution X into it.
Label another test-tube Y and add about 1cm3
of solution Y into it.
Label another test-tube Z and add about 1cm3
of solution Z into it.
Add dilute nitric acid slowly to each test-tube,
with shaking, until no further change is seen.
Keep the contents of test-tubes X, Y and Z for use in part (a)(ii).
|
No visible
reaction.
|
Observations with solution Y
|
Observations with solution Z
|
Effervescence
observed. Colourless, odourless gas forms white precipitate in limewater.
|
Effervescence
observed. Colourless, odourless gas forms white precipitate in limewater.
|
(a)(ii)
|
Add more diute nitric acid to the solution in
test-tube X until the test-tube is about half full. Shake the
test-tube.
Then add about 1cm3 of barium nitrate
solution and shake the mixture.
Repeat using test-tubes Y and Z.
|
White
precipitate forms.
|
White
precipitate forms.
|
No visible
reaction.
|
(iii) From the observations, suggest identities for
the anions present in each of the solutions
X, Y and Z.
X contains sulfate ions.
Y
contains sulfate ions and carbonate
ions.
Z
contains carbonate
ions.
Give evidence from your observations
to support your identification of the two anions.
Anion 1 is sulfate
as white precipitate of barium sulfate is formed when barium nitrate
solution is added.
Anion 2 is carbonate
as effervescence of carbon dioxide which forms white precipitate of
calcium carbonate in limewater, is given out when dilute nitric acid is added.
[2]
test
|
observations
|
|
(b)(i)
|
Add 1cm3 of solution P into a
clean test-tube.
Add sodium hydroxide solution slowly, with shaking,
until no further change is seen.
|
Green
precipitate forms.
Precipitate
is insoluble in excess.
|
Then add a further 1cm3 of sodium
hydroxide solution and shake the test-tube.
Gently
heat this test-tube for at least one minute but do not allow the
solution to boil.
|
Colourless,
pungent gas that turns red litmus blue is evolved.
Ammonia gas is given off.
|
|
(ii)
|
Measure 2cm3 of solution P in a 10cm3 measuring
cylinder. Make up to 10cm3 using deionised water. Pour this
diluted solution into a boiling tube and mix thoroughly.
Add 2cm3 of the diluted solution P into a clean boiling tube.
Add 2cm3 of sulfuric acid to this
solution.
Then slowly add potassium manganate(VII) solution,
with shaking.
Stop adding potassium manganate(VII) when the
solution has turned a permanent pale pink colour. This is solution Q.
You must retain solution Q for use in part (b)(iii).
|
When a
little purple potassium manganite(VII) solution is added, it is decolourised
/ turns colourless.
|
(iii)
|
Add 1cm3 of solution Q into a clean test-tube.
Add aqueous ammonia slowly, with shaking, until no
further change is seen.
|
Red-brown
precipitate forms.
Precipate
is insoluble in excess.
|
[6]
(c) From your observations in part (b), what can
you conclude about the cations present in solution P and solution Q?
Explain how your
observations support your conclusions.
The cations
in P is Fe2+ (or iron(II)) which forms green precipitate of iron(II) hydroxide
in sodium hydroxide solution; and NH4+ (or ammonium) which gives off ammonia
gas when warmed with sodium hydroxide solution.
The
cations in Q is Fe3+ (or iron(III)) which forms red-brown precipitate of
iron(III) hydroxide in sodium hydroxide solution.
Fe2+ in
P is oxidized to Fe3+ in Q
[3]
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