🧪 2018 P5 — Qualitative analysis of solutions Q and R(Version A: guided notebook)
📖 What's the chemistry?
The two unknown solutions
Solution Q is aqueous iron(III) nitrate — the Fe³⁺ makes it
orange-yellow. Solution R is aqueous sodium carbonate — colourless.
Part (a) — two negatives that matter
Dilute nitric acid just dilutes Q: the orange-yellow solution becomes paler and — crucially —
there is NO effervescence, so Q contains no carbonate. (You use this in (b)(iii).)
Silver nitrate → no observable change → there is no chloride in Q.
Part (b) — mixing Q and R
Fe³⁺ meets CO₃²⁻: a red-brown precipitate forms and carbon dioxide effervesces
(a white precipitate forms in the limewater). The examiner insists the limewater test is actually performed — the
(b)(ii) carbonate deduction has no evidence without it. Excess dilute sulfuric acid then dissolves the
precipitate to a yellow solution — solution X, which is labelled and
retained for part (c).
Part (c) — two tests on X
(c)(i) Zinc:effervescence — zinc + the excess acid in X gives hydrogen, which extinguishes
a lighted splint with a 'pop'. The yellow solution becomes paler (pale green, then almost colourless), and
after pouring off, a brown coating has formed around the zinc — zinc displaces iron because zinc is
more reactive than iron (that is (d)(ii)).
(c)(ii) NaOH: a red-brown precipitate, insoluble in excess → Fe³⁺. Adding
aluminium foil and warming gently reduces the nitrate: effervescence, and a pungent gas turns damp red
litmus paper blue → ammonia → NO₃⁻ present. (The supervisor also noticed a hydrogen 'pop' from Al + NaOH —
a correct extra, but not asked for.)
Examiner tips (from the real report)
Strong candidates described Q becoming paler with the acid — and "clear" is NOT
"colourless".
Do not invent a white precipitate with the silver nitrate — write "no observable change".
With the zinc, record the change in colour of the solution too, not only the bubbles and the coating.
Tie the (d) deductions to your own recorded observations, not memorised facts.