🧪 2018 P5 — Qualitative analysis of solutions Q and R (Version B: written answers)

📖 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 colour becomes paler and — crucially — there is NO effervescence, so Q contains no carbonate. (You use this in (b)(iii).)
• Silver nitrate → no visible change → there is no chloride in Q.

Part (b) — mixing Q and R

Fe³⁺ meets CO₃²⁻: a reddish-brown precipitate forms and carbon dioxide effervesces (limewater → milky). The examiner insists the limewater test is actually performed — the (b)(ii) carbonate deduction has no evidence without it. Dilute sulfuric acid then dissolves the precipitate to a yellow / brownish-orange solution — solution X, which is labelled and retained for part (c).

Part (c) — two tests on X

• (c)(i) Zinc: effervescence and warmth (zinc + the acid in X gives hydrogen), the solution turns colourless, and after pouring off, the zinc is coated with a greyish-black / brown deposit — zinc displaces iron because zinc is more reactive (that is (d)(ii)).
• (c)(ii) NaOH: a reddish-brown precipitate, insoluble in excess → Fe³⁺. Adding aluminium foil and warming gently reduces the nitrate: a pungent gas turns damp red litmus blue → ammonia → NO₃⁻ present. (The supervisor also noticed a hydrogen 'pop' from Al + NaOH — a correct bonus, but not marked.)

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 — there is no chloride here.
• Tie the (d) deductions to your own recorded observations, not memorised facts.