Part 1: Identifying ions and avoiding false positives

Use observations, ionic equations and a sensible test sequence to identify carbonate, sulfate, halides and ammonium.

Record reagent, observation and inference

Show the chemical change

Carbonate reacts with acid to release CO₂; limewater identifies the gas. Sulfate precipitates with barium ions. Warming an ammonium salt with hydroxide releases NH₃. Use damp indicator paper because gas dissolving in its moisture produces the alkaline response. Do not deliberately inhale the gas to test its smell.

Carbonate → sulfate → halide

Carbonate can give white BaCO₃ and interfere with a sulfate test; silver carbonate and silver sulfate can interfere with halide tests. The required anion-testing sequence is therefore carbonate, sulfate, then halide, with interfering ions removed where using a common sample.

A coherent sequential method is: add dilute HNO₃ and test the evolved gas; once carbonate is removed, add Ba(NO₃)₂ to test/remove sulfate; separate any precipitate before adding AgNO₃ to the clear solution. Nitric acid and barium nitrate avoid introducing chloride or sulfate. Confirm the resulting silver halide using ammonia.

In separate-aliquot testing, use fresh portions and appropriate acidification for each test; do not pour tested portions back into the stock. BaCl₂ can test sulfate on its own fresh aliquot, but that aliquot cannot then give a valid chloride test. Never acidify a sulfate test with H₂SO₄. This reasoning matters more than a memorised reagent list.

Worked identification and practical control

An unknown solution gives no gas with HNO₃, no precipitate with Ba(NO₃)₂, then a yellow precipitate with AgNO₃ that remains in concentrated ammonia. A fresh portion warmed with NaOH releases a gas that turns damp red litmus blue. These results support iodide and ammonium ions, so NH₄I is a compatible salt.

A negative observation is useful only if the reagent and method worked. Use small labelled samples, clean apparatus and a known reference if needed. Wear eye protection and follow controls for barium salts, silver nitrate and ammonia; collect heavy-metal waste as directed. These are PAG 4 qualitative skills.

Plan the samples before adding reagents

Begin with labelled portions of the unknown and a written sequence. Reserve a fresh portion for the ammonium test: adding ammonia earlier would introduce the gas you later intend to detect, while acid left in a used sample consumes added hydroxide. Keep the original stock untouched for repeats.

For sequential anion testing, remove carbonate with nitric acid, test/remove sulfate with barium nitrate, separate any barium sulfate and test the clear solution for halide with silver nitrate. On separate aliquots, still account for carbonate and sulfate interference where they may coexist. Merely splitting a mixture into tubes does not remove interfering ions from each tube.

Use a distilled-water reagent blank if contamination is suspected. A positive result in the blank indicates that the reagent or apparatus, rather than the unknown alone, may be responsible. A known positive reference checks whether an apparent negative result could arise from a failed method.

Worked investigation: more than one ion can be present

An invented mixture fizzes with dilute HNO₃ and the gas turns limewater cloudy: this supports carbonate. After effervescence ends, Ba(NO₃)₂ gives a white precipitate: this supports sulfate remaining in the acid-treated sample. Filter it off before proceeding.

The clear filtrate gives a cream precipitate with AgNO₃. It remains in dilute NH₃ but dissolves in concentrated NH₃: this supports bromide. Report carbonate, sulfate and bromide as identified anions. These results do not identify the cations or prove that the mixture contains three particular named salts.

A colour or gas result needs the matching reagent and conditions to be meaningful. “White precipitate” without saying which reagent produced it cannot distinguish BaSO₄, AgCl or the CaCO₃ formed in limewater.

Turn an observed problem into a specific improvement

If too little acid is added before Ba²⁺, residual carbonate can form BaCO₃ and mimic sulfate. Add sufficient appropriate dilute acid and allow gas evolution to finish. If suspended BaSO₄ is carried into the halide test, new cloudiness becomes hard to see: separate the existing precipitate first.

In the ammonium test, warm gently and keep damp red litmus near the mouth without touching the alkaline liquid. A splash of NaOH on the paper can turn it blue without ammonia being demonstrated. State a volatile alkaline gas was detected, using the controlled test, rather than deliberately smelling the tube.

An ionic equation should show the species undergoing change, with charges and phases appropriate to the observation. Soluble sodium and nitrate ions are usually spectators here; adding them creates work without improving the evidence.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources