Part 5: Identifying inorganic ions

Choose reagents that do not introduce the ion being tested, write net ionic equations and turn observations into justified identifications.

Separate the sample into fresh portions

Qualitative analysis identifies species from characteristic reactions. Record the reagent, relevant conditions, observation and inference separately. ‘A white precipitate forms’ is an observation; ‘sulfate is present’ is an inference that depends on using the correct reagent and excluding interference.

Use clean test tubes and separate fresh portions for different tests. Adding hydrochloric acid for a carbonate or sulfate test introduces chloride, so that used portion is unsuitable for a later halide test. Likewise, adding sodium hydroxide introduces sodium and interferes with a subsequent sodium flame test. Label the portions and include known controls when the result is uncertain.

A negative result is also evidence, provided the method was capable of detecting the ion. State ‘no precipitate after acidification and addition of barium chloride’ rather than omitting the test. A mixture can give more than one positive result, so do not force all observations into the formula of one pure salt without sufficient information.

Carbonate and hydrogencarbonate release carbon dioxide with acid

Add a dilute acid to a fresh sample. Effervescence suggests gas formation; pass the gas into limewater to establish that it is carbon dioxide. Limewater becomes cloudy because white calcium carbonate forms. Carbonate uses two H⁺ per ion, while hydrogencarbonate uses one.

The gas test does not distinguish carbonate from hydrogencarbonate by itself: both give CO₂. Additional quantitative information or another test is needed for that distinction. Do not label every fizz as carbonate without identifying the gas; metals with acid can release hydrogen instead.

If excess CO₂ is passed through limewater for long enough, cloudiness can disappear as soluble hydrogencarbonate forms. Record the initial positive cloudiness rather than mistaking a later clear solution for an absent carbonate result.

Acidified barium chloride tests for sulfate

Acidify a fresh aqueous portion with dilute hydrochloric acid, then add barium chloride solution. A white precipitate of BaSO₄ supports sulfate. Acid removes carbonate interference before Ba²⁺ is added, because otherwise white barium carbonate could be mistaken for barium sulfate.

Do not acidify with sulfuric acid: it would introduce the sulfate being tested and give BaSO₄ even for a sulfate-free unknown. In the ionic equation, omit spectator chloride and hydrogen ions and retain correct charges and states. Pearson 8CH0/01 June 2023 Q2(c) specifically linked the reagent choice to this net ionic equation.

Barium chloride is toxic; use small prescribed amounts, avoid contact and collect waste appropriately. ‘Barium sulfate is insoluble’ does not mean all barium reagents are safe or can be poured away without following the laboratory procedure.

Warm ammonium ions with aqueous hydroxide

Add aqueous sodium hydroxide to a fresh portion and warm gently. Ammonium ions donate a proton to OH⁻, forming ammonia and water. Hold damp red litmus paper in the gas near the mouth of the tube: ammonia turns it blue. Do not dip the paper into the alkaline liquid, which would turn it blue whether or not ammonium were present.

The paper must be damp so ammonia can dissolve and produce an alkaline environment. Avoid splashing NaOH onto it and do not identify ammonia by deliberate smelling. The gas may be confirmed in a prescribed demonstration by white ammonium chloride fumes with HCl, but the damp-litmus test is the routine ion-test observation.

This is an acid–base process, not redox. Nitrogen is −3 in both NH₄⁺ and NH₃. The plus sign belongs to the ammonium ion; the ammonia gas is neutral.

Worked unknown: connect independent evidence

Constructed unknown salt U is tested in fresh portions. Warming with NaOH releases a gas that turns damp red litmus blue, indicating NH₄⁺. A second portion acidified with HNO₃ gives a white AgNO₃ precipitate that dissolves in dilute ammonia, indicating Cl⁻. If U is specified to be a pure salt containing one cation and one anion, ammonium chloride is a justified identification.

If the sample could be a mixture, those tests establish ammonium and chloride in the mixture but do not prove they were originally paired in one compound. Similarly, carbonate gas tests alone do not identify the metal cation. Use a separate flame test and other supplied evidence to determine the remaining species.

For common Group 1/2 cations, flame colours are taught in Part 2. Mg²⁺ can also be supported by a white hydroxide precipitate: Mg²⁺ + 2OH⁻ → Mg(OH)₂. Potassium ions remain in solution with hydroxide. In the 2023 AS Q2(b) comparison, the report notes that requested negative results were sometimes omitted.

Evaluate the test, not just the final label

A useful practical answer explains reagent order, clean samples, contamination controls, the relevant hazard control and the chemical basis of the observation. If a result is ambiguous, repeat with a fresh portion and a known reference instead of changing the expected colour to fit a guess.

These reactions support the inorganic identification in Topic 7 Core Practical 7. Learning the written chemistry does not itself demonstrate competent laboratory technique; successful practical work also requires safe handling, accurate observations and reliable recording. No separate numbered core practical is assigned to Topic 4.

Quick checks

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

Sources