Part 8: Core Practical 15: identifying inorganic and organic unknowns

Design a defensible sequence of small-scale tests, record observations separately from interpretations, and use confirmatory evidence to identify unknowns.

Plan the evidence before using up the sample

CP15 asks you to investigate inorganic ions and organic functional groups, building on CP7 and the Year 13 reaction toolkit. Begin with labelled samples, candidate identities if supplied, hazard information, clean test tubes, dropping pipettes and separate small portions. A logical plan selects tests that distinguish the remaining candidates rather than adding every reagent to the same tube.

Record reagent, conditions and observation immediately, then write the inference in a separate column. “A white precipitate formed” is an observation; “chloride present” is an inference. Negative results are meaningful only if the reagent is working and the conditions allow the reaction. Use a known positive control and reagent blank when appropriate.

Use separate portions for incompatible tests. HCl used in a carbonate test would introduce chloride and invalidate a later halide test on that same portion. Carry-over on droppers or dirty glass can give false positives. Plan which solution is retained for confirmations.

Separate flame evidence from precipitation evidence

For a flame test, use a clean wire, check that it gives no residual colour, introduce a small sample and observe the flame. Sodium gives intense yellow, potassium lilac, lithium crimson/red, calcium orange-red/brick red and barium pale/apple green in the usual school descriptions. Sodium contamination can mask weaker colours, so flame evidence should be checked against other tests.

Add aqueous NaOH dropwise, observe the initial precipitate, then add excess and record whether it dissolves. Cu²⁺ gives blue hydroxide, Fe²⁺ green that may brown in air, Fe³⁺ red-brown, and Cr³⁺ green that dissolves in excess hydroxide. Group 2 hydroxide precipitation depends on solubility and concentrations: Mg²⁺ gives a white precipitate, Ca²⁺ may, and Ba²⁺ normally does not under the usual dilute test conditions.

A green precipitate alone does not uniquely identify an ion; its behaviour in excess and on standing can distinguish candidates. For ammonium, warm with aqueous alkali and test gas with damp red litmus: ammonia turns it blue. Do not identify gases by direct smelling.

Choose an acid that does not introduce the target ion

For halides, acidify with dilute nitric acid, then add silver nitrate. AgCl is white, AgBr cream and AgI yellow. Test solubility in ammonia: chloride dissolves in dilute ammonia, bromide needs concentrated ammonia, and iodide remains insoluble in both under the usual test conditions. Nitric acid removes carbonate interference without adding chloride.

For sulfate, acidify a fresh portion with dilute HCl, then add barium chloride: persistent white BaSO₄ supports sulfate. Acid removes carbonate that might otherwise form a white barium precipitate. Do not acidify with sulfuric acid: that would supply the ion being tested for.

For carbonate, add dilute acid to a fresh sample; effervescence plus a gas turning limewater milky supports CO₂. A gas-producing observation by itself does not identify carbonate among every conceivable anion. Relate the conclusion to the candidate list and the confirmatory gas test.

Select tests that answer different structural questions

For a carboxylic acid, a carbonate/hydrogencarbonate test with confirmed CO₂ is a useful small-scale test. For a carbonyl, 2,4-DNPH gives a precipitate; a subsequent aldehyde test on a separate portion distinguishes a typical aldehyde from a ketone. For an alkene, bromine water loses its colour by addition, but phenol and some reducing substances also react, so use the whole pattern.

An aldehyde reduces freshly prepared Tollens’ reagent on gentle water-bath warming, producing a silver mirror/deposit; Fehling’s/Benedict’s can support an aliphatic aldehyde through red Cu₂O. Phenol decolourises bromine water with a white precipitate; a methyl ketone gives yellow CHI₃ with warm alkaline iodine. Acidified dichromate turns orange to green with several oxidisable groups, so it is not a unique aldehyde test.

The published Pearson CP15 teaching sheet also gives acid-catalysed ester formation as supporting evidence for a carboxylic acid. A characteristic odour is subjective and never requires direct inhalation; follow the supervisor’s procedure and corroborate it with a chemical test. The school’s approved procedure and actual candidates determine the appropriate sequence.

A negative result may arise from an insoluble sample, degraded reagent, insufficient warming or too little analyte. Compare with a control before treating “no change” as proof of absence. Use small quantities, a hot-water bath for flammable organic samples and separate waste handling for heavy-metal reagents.

Worked identification keeps observation and inference separate

Original unknown A gives an orange-red flame and a cream silver precipitate after nitric acid/silver nitrate; the precipitate dissolves in concentrated but not dilute ammonia. The flame supports Ca²⁺ and the precipitate pattern supports Br⁻. Within a simple single-salt candidate set, calcium bromide is consistent; use the stoichiometric formula CaBr₂, not CaBr.

Original organic unknown B gives a DNPH precipitate, no silver mirror with a working Tollens’ control, and yellow CHI₃ with alkaline iodine. This supports a methyl ketone within the candidates. It does not identify the chain length; a molecular formula or spectrum is still needed.

Original unknown C effervesces with acid and its gas clouds limewater, while an independently prepared portion gives a yellow flame. Those observations support a sodium carbonate within the stated candidate set. Do not reuse the acidified portion to “confirm chloride”. A reliable report includes conflicting observations rather than forcing them to fit the first guess.

Make improvements specific to a false result

Cross-contamination that introduces chloride raises the chance of a false positive silver test; use clean droppers and fresh portions. Excess sample can mask a subtle precipitate colour; repeat with a controlled small amount and a white background. Air oxidation changes Fe(II) observations over time; record the initial colour promptly as well as the later change.

A test may identify an ion or functional group without uniquely identifying the whole compound. Mixtures can combine positive results, and broad reagent reactivity can create ambiguity. Follow-up tests should discriminate the surviving alternatives. Repetition improves confidence in observations but does not make an inherently non-selective test unique.

Hazard controls include eye protection, suitable containment for corrosive/volatile reagents, no flames near flammable samples, and prompt supervised disposal of Tollens’ reagent without storage or drying. Written planning and interpretation complement the hands-on observations required for the practical endorsement.

Quick checks

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

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