Plan complementary tests on separate samples, record observations before conclusions and combine ionic and functional-group evidence to identify unknowns.
Build an evidence table before testing
Core Practical 7 identifies organic liquids and inorganic solids using observations from several tests. Begin with a labelled table containing sample, reagent/conditions, observation and inference. A statement such as “bromide present” is an inference; “cream precipitate after adding silver nitrate” is an observation. Preserve the distinction so another person can evaluate the conclusion.
Use clean apparatus, separate portions and measured small quantities. A reagent added in one test can contaminate a later test or consume the target group. Run a suitable known positive sample or reagent check when a negative result is crucial; no colour change might otherwise mean an inactive reagent or inadequate warming. Unknown does not mean harmless.
Treat organic unknowns as harmful and flammable and inorganic unknowns as harmful until identified. Wear eye protection, use water-bath heating rather than a flame near organic liquids, and use the laboratory’s ventilation and waste arrangements. Dichromate(VI) is oxidising, corrosive and carcinogenic; halogen solutions, ammonia, silver nitrate and barium salts require their specified controls. Flame-test work belongs in a separate controlled area away from flammable organic tests.
Use the AS functional-group tests together
The official CP7 organic sequence includes bromine water, acidified dichromate, Fehling’s and halogenoalkane hydrolysis/silver nitrate. The carbonate test is a useful AS extension linked to the CP5 acid product. Do not move Year 13 tests such as a full 2,4-DNPH derivative identification scheme into required AS scope without labelling them as extension.
The sodium hydroxide treatment releases halide ions from a covalent C–X bond. Dilute nitric acid then neutralises excess OH⁻ before silver nitrate is added, preventing a silver oxide/hydroxide-related interference. HCl is unsuitable at this stage because it introduces the chloride ion being tested for.
| Test and conditions | Positive observation | Inference and limit |
|---|---|---|
| Bromine water, shake at room temperature | Orange/brown becomes colourless | Supports C=C in the candidate set; other reactive groups can also decolourise bromine |
| Acidified K₂Cr₂O₇, warm in water bath | Orange changes towards green | An oxidisable substance: primary/secondary alcohol or aldehyde, not uniquely an alcohol |
| Fehling’s solution, warm | Red/orange Cu₂O precipitate | Supports a simple aliphatic aldehyde; original alcohols/ketones normally negative |
| Warm with aqueous NaOH plus ethanol; acidify with dilute HNO₃, then AgNO₃ | White/cream/yellow precipitate | Hydrolysis released chloride/bromide/iodide from a halogenoalkane |
| Carbonate added to a suitable separate sample | Effervescence; gas turns limewater cloudy | Supports an acid in the candidate set; verify CO₂ |
Worked pattern: do not force a more precise identity than the tests allow
Suppose unknown A decolourises bromine water, but is negative with warmed dichromate and warmed Fehling’s. Among a stated set of an alkene, primary alcohol and bromoalkane, A is the alkene. These results establish a functional group within that set; they do not distinguish cyclohexene from every other alkene.
Unknown B changes dichromate orange to green, but gives no red precipitate with Fehling’s and does not decolourise bromine water. A primary or secondary alcohol is supported in a simple candidate set. IR O–H and the molecular ion can then help distinguish candidates. Dichromate alone cannot tell primary from secondary, and a tertiary alcohol would normally be negative in this test.
Unknown C is negative in those direct tests but gives a cream precipitate after hydroxide hydrolysis, nitric-acid acidification and silver nitrate. A bromoalkane is supported. An immediate silver-nitrate precipitate in an unhydrolysed aqueous sample would instead suggest free halide contamination or an ionic halide. Check the sample history before concluding that a C–Br bond was present.
Flame tests give cation evidence
Clean a suitable nichrome/platinum wire by the prescribed acid-and-flame procedure until it gives no persistent sample colour; use a fresh small solid portion. Flame heating excites electrons, and emission occurs as electrons return to lower energy levels. The emitted colours provide characteristic evidence for some metal ions, not for their counter-ions.
A yellow flame does not prove sodium chloride: it gives evidence for sodium, not chloride. If colour is faint or mixed, clean the wire and repeat with a fresh portion rather than selecting a familiar salt name by guesswork. Magnesium has no distinctive flame colour in this test.
| Ion | Typical observed colour | Interpretation |
|---|---|---|
| Li⁺ | Crimson/scarlet red | Compare against candidate set/reference |
| Na⁺ | Yellow | Very strong; contamination can mask another colour |
| K⁺ | Lilac | May be obscured by sodium |
| Ca²⁺ | Orange-red/brick red | Use anion tests to identify the salt |
| Sr²⁺ | Crimson red | Red colours need careful comparison |
| Ba²⁺ | Apple/pale green | Support with other evidence where needed |
Use acidification to remove interference without adding the target ion
Dissolve a fresh portion of the inorganic solid in distilled water where possible and split it. For carbonate, add dilute acid to a separate portion and test the evolved gas with limewater. Effervescence alone is not proof that the gas is CO₂. The ionic reaction is CO₃²⁻ + 2H⁺ → CO₂ + H₂O.
For halides, add dilute nitric acid before silver nitrate. Acid removes carbonate that could otherwise form a misleading silver precipitate. Chloride gives white AgCl, bromide cream AgBr and iodide yellow AgI. AgCl dissolves in dilute ammonia; AgBr requires concentrated ammonia for dissolution, while AgI remains insoluble under these usual conditions. CP7’s dilute-ammonia step therefore distinguishes chloride from a bromide/iodide precipitate but does not distinguish bromide from iodide by dissolution alone.
For sulfate, acidify a separate portion and then add barium chloride; white BaSO₄ supports sulfate. The CP7 worksheet uses nitric acid for this acidification. Suitable dilute HCl is also used in standard sulfate tests, but sulfuric acid would introduce sulfate and invalidate the test. Acidification removes carbonate that could form BaCO₃.
Halogen displacement supplies independent anion evidence
The official CP7 inorganic method also adds chlorine water to a fresh solution portion. Chlorine can oxidise bromide to bromine and iodide to iodine. Formation of an orange bromine-containing solution supports bromide; an iodine colour supports iodide, interpreted using the solvent and concentration specified. Chloride does not reduce chlorine to give a different halogen.
Explain the redox change: each halide ion loses one electron, while Cl₂ gains two electrons overall and becomes 2Cl⁻. The sample used for this test cannot then be used as a clean chloride-test sample because chlorine water and its reactions can introduce chloride.
Worked identification and formula checking
An illustrative unknown solid gives a lilac flame. A separate aqueous portion has no effervescence on acidification, then gives a cream precipitate with AgNO₃ that remains in dilute ammonia. Another fresh portion becomes orange with chlorine water. The flame supports K⁺; precipitation and displacement support Br⁻. The simplest neutral formula is KBr.
A second unknown gives an orange-red flame and a white precipitate with barium chloride after suitable acidification; its halide test is negative. These results support Ca²⁺ and SO₄²⁻, hence CaSO₄ within the candidate set. Limited solubility must be considered when preparing an aqueous sample; a negative test on almost no dissolved sample is weak evidence.
Write balanced ionic equations for the reactions used to justify each anion. For formulae, balance ionic charges: Ca²⁺ with Br⁻ would require CaBr₂, whereas K⁺ with SO₄²⁻ would require K₂SO₄. Flame and anion tests identify different pieces of the same salt.
Use negative evidence with controls and practical limits
A result table should record the initial colour, added reagent and conditions, final colour, gas or precipitate, and any behaviour with excess reagent. Report no visible change rather than “nothing happened”: the reaction could be below the detection limit. Repeat ambiguous tests on a new portion and compare with a control or known sample.
Specific improvements follow the suspected cause: clean wire for sodium contamination; fresh pipettes for halide carry-over; verified bath temperature and sufficient heating for a slow hydrolysis; matching small volumes and a white background for faint precipitates. Repeating the same contaminated sample many times does not remove systematic contamination.
If two substances fit all results, report the remaining ambiguity and choose a new discriminating test, molecular mass or infrared evidence. Written reasoning is assessed alongside practical understanding, but successful written answers do not replace observed safe laboratory competence for 9CH0/04.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.
Q1. Why should the bromine-water test and dichromate test use separate portions?Show answer
Each reagent changes the sample and may affect the other test, causing interference. Fresh portions let each observation be interpreted as a response of the original unknown.
Q2. A sample reduces dichromate but is negative with warmed Fehling’s. Is “primary alcohol proved” justified?Show answer
No. A primary or secondary alcohol could fit; the negative Fehling’s test helps exclude a simple aldehyde only if the reagent/conditions work. Additional evidence is needed to distinguish alcohol classes or exact structures.
Q3. Why is nitric acid used before AgNO₃ rather than HCl?Show answer
It removes interfering carbonate or excess hydroxide without adding halide. HCl adds Cl⁻ and could produce AgCl even if the original sample contained no chloride.
Q4. A solid gives a yellow flame and a white precipitate after acidification then barium chloride. Suggest a formula.Show answer
The flame supports Na⁺ and the acidified barium test supports SO₄²⁻. Two Na⁺ are needed per sulfate, so Na₂SO₄ is supported within the candidate set. Neither test alone identifies the whole salt.
Q5. Write the ionic equation for chlorine water liberating bromine, then explain why that sample cannot test for original chloride.Show answer
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. The test produces chloride ions itself; subsequent AgCl precipitation could therefore come from the test rather than the original unknown.
Sources
Sources and examiner guidance (reviewed 9 October 2026)
- Pearson Edexcel 9CH0 specification — Issue 3 — Topic 7 Core Practical 7 and links to Topics 4 and 6, printed page 21; reviewed 9 October 2026.
- Pearson Edexcel 8CH0 specification — Issue 3 — Topic 7 printed page 19; infrared correlation table printed page 48. AS scope verified.
- Chemrevise — Edexcel Mass spectra and IR — Pages 1–3; secondary explanation checked against Pearson. Original examples and practice.
- Pearson Core Practical 7 — teacher, student and technician sheets — Teacher printed pages 1–3 and student printed pages 1–2; actual AS organic and inorganic sequence reviewed 9 October 2026. Sample identities are not reused as the original worked unknowns.
- Chemrevise — Edexcel practical guide — Pages 16–17 and 20; cross-checked against Pearson CP7. The guide combines CP7/CP15: Year 13 tests were not adopted as required AS content.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.