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Welcome to GCSE Edexcel Science revision.

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Unit S C 25: Qualitative analysis: tests for ions.

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Qualitative analysis identifies substances rather than measuring their quantity.

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A useful identifying test needs a sufficiently distinctive result; combine tests when one observation could fit several ions.

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Use small separate portions of the unknown for different tests, clean apparatus and known reference samples where appropriate.

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Mixing reagents from different tests can create misleading precipitates or contaminate flames.

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For a flame test, put a small sample into the blue, non-luminous flame using a clean wire loop or an approved alternative.

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Clean a suitable loop with hydrochloric acid and heat it until it gives no flame colour.

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Follow the teacher’s safety procedure.

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Lithium ions give a red flame; sodium gives yellow; potassium gives lilac; calcium gives orange-red; copper gives blue-green.

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These are the specified flame colours; small sodium contamination can obscure other colours.

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The colour identifies the metal-ion component; use another test for the anion.

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Flame tests suggest a metal ion, not its accompanying anion.

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A yellow flame does not by itself distinguish sodium chloride from sodium sulfate.

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Wear eye protection and handle flames, acids and unknown salts according to the laboratory risk assessment.

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Observations should state colour and physical change precisely, rather than only saying the test was positive.

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Add sodium hydroxide solution dropwise to a fresh sample of salt solution, observe any precipitate, then add excess where needed.

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A precipitate is an insoluble solid formed from dissolved substances.

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Aluminium ions give a white aluminium-hydroxide precipitate that dissolves in excess sodium hydroxide.

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Calcium ions give a white precipitate that does not dissolve in excess under these test conditions.

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Warm an ammonium-containing sample with N A O H to test the ammonia gas separately.

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Copper(I I) ions give a blue precipitate; iron(I I) gives green; iron(I I I) gives red-brown.

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Record the initial result promptly, because iron(I I) compounds can oxidise on exposure to air.

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Ammonium ions produce ammonia when warmed with sodium hydroxide.

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Hold damp red litmus paper near the released gas; it turns blue.

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Do not identify ammonia by directly inhaling it.

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Hydroxide precipitates form because the metal cations react with hydroxide ions.

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For copper, C U, charge 2 plus, plus 2 O H, charge minus, produces C U open bracket O H close bracket subscript two; the solid is insoluble under the test conditions.

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Colour alone may not identify every possible unknown; use only the ions in the specified set and combine independent observations to reach a justified conclusion.

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To test for carbonate ions, add dilute acid to a fresh sample.

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Bubbles (effervescence) may show a gas is produced.

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Pass the gas into limewater: it turns cloudy if the gas is carbon dioxide.

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Confirm the gas rather than relying on bubbles alone.

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For sulfate ions, acidify a fresh solution with dilute hydrochloric acid, then add barium chloride solution.

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A white barium-sulfate precipitate indicates sulfate; acidification removes carbonate interference.

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For halide ions, acidify a fresh solution with dilute nitric acid, then add silver nitrate solution.

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Chloride gives a white precipitate, bromide cream, and iodide yellow.

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Acidify with dilute nitric acid before adding silver nitrate.

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Do not use hydrochloric acid to acidify a halide-test sample: it adds chloride ions and could cause a false-positive chloride result.

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Nitric acid supplies nitrate, which does not give the specified halide precipitates.

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The core practical identifies cations and anions in unknown salts using flame, gas and precipitation tests.

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Plan a sequence on separate samples, compare with controls and record each conclusion and its supporting observation.

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A salt’s full identity needs both its cation and anion.

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For example, a yellow flame together with a confirmed sulfate test supports sodium sulfate within the specified set of salts.

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Use balanced ionic equations where asked: A G, charge plus, plus C L, charge minus, produces A G C L and B A charge two plus plus S O 4, charge 2 minus, produces B A S O four.

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Spectator ions do not appear in these net ionic equations.

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Instrumental methods can identify ions and measure very small concentrations more sensitively and quickly than simple visual tests.

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Accuracy still depends on calibration, clean samples and correct operation.

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Compare a sample’s flame-photometer signal with reference data to identify metal ions.

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For concentration, measure standards of known concentration to make a calibration curve, then read the unknown’s concentration from its signal.

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Read an unknown concentration from points within the calibration curve’s measured range: this is interpolation.

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If a sample is too concentrated, dilute it into that range, measure it, then multiply by the dilution factor to find the original concentration.

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Use a blank to check background signal and repeated standards or unknowns to assess reliability.

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A straight-line calibration may be supplied, but do not assume every instrument is linear at every concentration.

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Evaluate a method using sensitivity, accuracy, speed, cost, required training and potential interference.

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Detailed internal workings of a flame photometer are not required for this course.

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That completes Qualitative analysis: tests for ions.

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Revisit the notes and test yourself on the revision website.