SC25 · Qualitative analysis: tests for ions · Topic 9 — Separate chemistry 2 | GCSE Edexcel Separate Sciences

Revise the key ideas

Qualitative analysis identifies substances rather than measuring their quantity. A useful identifying test needs a sufficiently distinctive result; combine tests when one observation could fit several ions.
Use small separate portions of the unknown for different tests, clean apparatus and known reference samples where appropriate. Mixing reagents from different tests can create misleading precipitates or contaminate flames.
For a flame test, put a small sample into the blue, non-luminous flame using a clean wire loop or an approved alternative. Clean a suitable loop with hydrochloric acid and heat it until it gives no flame colour. Follow the teacher’s safety procedure.
Lithium ions give a red flame; sodium gives yellow; potassium gives lilac; calcium gives orange-red; copper gives blue-green. These are the specified flame colours; small sodium contamination can obscure other colours.
The colour identifies the metal-ion component; use another test for the anion.
Flame tests suggest a metal ion, not its accompanying anion. A yellow flame does not by itself distinguish sodium chloride from sodium sulfate.
Wear eye protection and handle flames, acids and unknown salts according to the laboratory risk assessment. Observations should state colour and physical change precisely, rather than only saying the test was positive.

Add sodium hydroxide solution dropwise to a fresh sample of salt solution, observe any precipitate, then add excess where needed. A precipitate is an insoluble solid formed from dissolved substances.
Aluminium ions give a white aluminium-hydroxide precipitate that dissolves in excess sodium hydroxide. Calcium ions give a white precipitate that does not dissolve in excess under these test conditions.
Warm an ammonium-containing sample with NaOH to test the ammonia gas separately.
Copper(II) ions give a blue precipitate; iron(II) gives green; iron(III) gives red-brown. Record the initial result promptly, because iron(II) compounds can oxidise on exposure to air.
Ammonium ions produce ammonia when warmed with sodium hydroxide. Hold damp red litmus paper near the released gas; it turns blue. Do not identify ammonia by directly inhaling it.
Hydroxide precipitates form because the metal cations react with hydroxide ions. For copper, Cu²⁺ + 2OH⁻ → Cu(OH)₂; the solid is insoluble under the test conditions.
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.

To test for carbonate ions, add dilute acid to a fresh sample. Bubbles (effervescence) may show a gas is produced. Pass the gas into limewater: it turns cloudy if the gas is carbon dioxide. Confirm the gas rather than relying on bubbles alone.
For sulfate ions, acidify a fresh solution with dilute hydrochloric acid, then add barium chloride solution. A white barium-sulfate precipitate indicates sulfate; acidification removes carbonate interference.
For halide ions, acidify a fresh solution with dilute nitric acid, then add silver nitrate solution. Chloride gives a white precipitate, bromide cream, and iodide yellow.
Acidify with dilute nitric acid before adding silver nitrate.
Do not use hydrochloric acid to acidify a halide-test sample: it adds chloride ions and could cause a false-positive chloride result. Nitric acid supplies nitrate, which does not give the specified halide precipitates.
The core practical identifies cations and anions in unknown salts using flame, gas and precipitation tests. Plan a sequence on separate samples, compare with controls and record each conclusion and its supporting observation.
A salt’s full identity needs both its cation and anion. For example, a yellow flame together with a confirmed sulfate test supports sodium sulfate within the specified set of salts.
Use balanced ionic equations where asked: Ag⁺ + Cl⁻ → AgCl and Ba²⁺ + SO₄²⁻ → BaSO₄. Spectator ions do not appear in these net ionic equations.

Instrumental methods can identify ions and measure very small concentrations more sensitively and quickly than simple visual tests. Accuracy still depends on calibration, clean samples and correct operation.
Compare a sample’s flame-photometer signal with reference data to identify metal ions. For concentration, measure standards of known concentration to make a calibration curve, then read the unknown’s concentration from its signal.
Read an unknown concentration from points within the calibration curve’s measured range: this is interpolation. If a sample is too concentrated, dilute it into that range, measure it, then multiply by the dilution factor to find the original concentration.
Use a blank to check background signal and repeated standards or unknowns to assess reliability. A straight-line calibration may be supplied, but do not assume every instrument is linear at every concentration.
Evaluate a method using sensitivity, accuracy, speed, cost, required training and potential interference. Detailed internal workings of a flame photometer are not required for this course.