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*Revision Materials and Past Papers* 2.1.1 Atomic structure and isotopes 2.1.2 Compounds, formulae and equations 2.1.3 Amount of substance 2.1.4 Acids 2.1.5 Redox 2.2.1 Electron structure 2.2.2 Bonding and structure 3.1.1 Periodicity 3.1.2 Group 2 3.1.3 The halogens 3.1.4 Qualitative analysis 3.2.1 Enthalpy 3.2.2 Reaction Rates 3.2.3 Chemical equilibrium 4.1 Basic concepts and hydrocarbons 4.1.2 Alkanes 4.1.3 Alkenes 4.2.1 Alcohols 4.2.2 Haloalkanes 4.2.3 Organic synthesis 4.2.4 Analytical techniques 5.1.1 How fast? 5.1.2 How far? 5.1.3 Acids, bases and buffers 5.2.1 Lattice enthalpy 5.2.2 Enthalpy and entropy 5.2.3 Redox and electrode potentials 5.3.1 Transition elements 5.3.2 Qualitative analysis 6.1.1 Aromatic compounds 6.1.2 Carbonyl compounds 6.1.3 Carboxylic acids and esters 6.2.1 Amines 6.2.2 Amino acids, amides and chirality 6.2.3 Polyesters and polyamides 6.2.4 Carbon–carbon bond formation 6.2.5 Organic synthesis 6.3.1 Chromatography and qualitative analysis 6.3.2 Spectroscopy Required Practicals

3.1.4 Qualitative analysis

Qualitative analysis

Qualitative Analysis

Specification Reference 3.1.4 (a)

Quick Notes

  • Anion Tests
    • Carbonate (CO32−): Add dilute acid (e.g., HCl).
      • Effervescence from CO2 gas.
      • Confirm with limewater: turns cloudy.
    • Sulfate (SO42−): Add Ba2+ ions (e.g., BaCl2), with dilute HCl.
      • White precipitate of BaSO4 forms.
    • Halides (Cl−, Br−, I−): Add AgNO3, followed by NH3.
      • Cl−: white ppt, soluble in dilute NH3
      • Br−: cream ppt, soluble in conc. NH3
      • I−: yellow ppt, insoluble in NH3
  • Cation Test
    • Ammonium (NH4+): Warm with NaOH.
      • Releases ammonia gas (NH3).
      • Damp red litmus turns blue.
  • Test Sequence (important to avoid false positives):
    • Test for carbonates
    • Then test for sulfates
    • Finally test for halides

Full Notes

Tests for Anions

Common ions can be identified using test-tube reactions, with observations used to identify a given ion.

Carbonate Ions (CO32−)

Reagents: Dilute hydrochloric acid (HCl) or any aqueous acid.

Observation: Effervescence as carbon dioxide gas is released.

OCR (A) A-Level Chemistry carbonate ion test showing effervescence when dilute acid is added.

Equation examples:

CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l)

HCO3−(aq) + H+(aq) → CO2(g) + H2O(l)

Confirmation of CO2:
Bubble the gas through limewater (Ca(OH)2). If CO2 is present, limewater turns cloudy white due to formation of calcium carbonate.

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

Sulfate Ions (SO42−)

Reagents: Barium chloride solution (BaCl2) acidified with dilute hydrochloric acid (HCl).

Observation: White precipitate of barium sulfate (BaSO4) forms if SO42− is present.

OCR (A) A-Level Chemistry sulfate ion test showing white precipitate with barium chloride solution.

Ionic equation:

Ba2+(aq) + SO42−(aq) → BaSO4(s)

Why acidified?
To remove carbonate ions (CO32−), which could also form a white precipitate with Ba2+ and give a false positive.

Halide Ions (Cl−, Br−, I−)

Test: Add dilute nitric acid (HNO3) to remove carbonate ions. Then add silver nitrate (AgNO3).

OCR (A) A-Level Chemistry halide ion test showing white precipitate for chloride with silver nitrate. OCR (A) A-Level Chemistry halide ion test showing cream precipitate for bromide with silver nitrate. OCR (A) A-Level Chemistry halide ion test showing yellow precipitate for iodide with silver nitrate.

Observations:

Further test – Add NH3:

Equations:

Ag+(aq) + X−(aq) → AgX(s) (X = Cl−, Br−, I−)

Test for Cations

Ammonium Ions (NH4+)

Reagents: Sodium hydroxide solution (NaOH).

Method: Warm the mixture gently.

Observation: Ammonia gas (NH3) is released which has a pungent smell and turns damp red litmus paper blue.

OCR (A) A-Level Chemistry test for ammonium ions showing ammonia released and litmus paper turning blue.

Ionic equation:

NH4+(aq) + OH−(aq) → NH3(g) + H2O(l)

Photo of Matt
Matt’s exam tip

Remember the test sequence is important. Carbonate test first (because Ba2+ and Ag+ also form white carbonate precipitates). Sulfate test second (Ag+ forms insoluble white Ag2SO4 precipitate). Halide test last. Incorrect order can result in confusing precipitates such as BaCO3 or Ag2SO4.

Summary