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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

5.3.2 Qualitative analysis

Qualitative analysis

Qualitative Analysis

Specification Reference 5.3.2 (a)

Quick Notes – Summary Table

Ion Test Result
CO32− Dilute HCl Effervescence (CO2)
SO42− BaCl2 + HCl White ppt (BaSO4)
Cl− AgNO3 + HNO3 White ppt (AgCl), soluble in dilute NH3
Br− AgNO3 + HNO3 Cream ppt (AgBr), soluble in conc. NH3
I− AgNO3 + HNO3 Yellow ppt (AgI), insoluble
NH4+ NaOH + heat Ammonia gas (NH3)
Cu2+ NaOH or NH3 Blue ppt; deep blue in excess NH3
Fe2+ NaOH or NH3 Green ppt, darkens
Fe3+ NaOH or NH3 Orange-brown ppt
Mn2+ NaOH Light brown ppt, darkens
Cr3+ NaOH Green ppt; dissolves in excess

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)

Transition Meta Ions – Cu2+, Fe2+, Fe3+, Cr3+ and Mn2+

Reagents: Sodium hydroxide solution (NaOH) or ammonia (NH3).

Iron (II), Fe²⁺ Reactions

OCR (A) A-Level Chemistry test-tube reactions for Fe2+ metal–aqua ion showing expected precipitate colour changes.

Iron (III), Fe³⁺ Reactions

OCR (A) A-Level Chemistry test-tube reactions for Fe3+ metal–aqua ion showing expected precipitate colour changes.

Copper (II), Cu²⁺ Reactions

OCR (A) A-Level Chemistry test-tube reactions for Cu2+ metal–aqua ion showing expected precipitate colour changes.

Chromium (III), Cr³⁺ Reactions

OCR (A) A-Level Chemistry test-tube reactions for Cr3+ metal–aqua ion showing expected precipitate colour changes.

Manganese (II), Mn²⁺ Reactions

OCR (A) A-Level Chemistry test-tube reactions for Mn2+ metal–aqua ion showing expected precipitate colour changes.
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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.