Chemical Analysis: Identifying Substances

Scope note

Most of the tests in this lesson (flame tests, hydroxide precipitates, halide and sulfate tests, instrumental methods) are not on this course. What you need is the chromatography and the tests the specification names in chapter C5; the rest is useful background.

Why Do We Need Chemical Analysis?

Chemists often need to identify unknown substances. This might be to check whether a water supply is safe, to identify minerals in rocks, to verify the purity of a manufactured product, or to analyse an unknown sample in a forensic investigation. The tools available fall into two broad categories: traditional chemical tests that rely on visible observations, and modern instrumental methods that use machines for faster, more accurate results. This lesson covers the tests you need to know and how to apply them systematically.

Flame Tests

A flame test identifies metal ions by the colour they produce when heated in a Bunsen burner flame. Different metal ions emit light of different colours because their electrons absorb energy and then release it as visible light at characteristic wavelengths. To carry out a flame test: dip a clean nichrome wire loop in concentrated hydrochloric acid (to remove previous residues), then dip it into the sample, and hold it in a blue Bunsen flame. The wire must be cleaned thoroughly between tests to avoid contamination, which would give misleading results.

Key Fact

Flame test colours: Lithium — crimson red. Sodium — yellow. Potassium — lilac. Calcium — orange-red. Copper — green / blue-green.

Tests for Gases

Four common gases have reliable, distinctive tests that you must know:

• Hydrogen ($H_2$): hold a burning splint near the mouth of the test tube — a squeaky pop confirms hydrogen igniting with oxygen in the air.
• Oxygen ($O_2$): insert a glowing splint into the gas — it relights because oxygen supports combustion.
• Carbon dioxide ($CO_2$): bubble the gas through limewater — it turns milky or cloudy because calcium carbonate precipitates: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O$.
• Chlorine ($Cl_2$): hold damp litmus paper near the gas — it first turns red (chlorine is acidic) then is bleached white.

Testing for Metal Ions with Sodium Hydroxide

Adding sodium hydroxide solution (NaOH) to a solution containing metal ions produces coloured precipitates. The precipitate forms because metal ions react with hydroxide ions to produce an insoluble metal hydroxide. The colour of the precipitate identifies the metal ion present.

Calcium ions ($Ca^{2+}$) — white precipitate of calcium hydroxide.
Copper(II) ions ($Cu^{2+}$) — blue precipitate of copper hydroxide.
Iron(II) ions ($Fe^{2+}$) — green precipitate of iron(II) hydroxide.
Iron(III) ions ($Fe^{3+}$) — brown precipitate of iron(III) hydroxide.
Aluminium ions ($Al^{3+}$) — white precipitate that dissolves in excess NaOH (aluminium hydroxide is amphoteric).
Magnesium ions ($Mg^{2+}$) — white precipitate that does not dissolve in excess NaOH.
The distinction between aluminium and magnesium is frequently tested: both produce white precipitates, but only aluminium’s dissolves in excess sodium hydroxide.

Testing for Negative Ions (Anions)

Several anions can be identified using specific reagents:

• Carbonate ions ($CO_3^{2-}$): add dilute acid. Carbon dioxide gas is produced and turns limewater milky.
• Sulfate ions ($SO_4^{2-}$): acidify with dilute hydrochloric acid first, then add barium chloride solution. A white precipitate of barium sulfate ($BaSO_4$) confirms sulfate ions.
• Halide ions: acidify with dilute nitric acid, then add silver nitrate solution. Chloride ions ($Cl^-$) give a white precipitate of silver chloride; bromide ions ($Br^-$) give a cream precipitate of silver bromide; iodide ions ($I^-$) give a yellow precipitate of silver iodide.

Key Fact

Halide tests with silver nitrate (after acidifying with dilute nitric acid): chloride → white precipitate. Bromide → cream precipitate. Iodide → yellow precipitate. Always acidify first to prevent interference from carbonate or sulfate ions.

Separation and Purity

Analysis often begins with separating a mixture into its components. The technique used depends on the physical properties of the substances involved:

• Filtration — separates an insoluble solid from a liquid. The mixture is poured through filter paper; the solid (residue) stays on the paper and the liquid (filtrate) passes through.
• Distillation — separates a liquid from a dissolved solid by heating the solution. The liquid evaporates, is cooled in a condenser, and collected as a pure distillate. A mixture of liquids with different boiling points can be separated by fractional distillation.
• Chromatography — separates a mixture of dissolved substances by how strongly each component is attracted to the stationary phase (paper) versus how soluble it is in the mobile phase (solvent).

Each spot on a chromatogram has an Rf value that can be used to identify it.

$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}$$

Example: The solvent front travels 12.0 cm. A pigment spot travels 9.0 cm.
$R_f = \frac{9.0}{12.0} = 0.75$
Rf values are always between 0 and 1. A pure substance produces a single spot with a consistent Rf value under the same conditions. A mixture produces multiple spots.

A pure substance melts and boils at fixed, sharp temperatures. Impurities lower the melting point and broaden the melting range. Checking a substance’s melting point against a known value is therefore a simple way to test purity in the laboratory.

Instrumental Methods

Instrumental methods of analysis are faster, more sensitive and more accurate than traditional chemical tests. Gas chromatography separates the components of a mixture by passing it through a column in which each component travels at a different rate, producing separate peaks. The number of peaks tells you how many components are in the mixture, and the retention time identifies each substance. Mass spectrometry gives the relative molecular mass of a substance by measuring the mass-to-charge ratio of its ions. Gas chromatography linked with mass spectrometry (GC-MS) is particularly powerful because it both separates and identifies components in a single analysis.

Work systematically. First dissolve the sample in water. Use a flame test to suggest the cation (positive ion). Confirm the cation by adding sodium hydroxide and observing the precipitate colour. Then test for the anion: silver nitrate for halides, barium chloride for sulfates, dilute acid for carbonates. Record all observations carefully and compare with known reference data. If chemical tests are inconclusive, instrumental methods such as GC-MS can identify substances with greater certainty.