This chemistry grades 9-10 lesson on chemical analysis notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on chemical analysis and check yourself with the quiz.
Quick answer: Chemical analysis identifies what a sample contains. Pure substances have sharp melting points, chromatography separates mixtures and gives Rf = distance moved by spot ÷ distance moved by solvent, flame tests and sodium hydroxide precipitates identify metal ions, and simple tests identify gases, carbonates, sulfates and halides.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C8
Chemical analysis notes: the key ideas
How can you tell if a substance is pure?
In chemistry a pure substance is a single element or compound with nothing else mixed in. It is not the same as “natural” or “unprocessed” in everyday language.
A pure substance melts and boils at one sharp, fixed temperature. A mixture melts over a range of temperatures, and impurities usually lower the melting point. A formulation is a mixture designed on purpose, with exact amounts of each component, to give useful properties. Paints, fuels, medicines, fertilisers and alloys are formulations.
How does chromatography separate a mixture?
In paper chromatography a solvent (the mobile phase) moves up a sheet of paper (the stationary phase). Each substance in the sample spends time dissolved in the solvent and time held by the paper. Substances that dissolve well and cling little travel far; others stay near the start. Draw the baseline in pencil, because pencil lead is insoluble and does not run, and keep the starting line above the solvent level.
Rf = distance moved by the substance ÷ distance moved by the solvent front. Both distances are in cm, so Rf has no unit and lies between 0 and 1.
A spot moves 3.6 cm while the solvent front moves 8.0 cm. Rf = 3.6 ÷ 8.0 = 0.45. In the same solvent, a spot with Rf = 0.30 and a front at 9.5 cm travels 0.30 × 9.5 = 2.85 cm.
How do flame tests identify metal ions?
Dip a clean nichrome wire loop in dilute hydrochloric acid, then in the sample, and hold it in a blue Bunsen flame. Metal ions give light of characteristic colours.
How does sodium hydroxide identify metal ions?
Add a few drops of sodium hydroxide solution to the sample solution. Many metal ions form an insoluble metal hydroxide, a precipitate. The colour, and whether it dissolves in excess alkali, identifies the ion.
| Ion | Precipitate | In excess sodium hydroxide |
|---|---|---|
| Al³⁺ | white | dissolves, colourless solution |
| Ca²⁺ | white | no change |
| Mg²⁺ | white | no change |
| Cu²⁺ | pale blue | no change |
| Fe²⁺ | green | no change |
| Fe³⁺ | red-brown | no change |
The ionic equation for copper(II) ions is Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s). The charges balance: +2 − 2 = 0.
How do you test for gases and negative ions?
| Gas | Test and result |
|---|---|
| Hydrogen | lit splint: squeaky pop |
| Oxygen | glowing splint relights |
| Carbon dioxide | limewater turns cloudy |
| Chlorine | damp litmus paper is bleached white |
| Ion | Test | Result |
|---|---|---|
| Carbonate CO₃²⁻ | add dilute acid | fizzing, CO₂ made |
| Sulfate SO₄²⁻ | add dilute HCl, then barium chloride | white precipitate of BaSO₄ |
| Chloride Cl⁻ | add dilute HNO₃, then silver nitrate | white precipitate |
| Bromide Br⁻ | as chloride | cream precipitate |
| Iodide I⁻ | as chloride | yellow precipitate |
For sulfate, Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). For chloride, Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
A white solid gives a white precipitate with sodium hydroxide that dissolves in excess, no flame colour, and a white precipitate with acidified barium chloride. The precipitate behaviour shows Al³⁺, and the barium result shows sulfate. The compound is aluminium sulfate, Al₂(SO₄)₃.
Do not add barium chloride or silver nitrate before acidifying. Carbonates also give white precipitates with both reagents, so the acid removes carbonate first. Also remember that an Al³⁺ precipitate dissolves in excess alkali, but a Ca²⁺ or Mg²⁺ precipitate does not.
Use two tests for each unknown metal ion. Flame colour separates calcium from magnesium, which both give a white precipitate that stays in excess alkali.
- Pure substances melt at one sharp temperature; mixtures melt over a range, lower than the pure value.
- Rf = distance moved by spot ÷ distance moved by solvent front; it has no unit and is always less than 1.
- Flame colours: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ blue-green.
- Gas tests: H₂ squeaky pop, O₂ relights a glowing splint, CO₂ clouds limewater, Cl₂ bleaches damp litmus.
- Sulfate: acidified barium chloride, white ppt; chloride, bromide, iodide: acidified silver nitrate, white, cream, yellow.
Identifying substances with chemical tests: frequently asked questions
Why is the chromatography baseline drawn in pencil?
Pencil lead is insoluble in the solvent, so the line stays put and does not add extra spots. Ink from a pen would dissolve and travel up the paper with the sample.
Can an Rf value be greater than 1?
No. The solvent front always travels at least as far as any substance, so the distance moved by a spot is at most the distance moved by the front. Rf is between 0 and 1.
How can you tell calcium from magnesium?
Both give a white precipitate with sodium hydroxide that does not dissolve in excess. A flame test separates them: calcium gives an orange-red flame, while magnesium gives no flame colour.
Why is the sample acidified before testing for sulfate?
Carbonate ions would also give a white precipitate with barium ions. Adding dilute hydrochloric acid first destroys carbonate, so a white precipitate is then due to sulfate alone.
How can a melting point show that a substance is pure?
A pure substance melts sharply at a fixed temperature that can be compared with a data book value. A sample with impurities melts below that value and over a range of temperatures.
