Topic 8: Chemical analysis

Chemical analysis is how chemists work out what a sample actually contains and whether it is pure. This topic covers the meaning of purity, how to separate and identify the parts of a mixture using chromatography, simple tests for common gases, and the more advanced ion tests used to identify dissolved substances.

Pure substances and melting/boiling points

In everyday language 'pure' often means natural or unmixed, like pure orange juice. In chemistry the meaning is stricter: a pure substance is a single element or compound that is not mixed with anything else. The most reliable way to test purity is to measure melting point or boiling point. A pure substance melts and boils at a single, sharp, fixed temperature that matches accepted data book values. A mixture, by contrast, melts and boils over a range of temperatures, and impurities lower the melting point and raise the boiling point compared with the pure substance. So if a sample melts cleanly at the expected temperature, it is likely pure; if it softens and melts across several degrees, it contains impurities.

Formulations

A formulation is a mixture that has been designed as a useful product, with each component present in a carefully measured quantity so that the mixture has the exact properties needed. The proportions are controlled because changing them would change how the product performs. Everyday examples include paints (pigment, solvent, binder, additives), medicines (active drug plus fillers and coatings), cleaning products, fuels, fertilisers, alloys and processed foods. The key idea for the exam is that a formulation is deliberately made to a recipe, so it is a mixture rather than a pure substance, even though it is sold as a single product.

Chromatography

Paper chromatography separates the substances in a mixture, such as the dyes in an ink. It works because of two phases. The stationary phase is the chromatography paper, which stays still. The mobile phase is the solvent (often water or ethanol) that moves up the paper carrying the dissolved substances with it. Each substance is attracted to the two phases by different amounts: a substance more attracted to the mobile phase, and more soluble in the solvent, travels further up the paper. Components therefore separate into spots at different heights. A pure substance always produces just one spot in all solvents, while a mixture produces two or more spots. Always draw the start line in pencil, because pencil is insoluble and will not move in the solvent.

Calculating Rf values (worked example)

To compare results, chemists calculate the retardation factor, or Rf value, for each spot. The formula is: Rf = distance moved by the substance / distance moved by the solvent. Both distances are measured from the pencil start line; the solvent distance is measured to the solvent front (the highest point the solvent reached). Rf values are always between 0 and 1 and have no units. Worked example: a dye spot travels 4.5 cm from the start line while the solvent front travels 9.0 cm. Rf = 4.5 / 9.0 = 0.5. Because Rf depends on the substances and the solvent used, an unknown can be identified by matching its Rf, measured under the same conditions, to a reference value.

Tests for common gases

Four gas tests must be memorised. Hydrogen: hold a burning splint at the mouth of a test tube; the hydrogen burns rapidly with a squeaky pop. Oxygen: insert a glowing (not flaming) splint; oxygen relights it because it supports combustion. Carbon dioxide: bubble the gas through limewater (calcium hydroxide solution); the limewater turns cloudy or milky white. Chlorine: hold damp blue litmus paper in the gas; chlorine first turns it red because it is acidic, then bleaches it white. Remembering both stages of the chlorine test, and that the litmus must be damp, is a common source of marks.

Higher: flame tests and metal hydroxide precipitates

Some metal ions can be identified by the colour they give to a flame. A clean wire is dipped in the sample and held in a blue Bunsen flame: lithium gives crimson red, sodium yellow, potassium lilac, calcium orange-red, and copper green. A limitation is that a strong colour such as sodium yellow can mask a weaker one in a mixture. Metal ions can also be identified by adding sodium hydroxide solution, which forms a coloured insoluble metal hydroxide precipitate. Copper(II) gives a blue precipitate, iron(II) a green precipitate, and iron(III) a brown precipitate. Aluminium, calcium and magnesium all give white precipitates; aluminium hydroxide is distinguished because it redissolves in excess sodium hydroxide while the others do not.

Higher: tests for carbonates, halides and sulfates

These three anion tests are standard at Higher tier. Carbonates: add dilute acid; carbonates fizz, releasing carbon dioxide, which is confirmed by turning limewater cloudy. Halides (chloride, bromide, iodide): add dilute nitric acid then silver nitrate solution; a chloride gives a white precipitate, a bromide a cream precipitate, and an iodide a yellow precipitate. Sulfates: add dilute hydrochloric acid then barium chloride solution; a sulfate gives a white precipitate of barium sulfate. The acid is added first in the halide and sulfate tests to remove carbonate ions, which would otherwise give a false positive precipitate.

Higher: flame emission spectroscopy and instrumental methods

Modern laboratories increasingly use instrumental methods rather than chemical tests because they are faster, more sensitive (detecting tiny amounts), and more accurate. Flame emission spectroscopy is one example used for metal ions in solution. The sample is placed in a flame and the light given out passes into a spectroscope, producing a line spectrum. The pattern of lines identifies which metal ions are present (qualitative analysis), and the intensity of the lines shows their concentration (quantitative analysis). A major advantage over a simple flame test is that instrumental methods can identify several metal ions in the same mixture, even at low concentration.