Chemical analysis answers two questions: whether a substance is pure, and what it contains.
In chemistry pure means a single element or compound. A pure substance melts and boils at a specific temperature. A mixture usually melts and boils over a range of temperatures instead, and this is used to test purity.
A formulation is a mixture designed to do a job, with each component present in a measured quantity. Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all examples of formulations.
Chromatography separates substances by how strongly each is attracted to the stationary phase (the paper) compared with the mobile phase (the solvent). A substance more attracted to the solvent travels further.
Draw the baseline in pencil, because ink would dissolve and run, and keep the baseline above the solvent level. Under suitable conditions a pure substance normally gives one spot; a mixture may give more than one. Two different substances can share an Rf value or fail to separate in a particular solvent, so one chromatogram is not conclusive.
\(R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}}\), always between 0 and 1, and comparing Rf values under the same conditions can help identify a substance, though it does not prove identity on its own.
Chlorine bleaches damp litmus paper, whereas ammonia turns damp red litmus paper blue.
A clean wire loop dipped in the sample and held in a blue Bunsen flame gives a colour characteristic of the metal ion:
In a mixture, one flame colour may mask another. Sodium's intense yellow emission is particularly likely to do this.
Adding sodium hydroxide solution precipitates the metal hydroxide, and the colour narrows down the ion: calcium, magnesium and aluminium all give a white precipitate; copper(II) gives blue; iron(II) gives green; iron(III) gives brown.
To tell the three white ones apart, add excess sodium hydroxide: of these three, only the aluminium hydroxide redissolves. Some other hydroxides, zinc among them, also dissolve in excess.
Carbonates: add dilute acid; the mixture fizzes and the gas turns limewater milky.
Halides: add dilute nitric acid then silver nitrate: chloride gives a white precipitate, bromide cream, iodide yellow.
Sulfates: add dilute hydrochloric acid then barium chloride: a white precipitate forms.
Identifying a salt means finding both its ions, so a cation test and an anion test are both needed.
Use enough tests to tell the possible ions apart unambiguously; a single result that several ions could produce is not enough on its own. Adding acid first removes carbonate ions, which would otherwise give a white precipitate and be mistaken for a chloride or a sulfate. The choice of acid matters: nitric acid for the halide test, because hydrochloric acid would add chloride ions of its own, and hydrochloric acid for the sulfate test, where chloride does not interfere.
Instrumental methods are generally more accurate and more sensitive than simple laboratory tests, can work with very small samples and are often much faster, though the equipment is expensive and needs training to use.
Flame emission spectroscopy puts a sample into a flame and passes the light into a spectroscope. The output is a line spectrum whose pattern identifies the metal ions present, and whose intensity gives their concentration. It can identify metal ions in mixtures much more reliably than a simple flame test.
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