HookFour days, a park bench, and a spectrum that named a nerve agent
On the afternoon of 4 March 2018, a former spy named Sergei Skripal and his daughter Yulia were found slumped on a bench in a Salisbury shopping centre. Within four days, scientists at the Defence Science and Technology Laboratory at Porton Down — a few miles up the road — had identified the substance that poisoned them as a Novichok, a military nerve agent, with a level of certainty no colour-change test in a school lab could ever provide. They did it with instrumental methods: machines that read the unique molecular fingerprint of a sample from a trace too small to see.
That is the whole of C8 in one case. Chemical analysis answers a single question — what is this substance? — and it hands you two toolkits to answer it. The first is a set of quick bench tests: watch a colour change, listen for a pop, look for a cloudy precipitate. The second is the machine-based methods the professionals actually trust. This chapter walks from deciding whether a sample is even pure, through separating mixtures and identifying gases and ions by hand, to the instruments that do the same job faster, on less material, and with fewer mistakes.
ModelPure substances and formulations
In everyday English pure means natural or unadulterated — pure orange juice, pure wool. In chemistry it means something far stricter: a pure substance is a single element or a single compound, with nothing else mixed in. The test for it is a thermometer. A pure substance melts and boils at fixed, sharp temperatures — pure ice melts at exactly 0°C and pure water boils at exactly 100°C at normal pressure. An impurity does two things: it lowers the melting point and spreads it over a range. So a sharp melting point is a certificate of purity, and a low, smeared melting range is a fingerprint of contamination — exactly how a pharmaceutical lab checks a batch of a drug.
Most useful products are not pure at all, and are not meant to be. A formulation is a mixture designed on purpose, with each component present in a measured quantity to do a specific job. Paint is a formulation of pigment, binder, solvent and additives; a fuel is blended for a target volatility; fertilisers, cleaning products, cosmetics, alloys and medicines are all formulations. A paracetamol tablet is a few hundred milligrams of the active compound held in a formulation of binders, fillers and a coating so that it presses into a tablet and dissolves at the right rate.
Two samples are labelled aspirin. Pure aspirin melts sharply at 135°C. Sample A melts cleanly at 135°C; sample B softens and melts over the range 126–132°C. Which is pure, and what does the other tell you? Sample A is pure: a single, sharp melting point at the literature value. Sample B is impure: its melting point is both lower than 135°C and spread across several degrees, the two classic signatures of an impurity. You cannot say what the impurity is from this alone — only that the sample is not a single pure compound.
MechanismChromatography and the Rf value
Chromatography separates a mixture by making its components race across a surface, and it turns on a competition between two phases. The stationary phase stays put — in paper chromatography it is the paper and the water held in it. The mobile phase moves — it is the solvent that soaks up the paper and carries the dissolved substances with it. Each component is pulled between the two: the more strongly it is attracted to the moving solvent, and the less it clings to the paper, the further up the paper it travels. Components that travel at different speeds end up at different heights, and the mixture separates into spots.
This gives a purity test of its own: a pure substance produces a single spot in every solvent, whereas a mixture separates into two or more. To turn a chromatogram into an identity you calculate an Rf value, the ratio of how far the spot moved to how far the solvent moved: \[R_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent front}}\] An Rf value is always between 0 and 1 (the spot can never outrun the solvent that carries it), and for a given substance in a given solvent it is a constant you can look up — so matching Rf values identifies the components. In the required practical the details are marked: draw the start line in pencil (ink would dissolve and run), keep the spots above the solvent level so they are not washed straight off, and put a lid on to stop the solvent evaporating.
A spot of dye travels 4.5 cm up the paper while the solvent front travels 9.0 cm in the same time. Its Rf value is \[R_f = \frac{4.5}{9.0} = 0.50\] Because it is a ratio of two lengths it has no units, and because the spot cannot overtake the solvent it must be less than 1 — an Rf of 1.2 is always an arithmetic slip. To identify the dye you would compare this 0.50 against known Rf values measured in the same solvent.
MechanismThe four gas tests — say exactly what you see
Four gases have tests so standard that the marks are entirely in the precise wording of the observation. Hydrogen: hold a lighted splint at the mouth of the tube; the gas burns with a squeaky pop as it combines explosively with oxygen in a small volume, \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\). Oxygen: hold a glowing splint in the gas; because oxygen supports combustion, the splint relights. Do not mix these two up — hydrogen needs a flame and pops, oxygen needs a glow and reignites.
Carbon dioxide: bubble the gas through limewater (a solution of calcium hydroxide); it turns milky — a cloudy white — as insoluble calcium carbonate forms. Chlorine: hold damp litmus paper in the gas; it is bleached white (it may flash red first, because chlorine is acidic, before the bleaching removes all colour). In every one of these, an answer that names the reagent, the action and the result — damp litmus paper is bleached white — scores where a vague it changes colour does not.
Write the equation behind the carbon-dioxide test. Limewater is calcium hydroxide solution; the gas reacts to form insoluble calcium carbonate (the milkiness) and water: \[\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O}\] The white cloudiness is a suspension of solid \(\mathrm{CaCO_3}\) — the same compound as chalk and limestone — which is why turns limewater milky is a reliable, specific observation rather than a vague colour change.
CaseIdentifying metal ions — flame tests and hydroxides
Positive metal ions (cations) can be identified two ways. A flame test uses colour: clean a nichrome wire in acid, dip it in the sample and hold it in a roaring blue Bunsen flame. Five colours are examinable — lithium burns crimson, sodium yellow, potassium lilac, calcium orange-red and copper green. The method's weakness is mixtures: sodium's intense yellow is so strong it drowns out fainter colours, so a flame test on a mixture can hide everything but the sodium.
The second method adds sodium hydroxide solution and watches for a coloured precipitate — an insoluble metal hydroxide that forms as a solid in the liquid. Copper(II) ions give a blue precipitate, iron(II) a green one, and iron(III) a brown one — three distinctive colours you must know. Aluminium, calcium and magnesium ions all give a white precipitate, so colour alone cannot separate them; the tie-breaker is that aluminium hydroxide redissolves in excess sodium hydroxide, while calcium and magnesium hydroxides do not.
Predict what you would see when sodium hydroxide is added to copper(II) sulfate solution, and write the ionic equation. The copper(II) ions react with hydroxide ions to form insoluble copper(II) hydroxide, a blue precipitate: \[\mathrm{Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)}\] The state symbols carry the meaning: two dissolved ions \(\mathrm{(aq)}\) combine to make a solid \(\mathrm{(s)}\), and that new solid is the blue cloudiness you record.
CaseIdentifying negative ions — and cracking a full unknown
Negative ions (anions) each have their own test. Carbonates: add a dilute acid; the mixture fizzes and the gas released turns limewater milky, confirming carbon dioxide and therefore a carbonate. Halides (chloride, bromide, iodide): add dilute nitric acid then silver nitrate solution; a coloured precipitate of the silver halide forms — white for chloride, cream for bromide, yellow for iodide. Sulfates: add dilute hydrochloric acid then barium chloride solution; a white precipitate of barium sulfate forms.
The order matters and is heavily marked: you add the acid first to destroy any carbonate ions, which would otherwise form their own precipitate with silver or barium and give a false positive. Required practical 7 puts the whole toolkit together — you identify the two ions in an unknown single ionic compound by running one cation test and one anion test and reading both results as a pair. That is exactly what a real analyst does: no single test names a compound, but two together usually do.
An unknown white solid dissolves in water. A flame test burns bright yellow. A second sample is acidified with dilute hydrochloric acid and then barium chloride is added, giving a white precipitate. Identify the compound. The yellow flame is the signature of the sodium ion, \(\mathrm{Na^+}\). The white precipitate with acidified barium chloride is the signature of the sulfate ion, \(\mathrm{SO_4^{2-}}\), forming insoluble barium sulfate: \[\mathrm{Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)}\] A sodium cation with a sulfate anion means the compound is sodium sulfate, \(\mathrm{Na_2SO_4}\). One cation test plus one anion test has named the solid — the exact logic of the required practical.
DataThe machines — instrumental methods and flame emission spectroscopy
Bench tests are cheap and quick, but professionals reach for instrumental methods — machines — for four reasons the exam wants back: they are faster, more accurate, more sensitive (they work on tiny samples), and they can untangle complex mixtures that would defeat a colour test. They are the reason Porton Down could name a nerve agent from a trace, and the reason a hospital can read a blood sample in minutes.
The one you must know by name is flame emission spectroscopy. A sample solution is sprayed into a flame; the metal atoms absorb energy and then emit light at wavelengths unique to each element. That light is passed into a spectroscope, which spreads it into a line spectrum — a barcode of coloured lines. The pattern of lines identifies which metal ions are present, and the intensity of the lines measures their concentration. Unlike a simple flame test, it can identify several metal ions in the same mixture and measure very low concentrations, which is why it replaced the Bunsen-and-wire method wherever a real number is needed. The technique's own history proves the point: caesium and rubidium were both discovered in 1860–61 purely from unexpected lines in a flame spectrum, elements no wet test had ever revealed.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
This section is marked on precision of observation. It changes colour earns nothing; damp litmus paper is bleached white, limewater turns milky, and a lighted splint gives a squeaky pop earn the mark because they name the reagent, the action and the exact result. Learn the gas tests and the ion colours as fixed phrases.
For ion tests, always state the reagents in the right order and say why: acid before silver nitrate or barium chloride, to destroy carbonates that would give a false precipitate. In Rf calculations, write the fraction, put the substance distance on top, and sanity-check that your answer is below 1 — an Rf over 1 is always wrong. When a question gives you two clues about an unknown, treat them as a cation test plus an anion test and combine them into a formula.
The instrumental-methods questions have a stock answer: machines are faster, more accurate, more sensitive and work on smaller or mixed samples. For flame emission spectroscopy specifically, tie the line pattern to identity and the line intensity to concentration, and note its edge over a simple flame test — it can handle mixtures and very dilute solutions.