Past exam of the medicine course of the University of Oxford / 1865 / First examination / Practical chemistry (No. II.)

Original volume: Examination papers for the Degree of Bachelor of Medicine by the University of Oxford (1864–1867), PDF pages 31–31. Open the original paper in the scanned volume. Oxford, December, 1865.

Original questions are transcribed from the public-domain edition. Credit: Wellcome Collection; original supplied by Royal College of Physicians, London. Source and public-domain rights statement.

Each of the tubes, $a$, $b$, $c$, $d$, contains a single substance.

TODO: The single substances in tubes a, b, c and d are absent and no identifying test observations are given.

The identities of a, b, c and d remain undetermined. The assertion that each tube holds a single chemical substance narrows the type of problem but gives no chemical test result from which to choose a name.

A useful examination would give each specimen its own labelled record and preserve an untouched portion. Record physical appearance and behaviour in suitable solvents before choosing tests. A soluble inorganic salt calls for separate investigation of its cation and anion; an insoluble material requires a justified preparation step before solution tests can be interpreted. A colour change, precipitate or evolved gas must be recorded as an observation before it is used as evidence for a composition. Fresh portions prevent one reagent from introducing the ion sought by the next test.

In qualitative chemical analysis, a discriminating result is followed by an independent confirmation and comparison with a reagent blank and a known reference. For example, the behaviour of a metal hydroxide depends on both the cation and the chemical conditions; “a precipitate formed” is not a complete identification.^[ref] The record supplied here contains none of those observations, so no particular reaction or identity can be assigned to any of the four letters.

The solution $e$ is to be examined for hydrocyanic acid, and the solution $f$ for iron and magnesia.

TODO: Solutions e and f and their test observations are absent. Hydrocyanic acid, iron and magnesia are testing targets, not supplied positive findings; general test methods can still be described.

The presence or absence of hydrogen cyanide in e, and of iron or magnesium in f, remains unestablished. The named substances are targets of the examination. They are not positive findings.

The period name “hydrocyanic acid” refers to aqueous hydrogen cyanide. “Magnesia” is the historical name for magnesium oxide; examination of a chemical solution for magnesia normally concerns dissolved magnesium compounds, rather than a claim that intact magnesium oxide particles are dissolved unchanged. The modern ionic equations below express test principles and are not observations made on f.

For e, the analytical task is to obtain a response attributable to cyanide, distinguish it from responses of other constituents, and check the entire procedure against a reagent blank and a known reference. The distinction between free cyanide and cyanide held in a coordination complex matters: a test for one fraction need not measure the other. As a modern comparison, the Cyanide in Waters and Extracts, Method 9014 by United States Environmental Protection Agency provides titrimetric analysis and spectrophotometric analysis, with specified sample preparation, interferences and quality controls. This 2014 procedure illustrates how a result is validated; it is not being attributed to the 1865 candidate.^[ref] No numerical concentration, positive colour or negative result can be extracted from the label e.

An odour is neither an acceptable identifying experiment nor a reliable negative test. Hydrogen cyanide is volatile and highly toxic, and many people cannot detect its supposed characteristic smell. Any actual analysis belongs in a suitably contained laboratory method; uncontrolled acidification of a possible cyanide solution can release hydrogen cyanide.^[ref][ref]

For f, distinguish iron(II) from iron(III) when describing a test. On suitable separate portions, hydroxide can produce a green iron(II) hydroxide precipitate or a brown hydrated iron(III) hydroxide precipitate. Iron(II) products can oxidise during handling, so the time at which a colour is observed matters. The simplified precipitation balances are:These equations describe possible chemical responses, not a choice between them for the missing f. Tests intended for a particular oxidation state cannot be treated as universal tests of every form of iron.

Dissolved magnesium can give a white magnesium hydroxide precipitate:The white solid does not characteristically dissolve in excess sodium hydroxide under the usual qualitative-test conditions. This differentiates it from some amphoteric hydroxides, but is not uniquely specific for magnesium.^[ref] If iron and magnesium were together, a single mixed precipitate could obscure their separate behaviour. An analytical separation with controlled pH, followed by confirmation on the separated fractions, would be needed; indiscriminately adding excess alkali is not proof of separation.^[ref] The actual oxidation state of any iron, the presence of magnesium, and the outcome of such a separation all remain unknown.

Under what circumstances can you obtain a fixed gas from the solid substance $g$, and what is the nature of the gas ?

TODO: Solid substance g is an unidentified supplied specimen. Without its identity or relevant observations, the particular gas and conditions for obtaining it cannot be determined.

Neither the gas nor the conditions required to obtain it from g can be identified. The word “fixed” does not supply a chemical formula, and the existence of a labelled solid does not establish that it is a carbonate or any other particular chemical compound.

A defensible investigation would first characterise g and distinguish physical release of an absorbed gas from a chemical reaction that produces a new gas. Whether heating, contact with a solvent, or a selected reagent is relevant depends on that characterisation. An arbitrary thermal decomposition equation would therefore assume the very composition that is missing.

If an identified reaction produced a gas, the apparatus would be chosen for its actual properties: a reaction vessel and delivery system, a means of distinguishing the product from initially enclosed air, and a compatible collection or detection method. Collection over water would be unsuitable when dissolution or reaction with water materially changes the product; a gas syringe is an alternative only when compatible with the substances and conditions. The gas would then require discriminating chemical tests, and the residue could help check the proposed reaction. No evolution, odour, combustion behaviour, precipitation result or residue is reported for g, so none of those observations can be supplied as its answer.