= Solution
TODO: The single substances in tubes a, b, c and d are absent and no identifying test observations are given.
\b[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.<Testing Salts for Anions and Cations by the Royal Society of Chemistry>[Metal hydroxides and negative-ion tests: colour, behaviour in excess reagent and separate anion tests]{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.
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