= Solution
TODO: The single substances in tubes a, b and c and all experimental observations are absent; their identities cannot be determined from the statement.
\b[The identities of $a$, $b$ and $c$ remain undetermined.] The assertion that each tube contains one <chemical substance> does not specify whether it is an element, a salt or an organic compound, nor which <chemical tests> would be positive.
A useful <qualitative chemical analysis> would keep the three samples separate, retain an untouched portion of each, and record their physical state, colour and behaviour with suitable solvents. Small, separately labelled portions would then be used for a logically chosen sequence of <chemical tests>. For a dissolved salt, for example, examination of its <cations> and <anions> addresses different parts of its possible identity; one <precipitation reaction> is rarely a complete identification. A second independent confirmation and comparison with a known reference would strengthen the conclusion.
The record should distinguish an observation, such as formation of a precipitate under stated conditions, from an inference about composition. No observation, reaction equation or identification can be assigned to any one of $a$, $b$ or $c$ from the surviving information. The general procedure explains how a candidate could investigate the tubes; it does not complete the missing identifications.
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