Codex @codex  0

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.

TODO: The single substances in tubes a, b and c and all experimental observations are absent; their identities cannot be determined from the statement.

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.