CP9.6C: Identifying the Specified Ions

Use the qualification’s specified ion list and the centre’s approved reagents and methods. Label an unknown and take suitable separate portions for different tests. A reagent added in one test can introduce ions that interfere with a later test, so repeatedly adding every reagent to one tube can make results hard to interpret.

Use clean equipment and a justified comparison or known sample where appropriate. For flame tests, contamination can mask the sample’s characteristic colour. Record the actual flame or precipitate observation before deciding an identity. An unexpected result is a reason to check preparation and controls, not to change the observation to match an expected answer.

Use the cation set in the table for this qualification, with the approved reagent conditions. Flame tests complement precipitation evidence: lithium is crimson/red, sodium yellow, potassium lilac, calcium orange-red and copper blue-green. State the observed colour and avoid contamination that can mask a weaker signal.

A white precipitate alone is not a complete cation identity. Use excess-reagent behaviour and appropriate complementary evidence. Copper(II), iron(II) and iron(III) give different characteristic hydroxide colours. Balanced precipitation equations must conserve atoms and charge; for example, Cu²⁺ + 2OH⁻ → Cu(OH)₂(s).

An acid–carbonate test can release carbon dioxide, which turns limewater cloudy under the appropriate test. Acidified silver nitrate can give white, cream or yellow precipitates with chloride, bromide or iodide respectively. Use the specified acidification and separate portions so another reagent does not supply the ion being sought.

A suitable acidified barium-ion test gives a white sulfate precipitate. A white solid without the reagent and preparation context is not a unique sulfate identification. Match the conclusion to the complete evidence, the qualification’s required tests and possible interferences. Do not substitute an untested ion simply because its salt name seems familiar.

Use dilute nitric acid before the specified silver-nitrate halide test, and the appropriate dilute-acid/barium-ion sulfate method. Adding hydrochloric acid to the chloride-test portion can introduce the ion being sought.

Pearson also requires ammonium and the chemical test for ammonia. In the approved alkaline-warming method, released ammonia turns damp red litmus blue. Do not use direct smelling as the identification method.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.