Substance X is an ammonium salt - Edexcel - GCSE Chemistry - Question 6 - 2013 - Paper 1

To test for ammonium ions, add sodium hydroxide solution (NaOH) to substance X and gently heat the mixture. If ammonium ions are present, ammonia gas will be released. This gas can be detected by its characteristic smell or by turning damp red litmus paper blue.

The ionic equation for the reaction between aluminum ions and hydroxide ions is:

$Al^{3+} + 3OH^{-} → Al(OH)_{3} ext{ (s)}$

This equation shows that one aluminum ion reacts with three hydroxide ions to form a white precipitate of aluminum hydroxide.

To identify the substance, first dissolve the crystals in water. Then conduct the following tests:

1. Flame Test:

   • Dip a clean wire loop into the solution and place it in a flame. If the flame is yellow, sodium ions are present. If the flame is lilac, potassium ions are present.

2. Test for Iodide Ions:

   • Add dilute nitric acid followed by silver nitrate solution. A yellow precipitate indicates iodide ions: $Ag^+ + I^- → AgI ext{ (s)}$
   • If there is no precipitate, sulfate ions are present.

3. Test for Sulfate Ions:

   • Add dilute hydrochloric acid followed by barium chloride solution. A white precipitate indicates sulfate ions: $Ba^{2+} + SO_{4}^{2-} → BaSO_{4} ext{ (s)}$

By performing these tests, the technician can identify which substance was present.