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

To confirm the presence of ammonium ions, add a few drops of sodium hydroxide solution to substance X and gently heat the mixture. Ammonium ions will react with the sodium hydroxide to produce ammonia gas, which can be detected by its characteristic smell or by turning moist red litmus paper blue.

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

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

This reaction produces a white precipitate of aluminium hydroxide.

To identify the substance:

1. Flame Test: Dissolve a small amount of the crystals in water. Dip a clean splint into the solution and hold it in a Bunsen burner flame.

   • A yellow flame indicates sodium ions (sodium sulfate or sodium iodide).
   • A lilac flame indicates potassium ions (potassium sulfate or potassium iodide).

2. Iodide Test: If sodium ions are present, add dilute nitric acid followed by silver nitrate solution to check for iodide ions.

   • A yellow precipitate would indicate the presence of iodide ions.
   • No precipitate indicates sulfate ions are present.
   • The reaction can be written as: $Ag^+ (aq) + I^{-} (aq) → AgI (s)$

3. Sulfate Test: Add dilute hydrochloric acid followed by barium chloride solution. A white precipitate indicates sulfate ions are present. The reaction is: $Ba^{2+} (aq) + SO_{4}^{2-} (aq) → BaSO_{4} (s)$