A forensic scientist is analysing an unknown white powder found at a crime scene. The scientist dissolves the powder in water and carries out a series of chemical tests to identify the ions present. The scientist adds dilute hydrochloric acid to one portion of the solution and observes a fizzing. When a few drops of silver nitrate solution are then added to a second portion of the original solution, a cream-coloured precipitate forms. Explain what these two observations tell the scientist about the ions present in the unknown powder.

OCR GCSE Chemistry A: Gateway Science (J248) — C4.2 Identifying the products of chemical reactions · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A forensic scientist dissolves an unknown white powder in water and performs chemical tests. Adding dilute hydrochloric acid to the solution produces fizzing. Adding silver nitrate solution to a separate portion of the original solution produces a cream-coloured precipitate.

Model answer (4 marks)

The fizzing observed when dilute HCl is added shows that carbonate ions (CO₃²⁻) are present, because CO₃²⁻ reacts with H⁺ to give CO₂ gas:
CO₃²⁻ + 2H⁺ → CO₂(g) + H₂O
The evolution of CO₂ can be confirmed by passing the gas through limewater, which turns milky.

The cream‑coloured precipitate that forms when AgNO₃ is added indicates bromide ions (Br⁻) are present. Silver ions react with Br⁻ to give silver bromide:
Ag⁺ + Br⁻ → AgBr(s)
AgBr is a cream‑white precipitate, confirming the presence of Br⁻.

Examiner tips

  • Use the correct chemical equations and state the ions involved; the mark scheme rewards the specific reaction and the identity of the precipitate.
  • Mention the confirmation test for CO₂ (limewater) – this shows a deeper understanding of the reaction.”]
  • common_mistakes
  • :

Mark scheme (4 marks)

  1. The fizzing with dilute hydrochloric acid indicates the presence of carbonate ions (CO₃²⁻)
  2. The gas produced in the fizzing is carbon dioxide, which can be confirmed by testing with limewater turning milky/cloudy
  3. The cream-coloured precipitate with silver nitrate solution indicates the presence of bromide ions (Br⁻)
  4. The cream precipitate is silver bromide, formed because silver ions react with bromide ions

Key terms in this question

silver nitrate · precipitate

Related

More Identifying the products of chemical reactions questions

▶ Try answering this question with AI marking (free) →