Chlorine gas is bubbled into potassium bromide solution. The colourless solution turns orange-brown. Explain what has happened in terms of oxidation and reduction, identifying which species has been oxidised and which has been reduced.

Cambridge International IGCSE Chemistry (0620) — 6.4 Redox · Explain · 4 marks · View as Markdown

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

Chlorine gas is bubbled into potassium bromide solution. The colourless solution turns orange-brown.

Model answer (4 marks)

Bromide ions are oxidised.
Bromide ions lose electrons to form bromine.
Chlorine is reduced.
Chlorine gains electrons to form chloride ions.

Use the verb ‘oxidised’ for the species losing electrons and ‘reduced’ for the species gaining electrons.
Show the electron transfer clearly: Br⁻ → Br₂ + 2e⁻ and Cl₂ + 2e⁻ → 2Cl⁻.
Mention the colour change as evidence of bromine formation.
Keep the answer concise to fit the 4‑mark limit.

Writing ‘bromine is oxidised’ instead of ‘bromide is oxidised’. Failing to state that chlorine is reduced. Using the wrong charge on the species (e.g., Cl⁻ instead of Cl₂).