Chlorine water is added to a solution of potassium bromide. Describe what is observed and explain why this reaction occurs, referring to the reactivity of the halogens.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chlorine is a halogen in Group 7. When chlorine water is added to a solution containing bromide ions, a chemical reaction takes place.
Model answer (4 marks)
The solution turns orange‑brown.
Chlorine displaces bromide ions from potassium bromide, forming potassium chloride and bromine.
This occurs because chlorine is more reactive than bromine.
Halogens become less reactive down Group 7, so the higher‑up element chlorine can displace the lower‑down element bromine.
Chlorine displaces bromide ions from potassium bromide, forming potassium chloride and bromine.
This occurs because chlorine is more reactive than bromine.
Halogens become less reactive down Group 7, so the higher‑up element chlorine can displace the lower‑down element bromine.
Examiner tips
- Use the colour change to show observation; mention displacement reaction; state reactivity trend; link trend to displacement ability.
- Use exact terms: "chlorine displaces bromide" and "halogens become less reactive down the group".
Common mistakes
- Saying the solution turns green instead of orange‑brown; not mentioning displacement; confusing reactivity trend direction; omitting the halogen trend explanation.
Mark scheme (4 marks)
- The solution turns orange/brown
- Chlorine displaces bromide ions / chlorine displaces bromine from potassium bromide
- Chlorine is more reactive than bromine
- Halogens become less reactive going down Group 7, so chlorine (higher up the group) is more reactive and can displace bromine
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →