When zinc powder is added to copper sulfate solution, the blue colour of the solution fades and an orange-brown solid is deposited on the zinc. Explain why this reaction is a chemical change and not a physical change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc powder is added to copper sulfate solution. Over time, the blue colour of the solution fades and an orange-brown solid appears.
Model answer (4 marks)
The reaction produces new substances:
1. Copper metal is deposited as an orange‑brown solid.
2. Zinc ions are released into solution.
3. The blue Cu²⁺ ions are removed from the solution.
These new substances show that the reaction is a chemical change. Moreover, the deposition of copper is irreversible under the conditions given – it cannot be easily reversed by a simple physical process. In contrast, a physical change would not create new substances or would be reversible, which is not the case here.
1. Copper metal is deposited as an orange‑brown solid.
2. Zinc ions are released into solution.
3. The blue Cu²⁺ ions are removed from the solution.
These new substances show that the reaction is a chemical change. Moreover, the deposition of copper is irreversible under the conditions given – it cannot be easily reversed by a simple physical process. In contrast, a physical change would not create new substances or would be reversible, which is not the case here.
Examiner tips
- Mention new substances formed and irreversibility; use terms ‘deposition’, ‘copper metal’, ‘Cu²⁺ ions’
- Show that the blue colour disappears because Cu²⁺ is consumed
Common mistakes
- Saying the change is physical because colour changes; ignoring new substances
- Claiming the reaction is reversible without evidence
Mark scheme (4 marks)
- New substances are formed in the reaction
- The orange-brown solid is copper, which is a new substance not present at the start
- The change is irreversible / cannot be easily reversed
- In a physical change no new substances are formed / the change can be reversed, but this reaction does not meet those criteria
Key terms in this question
chemical change · physical change
Related
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