Explain why adding magnesium ribbon to dilute hydrochloric acid is a chemical change rather than a physical change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When magnesium ribbon is placed into dilute hydrochloric acid, bubbles are produced and the magnesium ribbon disappears. The test tube becomes warm.
Model answer (4 marks)
The reaction between magnesium and dilute hydrochloric acid is:
Mg(s)+2HCl(aq)→MgCl₂(aq)+H₂(g)
1. A new substance is formed – magnesium chloride is produced.
2. Hydrogen gas is evolved as a new substance.
3. The reaction is essentially irreversible; the magnesium is consumed and cannot be recovered.
4. The test tube warms, indicating an exothermic energy change, a hallmark of a chemical change.
Mg(s)+2HCl(aq)→MgCl₂(aq)+H₂(g)
1. A new substance is formed – magnesium chloride is produced.
2. Hydrogen gas is evolved as a new substance.
3. The reaction is essentially irreversible; the magnesium is consumed and cannot be recovered.
4. The test tube warms, indicating an exothermic energy change, a hallmark of a chemical change.
Examiner tips
- Use the reaction equation to show new products. Mention irreversibility and heat release. Keep the answer concise and use correct chemical symbols.
Common mistakes
- Failing to name the new products. Describing the process as merely a dissolution. Ignoring the exothermic temperature rise.
Mark scheme (4 marks)
- New substance(s) are formed in a chemical change
- Identification of a new substance formed, e.g. magnesium chloride or hydrogen gas
- The reaction is difficult / impossible to reverse (irreversible)
- Energy change (heat/temperature rise) is evidence of a chemical change
Key terms in this question
chemical change · physical change
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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