A student heats a piece of magnesium ribbon in air. The magnesium glows brightly and a white powder is formed. The student tries to reverse the change but cannot get the magnesium back. Describe the evidence that shows this is a chemical change, and state one general difference between a chemical change and a physical change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium ribbon is heated in air. It burns with a bright white flame and a white powder remains. The original magnesium cannot be recovered.
Model answer (4 marks)
A new substance is formed – the white powder is magnesium oxide.
The change cannot be reversed – the original magnesium cannot be recovered.
Energy is released – the bright white flame shows light and heat are given out.
In a chemical change a new substance is produced, whereas in a physical change the substance itself is unchanged.
The change cannot be reversed – the original magnesium cannot be recovered.
Energy is released – the bright white flame shows light and heat are given out.
In a chemical change a new substance is produced, whereas in a physical change the substance itself is unchanged.
Examiner tips
- Use the exact wording ‘new substance is formed’ and ‘cannot be recovered’ to hit the marks. Mention the energy release (light/heat) as evidence of a chemical change. Compare with a physical change by stating ‘no new substance is formed’. Keep the answer short – each point should be one sentence.
Common mistakes
- Writing that the magnesium simply ‘dissolved’ or ‘changed colour’ – this is a physical change. Forgetting to mention that the original magnesium cannot be recovered. Using vague phrases like ‘the reaction is irreversible’ without linking to the evidence.
Mark scheme (4 marks)
- A new substance is formed (white powder / magnesium oxide)
- The change is difficult / impossible to reverse (the original magnesium cannot be recovered)
- Energy is given out / light and heat are released (bright glow indicates energy change)
- In a physical change the substance is not changed / no new substance is formed, whereas in a chemical change a new substance is produced
Key terms in this question
chemical change · physical change
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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