Ethanol (C₂H₅OH) can be produced by the fermentation of glucose (C₆H₁₂O₆). During fermentation, yeast converts glucose into ethanol and carbon dioxide. Explain why fermentation is described as a chemical change rather than a physical change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanol is used as a fuel and as a solvent. It can be made from glucose by fermentation using yeast.
Model answer (4 marks)
A chemical reaction takes place when yeast ferments glucose.
New substances are produced – ethanol and carbon dioxide.
The products have a different chemical composition to the reactant, glucose.
A physical change only changes the form or state of a substance, not its composition – this does not occur here.
New substances are produced – ethanol and carbon dioxide.
The products have a different chemical composition to the reactant, glucose.
A physical change only changes the form or state of a substance, not its composition – this does not occur here.
Examiner tips
- Use the word ‘chemical reaction’ to show understanding of the definition.
- Show the formation of two new products – ethanol and CO₂ – to hit the product point.
- Explain the change in composition to satisfy the composition criterion.
- Mention that a physical change would not alter composition to avoid a common mistake.
Common mistakes
- Saying fermentation is a physical change because it looks like a simple mixture.
- Failing to mention that new substances are formed.
- Using vague terms like ‘change’ without specifying chemical composition.
Mark scheme (4 marks)
- A chemical reaction takes place / a chemical change requires a chemical reaction
- New substances / products are formed (ethanol and carbon dioxide)
- The products have a different chemical composition to the reactant(s) / glucose
- A physical change only changes the form / state of a substance, not its chemical composition — this does not happen here
Key terms in this question
chemical change · physical change · fermentation
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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