Hydrochloric acid reacts with sodium hydroxide solution to form sodium chloride and water. Explain why this reaction is a chemical change and not a physical change.

Eduqas GCSE Chemistry — 2.2 Acids, bases and salts · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Hydrochloric acid reacts with sodium hydroxide solution according to the equation: HCl + NaOH → NaCl + H₂O

Model answer (4 marks)

A chemical change occurs when a chemical reaction takes place.
The reaction HCl + NaOH → NaCl + H₂O produces new substances.
The products (NaCl and H₂O) have a different chemical composition from the reactants (HCl and NaOH).
A physical change would only change the state of matter, not the composition, so this is not a physical change.

Examiner tips

  • Use the word "chemical reaction" to show understanding of a chemical change.
  • Mention that the products are different substances with new compositions.
  • Explain that a physical change would not alter composition.
  • Keep the answer brief and to the point to fit the 4 marks.

Common mistakes

  • Saying the reaction is a physical change because it involves liquids.
  • Forgetting to state that the products are different substances.
  • Using vague language like "something changed" without specifying composition.

Mark scheme (4 marks)

  1. A chemical change involves a chemical reaction
  2. The products have a different chemical composition to the reactants
  3. A physical change only involves a change in state / the chemical composition remains the same in a physical change
  4. This reaction is not a physical change because the chemical composition has changed / different substances are present after the reaction

Key terms in this question

chemical change · physical change

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