A student dissolves citric acid powder in water to make a lemon-flavoured drink. The student notices that the outside of the glass feels noticeably cooler after mixing. Explain why the glass feels cooler, using your knowledge of bond breaking and bond making.

OCR GCSE Chemistry A: Gateway Science (J248) — C3.2 Energetics · Explain · 4 marks · View as Markdown

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

When citric acid dissolves and reacts in water, an overall energy change occurs between the student's hand and the glass.

Model answer (4 marks)

The dissolution of citric acid is endothermic.
1. Energy is absorbed from the surroundings.
2. More energy is required to break the bonds in the citric‑acid and water molecules than is released when new bonds form.
3. The energy taken from the surroundings is taken from the glass and the student’s hand.
4. Consequently the temperature of the glass falls, so it feels cooler.

Examiner tips

  • Use the word ‘endothermic’ to show understanding of the energy change.
  • Explain that energy is taken from the surroundings, not produced.
  • Show the energy balance: bond breaking > bond forming.
  • Mention the temperature drop as the result of energy loss.

Common mistakes

  • Saying the reaction is exothermic or that energy is released into the surroundings.
  • Failing to explain that the energy comes from the glass/hand.
  • Using vague terms like ‘heat’ without linking to bond breaking and forming.

Mark scheme (4 marks)

  1. The reaction is endothermic
  2. Energy is taken in from the surroundings (the glass / the student's hand)
  3. More energy is required to break bonds in the reactants than is released when bonds form in the products
  4. Because the surroundings lose energy, the temperature of the surroundings (the glass) decreases

Key terms in this question

bond breaking · bond making

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