Explain why dissolving a small amount of sugar in water results in no observable increase in the total volume of the solution compared with the volume of the pure water alone.

IB DP Chemistry Higher Level (2023 syllabus) — S1.1 Introduction to the particulate nature of matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sugar molecules are small enough to occupy the gaps between water molecules, which are not perfectly close‑packed. The solute therefore replaces pre‑existing empty space rather than adding new space, so the total volume of the solution remains essentially the same as that of the pure water.

Examiner tips

  • Use the word ‘gaps’ or ‘spaces’ to show understanding of particulate structure.
  • Explain that sugar molecules fit into these spaces, not creating new volume.
  • Mention that this illustrates the discontinuous nature of matter in liquids.

Common mistakes

  • Claiming that sugar increases volume without explaining the role of gaps.
  • Using vague terms like ‘mixing’ instead of ‘filling spaces’.
  • Ignoring the particulate nature of liquids in the explanation.

Mark scheme (4 marks)

  1. Particles (molecules/ions) in a liquid are not arranged in a perfectly close-packed structure and there are spaces/gaps between them.
  2. The small sugar molecules/particles can fit into the spaces/gaps between the water molecules.
  3. Because the solute occupies pre-existing spaces, no additional volume is created/needed.
  4. This behaviour demonstrates that matter is particulate/discontinuous in nature, with significant empty space between particles even in the liquid state.

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