Explain why changes of state, such as melting and boiling, are described as physical changes rather than chemical changes.

Eduqas GCSE Chemistry — 2.1 Bonding, structure and properties · Explain · 4 marks · View as Markdown

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

Water melts at 0 °C and boils at 100 °C. These temperatures depend on the strength of the forces between the particles in the substance.

Model answer (4 marks)

In a physical change the chemical composition of the substance remains unchanged – no new substance is formed.
A physical change involves only a change in the forces between particles; the bonds within the molecules are not broken.
The stronger the forces between the particles, the more energy is required to overcome them, so substances with stronger forces have higher melting or boiling points.
In a chemical change a chemical reaction occurs, producing substances with a different chemical composition; in a change of state no chemical reaction takes place.

Examiner tips

  • Use the phrase ‘chemical composition remains unchanged’ to hit the first point. Mention ‘forces between particles’ and ‘no bonds broken’ for the second. Link ‘stronger forces = higher energy required’ to the third. State ‘no chemical reaction’ for the fourth.
  • common_mistakes
  • :
  • Saying the substance ‘changes’ instead of ‘remains the same’. Confusing a physical change with a chemical reaction by mentioning new compounds. Omitting the point about forces between particles or energy required.

Mark scheme (4 marks)

  1. In a physical change the chemical composition of the substance remains the same / no new substance is formed
  2. A physical change involves a change in the forces between particles / involves breaking or overcoming forces between particles
  3. The stronger the forces between the particles, the more energy is needed to change state / the higher the melting point or boiling point
  4. A chemical change requires a chemical reaction and produces substances with a different chemical composition to the reactants / in a change of state no chemical reaction takes place

Key terms in this question

physical change · chemical change

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