2023 O Level Chem P1 Q7 — Kinetic Particle Theory Solution

Answer: B. The graph is a heating curve (time goes left to right; temperature rises). The four labels — vaporisation, melting, freezing, condensation — all sit at plateaus on the curve, each with an arrow pointing left (i.e. indicating the cooling, right-to-left direction).

For a label to be 'correctly' applied, two things must match: (i) the plateau temperature has to correspond to that phase change, and (ii) the arrow direction has to match whether the change is a heating or cooling process.

Freezing happens on cooling and at the freezing point (= melting point = lower plateau). The 'freezing' label sits at the lower plateau and its arrow points left — both the temperature and the direction are correct. ✓

Vaporisation and melting are heating processes, so their arrows should point right; pointing left is wrong direction. Condensation does happen on cooling (arrow direction right), but the 'condensation' label sits at the lower plateau — condensation occurs at the boiling point (upper plateau), so the temperature is wrong.