Paper 2 · November 2025 · Kinetic theory and states of matter
A pure substance is heated steadily from a solid. On its heating curve, B-C is a flat plateau (melting) and D-E is a later flat plateau (boiling), with the temperature rising continuously in between, over region C-D, and rising again after E. In region C-D, the substance is
Aliquid, since it lies between the melting plateau and the boiling plateau, where only one phase is warming up.
Bsolid, since region C-D comes soon after the melting plateau B-C and solids come before liquids on the graph.
Ca mixture of solid and liquid, since melting is still finishing off gradually through this region.
Dgas, since the temperature is rising and rising temperature always means the substance has become a gas.
Explanation
A flat plateau on a heating curve marks a change of state, where the temperature holds steady while energy breaks intermolecular forces instead of raising the temperature. Between the melting plateau (B-C) and the boiling plateau (D-E), only a single phase is present and warming steadily: the liquid formed at B-C, heating up towards its boiling point at D.
Derived from ZIMSEC Chemistry 4024/2, November 2025, Q1(a)(i)