O-LevelPhysicsThermal Physics5054/21 · Oct/Nov 2019 · Q35 Marks

A saucepan of water is placed on an electric hotplate and the water is brought to the boil. As the water boils, it changes from a liquid into a gas.

(a)Describe, in terms of the molecules, three ways in which a gas differs from a liquid.[3]
(b)In order to keep the liquid boiling, it is necessary to keep the hotplate switched on. Explain why energy must be supplied in order to turn a liquid at its boiling point into a gas.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[B3] any three from: molecules / they are further apart
[B3] any three from: forces between molecules / them smaller
[B3] any three from: molecules / they move in straight lines (between collisions)
[B3] any three from: potential energy of molecules / them greater
[B3] any three from: gas molecules move freely/randomly but liquid molecules slide over each other
[B1] (molecules separated against) intermolecular forces / bonds
[B1] work is done as molecules separate or bonds broken or latent heat supplied

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Topic: Thermal Physics, O-Level Physics (syllabus 5054). Structured question, 5 marks.
Source: O-Level Physics, Oct/Nov 2019 Paper 2 Variant 1, question 3.

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