O-LevelPhysicsThermal Physics5054/21 · May/June 2021 · Q36 Marks

A hot steel rod is cooled by plunging it into cold water, as shown in Fig. 3.1. [Figure 3.1]

(a)The steel rod has a mass of 2.0 kg and is initially at a temperature of 500 °C. It cools to 50°C when placed in the water. The specific heat capacity of steel is 460 J/(kg°C). Calculate the thermal energy (heat) lost by the steel rod as it cools to 50°C. thermal energy =[3]
(b)(i)A small mass of water boils when the rod is placed in the water. The remaining water then cools to room temperature and some of it evaporates. In both boiling and evaporation, water molecules escape into the air. State one way in which boiling is different from evaporation.[1]
(b)(ii)The rate of evaporation decreases as the water cools. Explain why this happens, using ideas about molecules.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[C1] (E =) mcT in any form algebraic or numerical
[C1] 2 x 460 x 450
[A1] 4.1 xx 10^5 J
[B1] any one of: boiling occurs at one temperature / boiling point or evaporation occurs at any temperature / below boiling point; boiling occurs within liquid or evaporation occurs on the surface; evaporation increased by draughts, more surface area (and boiling does not)
[B1] (average) speed / K.E. of molecules decreases (as temperature falls) or (molecules) do not have the energy to escape / break bonds
[B1] fewer fast/energetic molecules escape or (only) fastest / high KE molecules escape

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