O-LevelPhysicsThermal Physics5054/22 · Oct/Nov 2019 · Q46 Marks

In many countries, solid salt is produced by trapping sea-water in large, shallow ponds and letting the water evaporate. [Figure 4.1] shows salt being produced in this way.

(a)State two ways in which evaporation differs from boiling.[2]
(b)Describe, in terms of water molecules, what is happening as the water evaporates.[2]
(c)(i)State why the ponds used in this process have large surface areas.[1]
(c)(ii)State why this method of salt production does not work well in a country with a cold climate.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[B2] any two from: only occurs at the surface / no bubbles produced; occurs at any temperature; produces cooling
[B1] molecules escape (from liquid)
[B1] fast(est) molecules / molecules with great(est kinetic) energy escape or slow(est) molecules / molecules with small(est kinetic) energy remain or (average) speed of remaining molecules decreases or molecules gain (kinetic) energy and escape
[B1] rate of evaporation increases (with increasing area)
[B1] rate of evaporation decreases with decreasing temperature or takes too much time

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