Fig. 7.1 shows the apparatus a student sets up to obtain a cooling curve for a pure solid that melts a little above room temperature. A boiling tube contain...

Labelled answer diagram:

This tests the practical method for obtaining a cooling curve and the reasoning behind good technique.

(a) Method [3]: first melt the substance so the thermometer bulb is fully covered by the liquid [1]; then allow it to cool and record the temperature at regular fixed time intervals (for example every minute) using a stopwatch [1]; continue until the substance has fully solidified / the temperature has fallen well below the freezing point, then plot temperature against time [1]. Regular timed readings are what let you draw a smooth curve and spot the flat freezing section.

(b) The two quantities measured are the temperature of the substance [1] and the time [1] - these become the y-axis and x-axis of the cooling curve.

(c) As it cools from liquid to solid [3]: the liquid particles lose kinetic energy and move more slowly [1]; the attractive forces pull the particles together into a regular fixed arrangement [1]; the particles can then only vibrate about fixed positions, so a solid forms [1].

(d) Using a water bath rather than a direct flame: the heating is more even and gentle, so the substance does not overheat above its melting point [1]; and it is safer because the substance is flammable and there is no naked flame near it (it also avoids local overheating) [1].

(e) Improvements for more reliable temperatures (any two, plus a valid third) [3]: stir the liquid so the temperature is even throughout [1]; use a thermometer with finer / smaller divisions or read at eye level to avoid parallax error [1]; and repeat the experiment and average, or take readings at more frequent intervals [1].