When some solids dissolve in water the temperature falls, and when others dissolve it rises. In this experiment you find out whether dissolving each of two ...

This is a thermometric practical: the sign of the temperature change tells you the energy direction, and the size and direction of the change let you match the unknown to a named solid. A worked set of readings consistent with the method is used below; your own starting temperatures will differ, but the pattern of a fall for one solid and a rise for the other is what earns the marks.

(a) [6] Record every starting and final temperature to the same precision, then subtract to get each change and label it a rise or a fall. A consistent set of readings looks like this:

Marks: the starting and final temperatures for all three solids recorded [1][1][1][1]; the three changes worked out by subtraction [1]; each correctly labelled a rise or a fall [1].

(b) [2] For solid D the temperature fell from \(20.0\,^{\circ}\mathrm{C}\) to \(14.0\,^{\circ}\mathrm{C}\), so dissolving solid D is endothermic [1]. The reasoning is the energy bookkeeping: to pull the solid apart into dissolved particles more energy is absorbed than is given out when the particles are hydrated, so energy is taken from the water and the water cools [1]. (Had the temperature risen, it would have been exothermic because energy was released to the water.)

(c) [2] Solid X gave a temperature fall of \(5.5\,^{\circ}\mathrm{C}\), the same direction as solid D and close in size to D's \(6.0\,^{\circ}\mathrm{C}\) fall, whereas E gave a rise. Therefore solid X is solid D [1], justified directly from the recorded changes: X matches D's cooling and does not match E's warming [1].

(d) [2] The size of the temperature change depends on how much solid dissolves [1]; using the same \(5.0\,\mathrm{g}\) mass (and the same \(25.0\,\mathrm{cm}^3\) of water) each time keeps the comparison fair, so any difference in the readings is caused by the identity of the solid and not by using more or less of it [1].

(e) [2] Stirring helps the solid dissolve fully and spreads the heat evenly through the liquid [1], so the thermometer reads the temperature of the whole mixture rather than a local hot or cold spot next to an undissolved lump [1].

(f) [3] The change for X may differ slightly from the solid it matches for any three of the following reasons (max 3): the solids may not have been perfectly dry or pure, so a little extra mass is water or impurity [1]; some heat is lost to (or gained from) the surroundings and that loss is not identical between runs [1]; the solid may not have fully dissolved in the time allowed, so not all of the energy change was released [1]; the water's starting temperature differed slightly between the runs [1].