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WJEC GCSE Additional Science Teacher’s Notes b 2 g/100 cm 3 potassium chlorate at 45 °C Unsaturated c 155 g/100 cm 3 potassium iodide at 15 °C Saturated d 100 g/100 cm 3 sodium nitrate at 30 °C Supersaturated Note that pupils are not required to recall any information about supersaturated solutions. The information is included here to test interpretation of the data. 6. Compare the solubility curves for potassium iodide and potassium nitrate. Potassium iodide is more soluble than potassium nitrate over the whole range of temperatures. The solubility of both substances increases with temperature. The solubility of potassium iodide increases at a constant rate (approx. 15 g/100 cm 3 for every 10 °C), whereas the solubility of potassium nitrate increases at a greater rate as the temperature increases. _What makes water hard or soft (pages 147–49)____ PRACTICAL How can we tell the difference between hard and soft water (pages 147–48) It is envisaged here that four different water samples are made up in the following ways: soft water – e.g. deionised water permanent hard water – e.g. using MgSO 4 temporary hard water – e.g. made by bubbling CO 2 into lime water until the initial precipitate just clears water sample with a mixture of permanent and temporary hardness. 1. Use your results to describe the hardness of each water sample, and whether this hardness is temporary or permanent. Explain the reasons for your identification. The answer will depend on the labelling of solutions and possibly the results. Temporary hard water should be identified as removable by boiling. 2. This method works fairly successfully for comparing the hardness of the water samples. Suggest an improvement that could be made if you wanted a more accurate measure of water hardness. It would be necessary to use water samples of known hardness and to draw a calibration curve to enable the results to be related to actual hardness. TASK Is hard water good for your health (page 149) 2. Why do you think the fact that hard water areas of the world tend to have lower rates of heart disease than soft water areas is considered rather weak evidence for the hypothesis that drinking hard water reduces heart disease The areas (both hard water and soft water) are likely to be very different from each other so a truly ‘fair test’ is not possible. The differences are small. The results of different studies are variable/not very repeatable. 47
WJEC GCSE Additional Science Teacher’s Notes Discussion point On the basis of the evidence, do you think there is a case for adding calcium and magnesium salts to drinking water in soft water areas There appears to be no evidence for a specific link between calcium salts in water and heart benefits. There is some evidence that magnesium salts (>10mg/l) have a protective effect. No information is presented on any possible side-effects of these salts. No evidence is presented on the effect of magnesium salts at the level suggested on the taste of water. 48
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