Teacher's notes and answers to questions in the book - Hodder Plus ...

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.
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