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Answer the following questions with the help of the data given in Table 5.4.
| Salts | 10 °C | 20 °C | 30 °C | 40 °C | 60 °C | 80 °C |
|---|---|---|---|---|---|---|
| Potassium nitrate | 21 | 32 | 45 | 62 | 106 | 167 |
| Sodium chloride | 36 | 36 | 36.3 | 36.5 | 37 | 37 |
| Potassium chloride | 35 | 35 | 37.4 | 40 | 46 | 54 |
| Ammonium chloride | 24 | 37 | 41 | 41 | 55 | 66 |
(i) What mass of potassium nitrate would be needed to prepare its saturated solution in 50 g of water at 40 °C?
(ii) A student makes a saturated solution of potassium chloride in water at 80 °C and leaves the solution to cool at room temperature (25 °C). What would she observe as the solution cools? Explain.
(iii) What is the effect of a change in temperature on the solubility of salts? Also, compare the changes in the solubility of the four given salts with increasing temperature from 10 °C to 80 °C.
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Part (i)
Given: at 40 °C the solubility of potassium nitrate is 62 g per 100 g of water. To find: the amount for 50 g of water.
Part (ii)
She will see crystals of potassium chloride come out of the solution and collect at the bottom.
- At 80 °C the solubility is 54 g per 100 g of water.
- At 20 °C it is 35 g and at 30 °C it is 37.4 g, so at 25 °C it is about 36 g.
- After cooling, the water can only hold about 36 g, so roughly 18 g must come out as crystals. That is crystallization.
Part (iii)
| Salt | Solubility change | Increase |
|---|---|---|
| Potassium nitrate | 21 → 167 g | 146 g — the largest |
| Ammonium chloride | 24 → 66 g | 42 g |
| Potassium chloride | 35 → 54 g | 19 g |
| Sodium chloride | 36 → 37 g | 1 g — the smallest |
So potassium nitrate is the best of the four for crystallization, because cooling drops the most solid. Sodium chloride is the hardest to crystallise, because its solubility barely changes with temperature — which is exactly why salt is taken from sea water by evaporation rather than by cooling.
