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Use the salt solubility data in Table 5.4. (i) What mass of potassium nitrate is needed for a saturated solution in 50 g of water at 40 °C? (ii) A saturated potassium chloride solution made at 80 °C is cooled to 25 °C — what is observed? (iii) What is the effect of temperature on solubility, and compare the four salts.
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(i) At 40 °C, potassium nitrate's solubility is 62 g per 100 g water. For 50 g of water, mass needed = 62 ÷ 2 = 31 g .
(ii) As the potassium chloride solution cools from 80 °C to 25 °C, solubility falls, so the excess salt crystallises out and is seen as solid crystals.
(iii) Solubility generally increases with temperature. The rise is steepest for potassium nitrate (21 → 167), moderate for ammonium chloride and potassium chloride, and very small for sodium chloride (about 36 → 37), which barely changes.
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