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Solubility and solubility curves
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Solubility tells you how much of a solid dissolves. A solubility curve shows how this changes with temperature.
Solubility
Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a given temperature.
Units: g per 100 g of solvent.
If 15 g dissolves in 50 g of water, the solubility is 15 × 2 = 30 g per 100 g of water.
Units: g per 100 g of solvent.
If 15 g dissolves in 50 g of water, the solubility is 15 × 2 = 30 g per 100 g of water.
Solubility curves
A solubility curve plots solubility against temperature.
For most solids, solubility increases as temperature rises.
If a saturated solution is cooled, the extra solute comes out as crystals.
Mass of crystals = solubility at the higher temperature − solubility at the lower temperature (for 100 g of water).
For most solids, solubility increases as temperature rises.
If a saturated solution is cooled, the extra solute comes out as crystals.
Mass of crystals = solubility at the higher temperature − solubility at the lower temperature (for 100 g of water).
Measuring solubility
Make a saturated solution at a known temperature (add solid until some stays undissolved), then filter.
Weigh an evaporating basin, add some of the solution and weigh again.
Evaporate to dryness and weigh the solid. Mass of water = mass of solution − mass of solid.
Solubility = mass of solid ÷ mass of water × 100.
Weigh an evaporating basin, add some of the solution and weigh again.
Evaporate to dryness and weigh the solid. Mass of water = mass of solution − mass of solid.
Solubility = mass of solid ÷ mass of water × 100.
A saturated solution of salt X in 100 g of water is cooled from 60 °C to 20 °C. Using the table, what mass of crystals forms?
- At 60 °C, 65 g is dissolved.
- At 20 °C, only 30 g can stay dissolved.
- 65 − 30
Answer: 35 g
Solubility is the mass of solute that dissolves in 100 g of solvent at a given temperature (g per 100 g). For most solids it increases with temperature. Cooling a saturated solution forms crystals: the difference in solubility.
The interactive lesson includes the diagrams for this topic.
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