Solubility (Edexcel IGCSE Chemistry (Modular))
Revision Note
Written by: Stewart Hird
Reviewed by: Lucy Kirkham
Solubility
Solubility is a measurement of how much of a substance will dissolve in a given volume of a liquid
The liquid is called the solvent
The solubility of a gas depends on pressure and temperature
Different substances have different solubilities
Solubility can be expressed in g per 100 g of solvent
Solubility of solids is affected by temperature
As temperature increases, solids usually become more soluble
Solubility of gases is affected by temperature and pressure; in general:
As pressure increases, gases become more soluble
As temperature increases, gases become less soluble
Solubility curves
Solubility graphs or curves represent solubility in g per 100 g of water plotted against temperature
To plot a solubility curve, the maximum mass of solute that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures
Solubility curve for three salts
While the solubility of most salts increases with temperature, sodium chloride, or common salt, hardly changes at all
Worked Example
Use the solubility curve to answer these questions:
Determine how much potassium nitrate will dissolve in 20 g of water at 50 °C?
200 cm3 of saturated lead(II) nitrate solution was prepared at a temperature of 90 °C. What mass of lead(II) nitrate crystals form if the solution was cooled to 40 °C?
Answers:
At 50 °C, the solubility of potassium nitrate is 68 g per 100 g of water
So scaling, 68 x (20 / 100) = 13.6 g of potassium nitrate will dissolve in 20 g of waterSolubility of lead(II) nitrate at 90 oC is 118 g / 100 g water, and 64 g / 100 g water at 40 °C.
Therefore for mass of crystals formed = 118 – 64 = 54 g (for 100 cm3 of solution).
However, 200 cm3 of solution was prepared,
So total mass of lead(II) nitrate crystallised = 2 x 54 = 108 g
Examiner Tips and Tricks
As temperature increases, solids usually become more soluble and gases become less soluble.
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