Solubility Curves
Learning Objectives
The use of solubility tables and experimental measurement of solubility in gram per 100 g of water
The quantitative relationship between temperature and solubility of a given solid, liquid or gas in water
The use of solubility curves as a quantitative and predictive tool in selected biological, domestic or industrial contexts
Summary Notes
Aqueous solutions are solutions that contain a solid, liquid or gas that has been dissolved in water.
Aqueous solutions can be classified as saturated, unsaturated and supersaturated.
The solubility of a substance in water is measured in g per 100 g of water.
Solubility curves can be drawn to show the maximum amount of solute which can be dissolved at different temperatures.
Levels of saturation
Solubility curve (Alamy)
Solubility curves is a graph of solubility, measured in g/100 g water, against temperature in °C.
From the graph shown, at 50 °C, 51 g of NH₄Cl can be dissolved in 100 g of water while 115 g of NaNO₃ can be dissolved in 100 g of water. Any more than that and the solution will be supersaturated while any less will make the solution unsaturated.
Note that with heating more solid can generally dissolve (graphs increase). This is the opposite for gases.
When cooling or evaporating a saturated solution, the solution becomes supersaturated. The extra solute present in the solution will crystallise out to form solids.
Calculating Solubility Ratios
Solubility of compounds are often represented using ratios. Using the example of NH₄Cl and NaNO₃, the saturated solutions will be represented as 51 g/100 g and 115 g/100 g respectively.
Despite solubility curves always being presented as having 100g of water as solvent, this is not necessarily the case.
Eg. A saturated solution of sodium nitrate contains 70 g of sodium nitrate in 40 g of water at 60 °C. What is the solubility of sodium nitrate at this temperature?
50 g dissolves in 40g of water
x g dissolves in 100g of water.
x / 50 = 100 / 40
x = (100 x 50) / 40
x = 125 g
Solubility: 125 g / 100 g
Interpreting Solubility Curves
Working through Solubility Curve Questions
Importance of Temperature and Solubility Relationship
Practice Material
Teachers' Tip:
Always check to be sure how much solvent the solutes are being dissolved in. Very often, it is not 100 g despite the graph showing it to be.