Specification Point 1.5C (Paper 2C Only):
Know what is meant by the term solubility in the units g per 100 g of solvent

Terminology: Solubility

  • 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 become more soluble.
  • Solubility of gases is affected by temperature and pressure. As pressure increases, gases become more soluble. As temperature increases, gases become less soluble, in general.
Specification Point 1.6C (Paper 2C Only):
Understand how to plot and interpret solubility curves

Solubility Curves

  • Solubility graphs represent solubility in g per 100 g of water plotted against temperature.
  • To plot a solubility curve, the maximum mass of solvent that can be dissolved in 100 g of water before a saturated solution is formed, is determined at a series of different temperatures.

Solubility graph for salts:

Salts solubility graph Edexcel IGCSE Chemistry

Example question 1:

How much potassium nitrate will dissolve in 20g of water at 34 °C?

At 34 °C the solubility is 49g per 100g of water

So scaling, 49 x 20 / 100 = 9.8 g of potassium nitrate will dissolve in 20 g of water.

Example question 2:

200 cm3 of saturated copper solution was prepared at a temperature of 90 °C. What mass of copper sulphate crystals form if the solution was cooled to 20 °C?

Solubility of copper sulphate at 90 oC is 67g/100g water, and 19g/100g water at 20 °C.

Therefore for mass of crystals formed = 67 – 19 = 48g (for 100 cm3 of solution).

However, 200 cm3 of solution was prepared,

So total mass of copper sulphate crystallised = 2 x 48 = 96g

Solubility graph for gases:

Unlike salts, shown in the previous graph, gases become less soluble as temperature increases. E.g. fizzy drinks become flat more quickly when left at a warmer temperature.

Specification Point 1.7C (Paper 2C Only):
Practical: investigate the solubility of a solid in water at a specific temperature

Solubility Experiment

Method:

  • Take down volume of water and heat to specific temperature, e.g. in a water bath set to desired temperature. Keep thermometer in water to make sure temperature is maintained throughout.
  • Add known masses of solvent bit by bit, until saturated solution formed – when solid stops dissolving and remains as solid in solution.
  • Record mass of solid that was soluble.
  • Repeat with different temperature.
  • Can plot solubility curve as seen above.

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.