### 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.

### 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: 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.

### 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.