Solubility curves Edexcel International GCSE Chemistry revision
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In plain words
There's a limit to how much sugar you can stir into a cup of tea. When no more will dissolve, the solution is saturated. Hot water usually holds more than cold, so if a hot saturated solution cools, the extra has to come back out as crystals.
Three things to know
- Solubility is the mass of solute that dissolves in 100 g of solvent at a given temperature. Units: g per 100 g of solvent.
- A saturated solution can dissolve no more solute at that temperature. For most solids, solubility increases as the temperature rises.
- Cool a hot saturated solution and crystals form. The mass of crystals is the difference between the two solubilities.
Worked example
A salt has a solubility of 80 g per 100 g of water at 70 °C and 30 g per 100 g at 20 °C. A saturated solution made with 100 g of water at 70 °C is cooled to 20 °C. What mass of crystals forms?
- At 70 °C the water holds 80 g. At 20 °C it can only hold 30 g.
- The rest comes out: 80 − 30.
- = 50 g of crystals.
Tips and tricks
- Read a solubility curve up from the temperature and across to the mass. Use a ruler.
- Check how much water is in the question. The curve is for 100 g; for 50 g, halve the answer.
It lands in your notebook with its questions as flashcards.
Solubility curves: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What are the units of solubility?
It is a mass of solute for a fixed mass of solvent.
How does the solubility of most solids change as the temperature rises?
More solid dissolves in hot water than in cold.
What is a saturated solution?
Any extra solute stays undissolved.
The solubility of a salt is 36 g per 100 g of water. How much dissolves in 50 g of water at the same temperature?
Half the water dissolves half the mass.
What happens when a hot saturated solution is cooled?
The solubility falls, so the extra solute comes out of solution.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Solubility curves
Edexcel International GCSE Chemistry 4CH1 · 7 marks · papermunch.org
Name ______________________________ Date ______________
State what is meant by the solubility of a substance, and give its units.[2]
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The mass of the substance that dissolves in 100 g of solvent at a particular temperature; g per 100 g of solvent.
The solubility of potassium nitrate is 110 g per 100 g of water at 60 °C and 32 g per 100 g at 20 °C. A saturated solution in 100 g of water at 60 °C is cooled to 20 °C. Calculate the mass of crystals formed.[3]
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78 g. 110 − 32.
Describe how to make a saturated solution of salt in water.[2]
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Keep adding salt to the water and stirring until no more dissolves and some solid is left at the bottom.
Answers: Solubility curves
- 1. The mass of the substance that dissolves in 100 g of solvent at a particular temperature; g per 100 g of solvent.
- 2. 78 g. 110 − 32.
- 3. Keep adding salt to the water and stirring until no more dissolves and some solid is left at the bottom.