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Chemistry · Chapter 2: Separation and Purification
Crystallisation
Obtaining pure crystals from a solution: crystallisation point, slow and fast cooling, and why it is better than evaporating to dryness.
Key termRememberExampleWatch out
01Concept
Crystallisation is the method used to obtain a pure solid from its solution in the form of crystals. It is used to get pure salt, sugar and copper sulfate crystals.
Steps
- Dissolve the impure solid in the smallest amount of hot water.
- Filter the hot solution to remove any insoluble impurities.
- Heat the filtrate gently to evaporate some water until it reaches the crystallisation point. To test: dip a glass rod in and take it out. If small crystals form on the rod as it cools, the solution is ready.
- Cool the solution slowly. Crystals form because a solid is less soluble in cold water than in hot water.
- Filter to collect the crystals, wash them with a little cold distilled water, and dry them between filter papers.
Crystal size
| Slow cooling | Fast cooling |
|---|---|
| Large, well-shaped crystals | Many small crystals |
| Leave the solution on the bench for a day | Cool the dish in ice water |
Why not just evaporate to dryness?
- Soluble impurities would stay mixed with the solid. In crystallisation they stay behind in the remaining solution.
- Some salts decompose or lose their water of crystallisation if heated to dryness. Blue copper sulfate crystals turn into white powder.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Salt pans near Karachi
Along the coast at Maripur and Hawkes Bay, seawater is let into shallow pans. The sun evaporates the water and salt crystals form, which are raked up.
Sugar mills
Sugar mills in Sindh and Punjab boil sugarcane juice to a thick syrup and let sugar crystallise. The leftover syrup is molasses.
Khewra salt
Rock salt from Khewra is dissolved, filtered and crystallised again to make white table salt.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is crystallisation?Show answer
Answer
Crystallisation is the process of obtaining a pure solid in the form of crystals from its solution, by making a hot saturated solution and letting it cool.
Q2.How do you know when a solution has reached its crystallisation point?Show answer
Answer
Dip a glass rod into the solution and take it out. If small crystals form on the rod as it cools, the solution has reached its crystallisation point.
Q3.Why does slow cooling give larger crystals?Show answer
Answer
Because the crystals have more time to grow in an orderly way. Fast cooling makes many tiny crystals start at once.
Q4.Why is crystallisation better than evaporating to dryness?Show answer
Answer
- Soluble impurities stay in the solution, so the crystals are purer.
- Some substances decompose or lose water of crystallisation when heated to dryness.
Q5.Define a saturated solution.Show answer
Answer
A saturated solution is a solution that cannot dissolve any more solute at a given temperature.
Long questions
Q6.Describe how you would obtain pure crystals of copper sulfate from impure copper sulfate containing sand.Show answer
Answer
- Dissolve the impure copper sulfate in hot water and stir.
- Filter to remove the sand (residue).
- Heat the blue filtrate gently until the crystallisation point is reached (test with a glass rod).
- Leave the solution to cool slowly. Blue crystals of copper sulfate form.
- Filter off the crystals, wash with a little cold water and dry them between filter papers.