4. Purification by Crystallisation
The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. The process of crystallisation is used for obtaining a pure solid substance from impure sample. This is done as follows :
1.The impure solid substance is dissolved in the minimum amount of water to form a solution.
2.The solution is filtered to remove insoluble impurities.
3.The clear solution is heated gently on a water bath till a concentrated solution or saturated solution is obtained (This can be tested by dipping a glass rod in hot solution from time to time. When small crystals form on the glass rod, the solution is saturated). Then stop heating.
4.Allow the hot, saturated solution to cool slowly.
5.Crystals of pure solid are formed. Impurities remain dissolved in solution.
6.Separate the crystals of pure solid by filtration and dry.
As an example, we will describe the purification of an impure sample of copper sulphate to obtain pure copper sulphate.
Notes
Matter in Our Surroundings - Notes
1. 7. Metals are Solids at the Room Temperature
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6. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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8. 11. Non-Metals Have Many Different Colours.
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9. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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16. 8. Metals Generally Have High Melting Points and Boiling Points.
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19. 8. Non-Metals Have Comparatively Low Melting Points and Boiling Points
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23. To Study the Properties of a Suspension
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24. Effect of Temperature and Pressure on Solubility
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25. Chemical Formula for daily use material
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26. 1. Non-Metals are Not Malleable. Non-Metals are Brittle.
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28. Pure Substances : Elements and Compounds
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29. Solutions, Suspensions And Colloids
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33. 1. Separation by a Suitable Solvent
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40. 3. Metals are Good Conductors of Heat and Electricity.
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43. To Separate the Salt-Water Mixture (or Salt-Solution)
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44. To Study the Properties of a Compound of Iron and Sulphur
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48. To Distinguish a Colloid from a Solution
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