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. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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3. The Case of a Liquid Solute Dissolved in a Liquid Solvent
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4. 11. Metals Usually Have a Silver or Grey Colour
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8. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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9. To Study the Properties of a Colloid
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10. 7. Metals are Solids at the Room Temperature
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11. Separation Of Mixture Of a Solid And a Liquids
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12. 6. Non-Metals are Not Strong. They Have Low Tensile Strength.
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17. 11. Non-Metals Have Many Different Colours.
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18. To Study the Properties of a Compound of Iron and Sulphur
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20. 1. Separation by a Suitable Solvent
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27. 6. Metals are Usually Strong. They Have High Tensile Strength.
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28. To Study the Properties of a Solution
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32. Separation of Mixture Of Two Solids
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39. 3. Metals are Good Conductors of Heat and Electricity.
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40. Chemical Formula for daily use material
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43. Solutions, Suspensions And Colloids
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45. To Distinguish a Colloid from a Solution
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48. To Separate a Mixture of Alcohol and Water
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50. To Study the Properties of a Suspension
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