Water of crystallization (O-level chemistry)
WATER OF CRYSTALISATION:
This is the amount of water, which must be present in order for a solid to form crystals. When this water of crystallization is removed, the crystals lose their shape.
- A crystalline compound containing water is said to be Hydrated and water crystallization is sometimes called water of hydration e.g. copper sulphate
- Salts without water of crystallization are said to be anhydrous e.g. sodium chloride,
- When hydrated crystalline compounds are heated strongly, they lose water of crystallization and turn into anhydrous powders.
- The process of loss where hydrated crystals lose water of crystallisation and become anhydrous when exposed to air is called efflorescence. A salt that spontaneously loses its the water of crystallization when exposed to air is called efflorescent, examples are Na2SO4.10H2O, Na2CO3.10H2O
- Anhydrous compounds with different colours from hydrated compound can be used to test for water
For example,
Anhydrous copper sulphate of white turns blue on the addition of water because hydrated copper II sulphate is blue. Similarly, anhydrous cobalt chloride is blue will turn pink on the addition of water due to the formation of hydrated cobalt II) chloride
Therefore, anhydrous copper II sulphate, anhydrous cobalt II chloride are used to test for the presence of water.
EXPERIMENT: To prove that copper sulphate contains water of crystallisation.
You are provided with copper sulphate crystals. Place the crystals in a dry test tube and heat first gently and then strongly.
Put two drops of water to the residue.
Record your observation and conclusions.
Result
Blue crystals turn white on heating
CuSO4.5H2O (s) → CuSO4 (s) + 5H2O(l)
Blue white
Addition of a drop of water to the product (anhydrous copper sulphate) the white crystal turn blue.
Conclusion hydrated copper sulphate contains water of crystallization
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