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Chapter: 10th Science : Chapter 9 : Solutions

Hydrated salts and Water of Crystallization

On heating these hydrated crystalline salts, they lose their water of crystallization and become amorphous or lose their colour (if they are coloured).

Hydrated salts and Water of Crystallization

When ionic substances are dissolved in water to make their saturated aqueous solution, their ions attract water molecules which then attached chemically in certain ratio. This process is called hydration. These ionic substances crystallize out from their saturated aqueous solution with a definite number of molecules of water. The number of water molecules found in the crystalline substance is called water of crystallization. Such salts are called hydrated salts.

On heating these hydrated crystalline salts, they lose their water of crystallization and become amorphous or lose their colour (if they are coloured). Table 9.3 shows some common hydrated salts:


 

1. Copper sulphate pentahydrate CuSO4.5H2O (Blue vitriol)

The number of water molecules in blue vitriol is five. So its water of crystallization is 5. When blue coloured copper sulphate crystals are gently heated, it loses its five water molecules and becomes colourless anhydrous copper sulphate.


If you add few drops of water or allow it to cool, the colourless anhydrous salt again turns back into blue coloured hydrated salt.


 

2. Magnesium sulphate heptahydrate MgSO4.7H2O (Epsom salt)

Its water of crystallization is 7. When magnesium sulphate heptahydrate crystals are gently heated, it loses seven water molecules, and becomes anhydrous magnesium sulphate.


If you add few drops of water or allow it to cool, the colourless anhydrous salt again turns back into hydrated salt.


 

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10th Science : Chapter 9 : Solutions : Hydrated salts and Water of Crystallization |


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