Page 185 - ICSE Chemistry 8

Procedure:
• In the other bo le, pour 1 cup of dis lled water and add a few drops of liquid detergent in it. Label
it bo le 2.

• Screw the bo le cap ghtly and shake the bo le well for 10 seconds.
Observa on: In bo le 2, you will observe the forma on of lather on the surface of water.

Conclusion: This indicates that the water in bo le 2 is so water.

Causes of Hardness of Water

Hardness of water is caused by the presence of dissolved salts of calcium and magnesium such as calcium

chloride (CaCl ), calcium sulphate (CaSO ), calcium bicarbonate [Ca(HCO ) ], magnesium chloride (MgCl ),
4
2
2
3 2
magnesium sulphate (MgSO ) and magnesium bicarbonate [Mg(HCO ) ]. Hardness is of two types—
4
3 2
temporary and permanent.
• Temporary hardness of water: Water which contains bicarbonates of calcium and magnesium is
called temporary hard water. Temporary hardness can be easily removed by simple means such as
boiling and fi ltra on. On boiling, the soluble bicarbonates of calcium and magnesium change into
insoluble carbonates which can then be fi ltered to give so water.
• Permanent hardness of water: Water which contains chlorides and sulphates of calcium and
magnesium is called permanent hard water. Permanent hardness cannot be removed by simple
means such as boiling. This is because soluble chlorides and sulphates are stable to heat.

Disadvantages of Hard Water

• Hard water is not suitable for drinking and cooking purposes.

• Hard water is unfi t for washing as it results in wastage of soap. The scum formed is diffi cult to
remove and leaves dirty stains on clothes.
• The scum deposits as scales on metals and they soon lose their shine in hard water.

• Hard water is also unfi t for use in factories and industries as it forms scales that damage machines
and also causes wastage of fuel.

Removal of Hardness of Water (Softening of Water)

There are methods which can be used to remove the hardness of water. These methods are discussed
here.
• Temporary hardness of water can be removed by boiling. On hea ng, the soluble bicarbonates of
calcium and magnesium decompose to insoluble carbonates which are precipitated and removed
by fi ltra on.
heat
Ca(HCO ) (aq) CaCO (s)  + H O(l) + CO (g) 
2
2
3 2
3
Calcium Calcium Water Carbon
bicarbonate carbonate dioxide

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