Section-D: Comprehension Type
A water is said to be soft water if it produces sufficient foam with the soap and water that does not produce foam with soap is known as hard water. Hardness has been classified into two type (i) Temporary hardness (ii) Permanent hardness.
Temporary hardness is due to presence of calcium and magnesium bicarbonate. It is simply removed by boiling as
Ca(HCO3)2CaCO3+CO2 + H2O
Mg(HCO3)2MgCO3+CO2 + H2O
Temporary hardness can also be removed by addition of slaked lime, Ca(OH)2
Ca(HCO3)2 + Ca(OH)22CaCO3 + 2H2O
Permanent hardness is due to presence of sulphate and chlorides of Ca, Mg, etc. It is removed by washing soda as
CaCl2 + Na2CO3CaCO3 + 2NaCl
CaSO4 + Na2CO3CaCO3 + Na2SO4
Permanent hardness also removed by ion exchange resin process as
2RH + Ca2+→ R2Ca + 2H+
2ROH + → R2SO4 + 2OH–
The degree of hardness of water is measured in terms of ppm of CaCO3. 100 ppm means 100 g of CaCO3 is present in 106 g of H2O. If any water contain 120 ppm of MgSO4, its hardness in terms of CaCO3 = 100 ppm.
One litre of a sample of hard water (d = 1g/mL) contains 136 mg of CaSO4 and 190 mg of MgCl2. What is the total hardness of water in terms of CaCO3?