Correct Answer - Option 4 : Both (A) and (C)
The hardness of water: The hardness of water is that characteristic that prevents the formation of sufficient leather or foam.
- It occurs due to the presence of carbonates, bicarbonates, sulphates, chlorides of calcium and magnesium(Ca and Mg).
- Hardness of water is due to the presence of dissolved minerals of Ca2+, Mg2+, Al3+, Iron and other heavy elements. Specifically due to Calcium Sulphate, Calcium Bicarbonate, Calcium chloride, Magnesium Sulphate, Magnesium Bicarbonate and Magnesium chloride.
Hardness is of following two types-
Temporary hardness (Carbonate hardness):
- Temporary hardness is caused by the presence of dissolved bicarbonates of Calcium (Ca(HCO3)2 ), Magnesium (Mg(HCO3)2 ), Iron and other heavy elements.
- It can be eliminated by mere boiling.
Ca(HCO3)2 → CaCO3↓ + H2O + CO2 ↑
Mg(HCO3)2 → MgCO3 + H2O + CO2 ↑
MgCO3 → Mg(OH)2↓ + CO2 ↑
Permanent Hardness (non-carbonate hardness):
- Permanent Hardness is due to the presence of dissolved chlorides and sulphates of Calcium, Magnesium, Iron and other heavy elements.
- It cannot be eliminated by boiling.
Hence, we can conclude that Permanent hardness of water is due to the presence of Both (A) and (C).
Degree of Hardness
- Degree of hardness is extent of hardness.
- It is expressed as weight in milligrams of CaCO3 equivalent to all hardness causing substance in one million milligram of water. That is parts per million (ppm). Or it can be expressed as weight in milligrams of CaCO3 equivalent to all hardness causing substance in one litre of water.
CaCO3 as reference for Hardness
- a. CaCO3 is stable, non-hygroscopic and is obtained in pure form. Therefore a standard hard water solution can be prepared by dissolving accurately weighed CaCO3 in dilute HCl and make up to a known volume.
- b. CaCO3 is insoluble in water. Therefore it can be easily precipitated in water treatments.
- c. Molecular weight of CaCO3 is 100, so mathematical calculations are easy.
