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Water hardness is caused mainly by dissolved salts of calcium and magnesium. Hardness is classified into two types: temporary hardness and permanent hardness. Temporary hardness is due to bicarbonates of calcium and magnesium and can be removed by simple boiling. Permanent hardness, however, is caused by chlorides and sulfates of calcium and magnesium, which do not decompose on boiling.
To remove permanent hardness, chemical treatment is necessary. One effective method is the addition of washing soda (sodium carbonate, Na₂CO₃). When washing soda is added to hard water, it reacts with the calcium and magnesium salts to form insoluble carbonates that precipitate out of the solution. For example:
CaSO₄ + Na₂CO₃ → CaCO₃ ↓ + Na₂SO₄
MgCl₂ + Na₂CO₃ → MgCO₃ ↓ + 2NaCl
In these reactions, calcium carbonate and magnesium carbonate are formed as insoluble precipitates, which settle down and can be removed by filtration. The water left behind becomes soft and suitable for household and industrial use.
Other listed substances in the options do not remove permanent hardness. Chlorine is mainly used for water disinfection, not hardness removal. Bleaching powder (Ca(OCl)₂) is also used as a disinfectant and bleaching agent, but it does not address hardness. Therefore, they are not correct choices.
The use of washing soda is a traditional and inexpensive method of softening hard water. In industries and municipal treatment plants, other methods such as the zeolite process and ion exchange resins are also employed, but for domestic purposes, washing soda is an effective solution.
Hence, permanent hardness of water is best removed by adding washing soda (sodium carbonate).
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