Which one of the following sodium compounds is used for softening hard

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Which one of the following sodium compounds is used for softening hard water ?

Na₂CO₃
NaHCO₃
NaOH
Na₂SO₄
This question was previously asked in
UPSC CDS-1 – 2021
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Hard water contains dissolved minerals, primarily calcium and magnesium ions. Sodium carbonate (Na₂CO₃), also known as washing soda, is commonly used to soften hard water by removing these ions.
When sodium carbonate is added to hard water, the calcium and magnesium ions react with the carbonate ions to form insoluble calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). These precipitates settle down or can be filtered out, effectively reducing the concentration of hardening ions in the water. The overall reaction with calcium ions is: Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na⁺(aq). The resulting sodium ions (Na⁺) do not contribute to water hardness.
Other methods for water softening include using ion-exchange resins, which swap calcium and magnesium ions for sodium ions, or using chelating agents that bind the metal ions. While boiling can remove temporary hardness (caused by bicarbonates), it does not remove permanent hardness (caused by sulfates and chlorides), which sodium carbonate can address.

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