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

Sodium carbonate (Na2CO3), commonly known as washing soda or soda ash, is the primary sodium compound used for softening hard water [1]. Hardness in water is caused by the presence of calcium and magnesium ions. Sodium carbonate works by reacting with these dissolved calcium and magnesium salts to form insoluble precipitates, such as calcium carbonate (CaCO3) and magnesium hydroxide, which can then be removed through sedimentation or filtration [1]. Specifically, it is highly effective in removing permanent hardness (non-carbonate hardness) by converting soluble sulfates or chlorides of calcium and magnesium into insoluble carbonates [1]. While sodium hydroxide (caustic soda) can also be used in specific industrial precipitation processes, sodium carbonate is the standard reagent cited for general water softening and domestic cleaning purposes [1]. Sodium bicarbonate (NaHCO3) is primarily used in baking and as an antacid, and does not serve as a softening agent.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Uses of washing soda > p. 32