Which of the following methods can be used to remove temporary hardness from water?

1. Analyze the question You're asked which method removes temporary hardness from water.

2. Explain the concept

• Temporary hardness is caused by dissolved calcium or magnesium hydrogencarbonates, Ca(HCO₃)₂ and Mg(HCO₃)₂.
• Permanent hardness is caused by dissolved sulphates and chlorides of calcium and magnesium (CaSO₄, MgCl₂, etc.). Boiling does not remove it.

Temporary hardness is "temporary" because heating removes it.

3. Solution When the water is boiled, the hydrogencarbonates decompose into insoluble carbonates:

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

The insoluble CaCO₃ precipitates out (this is the "scale" or "fur" in kettles), and the water is softened.

Now check the other options:

• B. Filtration only removes suspended solids. It cannot remove dissolved Ca²⁺ and Mg²⁺ ions.
• C. Sedimentation lets suspended particles settle. It does not remove dissolved ions.
• D. Chlorination kills germs and does not soften water.

4. Answer The correct option is A. Boiling.

5. Tip Remember: "Temporary = Thermal." Heat (boiling) removes temporary hardness. Other methods for softening water include adding washing soda (Na₂CO₃), using slaked lime (Clark's method, for temporary hardness only), and ion exchange (for both types). Permanent hardness needs washing soda or ion exchange, since boiling won't work.