Boiling removes TEMPORARY hardness of water but not PERMANENT hardness. The chemical basis for this difference is:

• A Temporary hardness salts volatilise as vapour on heating
• B Calcium and magnesium bicarbonates decompose on heating into insoluble carbonates that precipitate out ✓
• C Sulphates and chlorides of calcium are destroyed by heat
• D Boiling converts all dissolved salts into soluble hydroxides

Explanation

Temporary hardness results from bicarbonates of calcium and magnesium, which decompose on heating to form insoluble carbonates (and water and carbon dioxide) that precipitate, leaving the water softened. Permanent hardness arises from sulphates, chlorides and nitrates of calcium and magnesium, which are thermally stable and unaffected by boiling; these require methods such as base exchange (zeolite) process or addition of washing soda. Salts do not volatilise or get destroyed by heat.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.