What is meant by the temporary and permanent hardness of water and how is it caused?

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Detailed Solution

Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium, while permanent hardness is caused by their chlorides and sulfates.

Temporary hardness: Due to Ca(HCO₃)₂ and Mg(HCO₃)₂. When you boil such water, bicarbonates break into insoluble carbonates that settle out, so hardness reduces. This is why kettle scale forms.
Permanent hardness: Due to CaSO₄, MgSO₄, CaCl₂, and MgCl₂. Boiling does not remove this hardness because these salts do not decompose on heating.

Removal methods

Boiling: For temporary hardness only.
Lime-soda process: Adds Ca(OH)₂ and Na₂CO₃ to form insoluble carbonates.
Ion exchange resins: Swap Ca²⁺/Mg²⁺ with Na⁺/H⁺ for permanent hardness.
Reverse osmosis: Membrane removes dissolved salts.

Hard water wastes soap and leaves scale in pipes and heaters. Softening improves washing and saves energy. However, moderate hardness adds calcium and magnesium, which are useful minerals for health.

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