Water Hardness and Treatment
~8 min read
- Hardness: Presence of dissolved Ca²⁺ and Mg²⁺ salts. Doesn't lather with soap.
- Two types: Temporary (Ca(HCO₃)₂, Mg(HCO₃)₂ — removed by boiling). Permanent (CaSO₄, MgSO₄, CaCl₂ — needs chemical treatment).
- Removal: Boiling, Clark's process (adding lime), washing soda, ion exchange (zeolite/permutit), reverse osmosis.
Hard water is the silent enemy of soap, boilers and kettles. CDS/OTA tests its causes, two types and removal methods.
What Makes Water Hard
- Hardness is caused by dissolved calcium and magnesium salts picked up as water flows through limestone, chalk and gypsum.
- Hard water does not lather easily — soap reacts with Ca²⁺/Mg²⁺ to form an insoluble scum (Ca/Mg stearate). Wastes soap.
- Forms scale (CaCO₃) inside kettles, geysers and boilers, reducing efficiency and risking explosions in industrial boilers.
Temporary and Permanent Hardness
| Type | Cause | Removed by |
|---|---|---|
| Temporary | Bicarbonates of Ca and Mg — Ca(HCO₃)₂, Mg(HCO₃)₂ | Boiling (decomposes to insoluble carbonate); Clark's process — adding Ca(OH)₂ in calculated amount |
| Permanent | Sulphates and chlorides — CaSO₄, MgSO₄, CaCl₂, MgCl₂ | Washing soda (Na₂CO₃) — precipitates CaCO₃; ion-exchange resins (zeolite/permutit); reverse osmosis |
Methods of Removal
- Boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. Removes temporary hardness only.
- Clark's process: Adding slaked lime Ca(OH)₂ to bicarbonate water precipitates CaCO₃ and MgCO₃.
- Washing soda (Na₂CO₃): Na₂CO₃ + CaSO₄ → Na₂SO₄ + CaCO₃↓. Removes permanent hardness.
- Ion exchange (Zeolite/Permutit): Na-zeolite swaps Na⁺ for Ca²⁺/Mg²⁺. Regenerated by brine.
- Reverse osmosis (RO): Pressure forces water through a semi-permeable membrane that blocks ions and impurities. Used in domestic purifiers.
Municipal Water Purification
- Sedimentation — large particles settle by gravity.
- Coagulation — alum added; fine particles clump together and settle.
- Filtration — through layers of sand and gravel.
- Disinfection — chlorination (most common), ozonation, UV irradiation.
- Fluoridation — small fluoride dose prevents dental caries (some municipalities).
CDS/OTA PYQ Examples
Q: Temporary hardness of water is due to:
(a) Chlorides of Ca and Mg (b) Sulphates of Ca and Mg (c) Bicarbonates of Ca and Mg (d) Nitrates of Ca and Mg
Answer: (c) Bicarbonates of Ca and Mg.
Q: Temporary hardness can be removed by:
(a) Filtration (b) Boiling (c) Adding common salt (d) Adding sand
Answer: (b) Boiling — decomposes bicarbonates to carbonates.
Q: Disinfection of municipal water supply is most commonly done by:
(a) Boiling (b) Filtration (c) Chlorination (d) Sedimentation
Answer: (c) Chlorination.
Q: Permanent hardness can be removed by:
(a) Boiling (b) Adding washing soda (c) Adding common salt (d) Filtration
Answer: (b) Washing soda — Na₂CO₃.
Drill Water Hardness and Treatment for CDS/OTA
CDS/OTA-pattern items on Water Hardness and Treatment with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does hard water not lather with soap?
Ca²⁺ and Mg²⁺ react with soap's fatty acid ions to form insoluble scum. More soap is wasted before lather forms.
Why is RO water sometimes called 'too pure'?
RO removes essential minerals (Ca, Mg, K) along with impurities. Long-term exclusive use of RO water without re-mineralisation may reduce dietary mineral intake.
Is rain water hard or soft?
Soft — it has not yet picked up minerals. It becomes hard after percolating through limestone or chalk.