Last reviewed 16 Sept 2026 · 10 min read
Sources and impurities
- Sources: rain water (purest natural), surface water (rivers, lakes — suspended matter, organics, microbes), groundwater (springs, wells — dissolved salts, hardness), sea water (high salinity).
- Impurities:
- Suspended — silt, clay, organic particles (turbidity).
- Colloidal — fine clay, colour-causing organics.
- Dissolved — salts of Ca, Mg, Na (bicarbonates, chlorides, sulphates, nitrates), iron, fluoride, arsenic; dissolved gases (O₂, CO₂, H₂S).
- Biological — bacteria, viruses, algae.
Hardness of water
Hardness — the soap-consuming capacity of water, mainly due to dissolved calcium and magnesium salts (also Fe, Mn, Al to a small extent). Hard water does not readily form lather with soap; insoluble scum forms:
| Type | Cause | Removal |
|---|---|---|
| Temporary (carbonate) hardness | Bicarbonates of Ca and Mg (Ca(HCO₃)₂, Mg(HCO₃)₂) | Boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂; Mg(HCO₃)₂ → Mg(OH)₂↓ + 2CO₂; also by lime |
| Permanent (non-carbonate) hardness | Chlorides and sulphates (and nitrates) of Ca and Mg | Not removed by boiling — lime–soda, zeolite, ion exchange processes |
Total hardness = temporary + permanent.
Units of hardness
| Unit | Definition |
|---|---|
| mg/L or ppm as CaCO₃ | Parts of CaCO₃ equivalent hardness per million parts of water (most common) |
| Degree Clarke (°Cl) | Parts per 70 000 (grains per imperial gallon) |
| Degree French (°Fr) | Parts per 10⁵ |
Conversions: 1 ppm = 0.07 °Cl = 0.1 °Fr (1 °Fr = 10 ppm; 1 °Cl ≈ 14.3 ppm).
Hardness as CaCO₃ equivalent
(or mass × 100 / molar mass for divalent salts). CaCO₃ is chosen because its molar mass is 100 (equivalent weight 50) and it is insoluble.
| Salt | Molar mass | Multiplication factor (to CaCO₃) |
|---|---|---|
| Ca(HCO₃)₂ | 162 | 100/162 |
| Mg(HCO₃)₂ | 146 | 100/146 |
| CaSO₄ | 136 | 100/136 |
| CaCl₂ | 111 | 100/111 |
| MgSO₄ | 120 | 100/120 |
| MgCl₂ | 95 | 100/95 |
| Mg(NO₃)₂ | 148 | 100/148 |
(NaCl, KCl and other sodium/potassium salts do not cause hardness.)
Alkalinity
- Alkalinity — capacity to neutralise acids, due to OH⁻, CO₃²⁻ and HCO₃⁻ ions.
- Determined by titration with standard acid using phenolphthalein (P) and methyl orange (M) indicators; the relative values of P and M give OH⁻, CO₃²⁻ and HCO₃⁻ contents.
- OH⁻ and HCO₃⁻ cannot coexist in significant amounts (they react to form CO₃²⁻).
- High alkalinity causes caustic embrittlement in boilers; alkalinity also affects coagulation in treatment.
Estimation of hardness — EDTA method
- EDTA (ethylenediamine tetra-acetic acid, disodium salt) forms stable complexes with Ca²⁺ and Mg²⁺.
- Water sample buffered at pH ≈ 10 (NH₄Cl–NH₄OH buffer) with Eriochrome Black T (EBT) indicator.
- EBT forms a wine-red complex with Ca/Mg; at the end point all metal ions are complexed by EDTA and the solution turns blue.
- Total hardness (mg/L as CaCO₃) .
- Permanent hardness is found after boiling and filtering; temporary = total − permanent.
Disadvantages of hard water
Domestic
- Wastage of soap; poor washing; stains.
- Cooking: pulses do not cook well; tea/coffee taste affected.
- Scaling in kettles, geysers and pipes → reduced flow and heat transfer.
Industrial (especially boilers)
| Problem | Cause and effect |
|---|---|
| Sludge | Soft, loose precipitates (MgCO₃, MgCl₂, CaCl₂) in cooler parts — poor heat transfer, choking of pipes; removed by blow-down |
| Scale | Hard, adherent deposits (CaSO₄, CaCO₃ in high-pressure boilers, Mg(OH)₂, CaSiO₃) on heating surfaces — poor thermal conductivity → fuel wastage, overheating and tube failure (bulging, explosion hazard) |
| Priming | Carryover of water droplets with steam — due to high steam velocity, sudden boiling, high water level; wet steam damages turbines |
| Foaming | Persistent bubbles due to oils, dissolved salts — causes priming |
| Caustic embrittlement | Intergranular cracking of boiler steel at stressed points due to concentrated NaOH (from Na₂CO₃ hydrolysis at high pressure); prevented by sodium sulphate, tannin, lignin additions or phosphate treatment |
| Boiler corrosion | Dissolved oxygen, CO₂ (carbonic acid), and acids from MgCl₂ hydrolysis — removed by deaeration, sodium sulphite/hydrazine, alkalinity control |
Other industries: textiles and dyeing (uneven dyeing), sugar (crystallisation issues), paper, concrete (see water for concrete below), cooling towers (scaling).