Water Chemistry and Hardness
~8 min read
- Pure water: Universal solvent. Boiling point 100°C, freezing 0°C. Density max at 4°C (anomalous).
- Hard water: Contains dissolved Ca²⁺ and Mg²⁺. Doesn't lather easily with soap. Forms scum and scale.
- Two types of hardness: Temporary (bicarbonates of Ca, Mg) — removed by boiling. Permanent (sulphates, chlorides) — needs ion exchange or washing soda.
Water hardness is the most common NDA Chemistry topic on water. NDA tests temporary vs permanent hardness, treatments, and water's unusual properties.
Properties of Water
- Universal solvent — dissolves more substances than any other liquid.
- Boiling point 100°C, freezing 0°C (at 1 atm).
- Anomalous expansion: Water is densest at 4°C. Below 4°C, it expands. Why ice floats. Why aquatic life survives in frozen lakes.
- High specific heat (4186 J/kg·K) — moderates climate.
- High latent heat of vaporisation (2260 kJ/kg) — sweating cools effectively.
- Polar molecule (V-shaped, 104.5° angle) — gives rise to hydrogen bonding.
Hard Water vs Soft Water
- Soft water: Lathers easily with soap. Examples: rainwater, distilled water, water passed through ion exchange resin.
- Hard water: Contains dissolved Ca²⁺, Mg²⁺ (also Fe²⁺ in some cases). Doesn't lather; forms a scummy white precipitate with soap.
- Cause of hardness: Water passing through limestone, gypsum, dolomite dissolves Ca/Mg salts.
- Problems:
- Wastes soap (forms scum).
- Scale deposits in kettles, pipes, boilers — reduces efficiency.
- Causes stains on washed clothes.
- Affects taste of food.
Temporary vs Permanent Hardness
| Hardness | Cause | Removal |
|---|---|---|
| Temporary | Bicarbonates of Ca and Mg (Ca(HCO₃)₂, Mg(HCO₃)₂) | Boiling — decomposes bicarbonate, precipitating CaCO₃/Mg(OH)₂. Also: Clark's method (adding slaked lime) |
| Permanent | Sulphates and chlorides of Ca and Mg (CaSO₄, MgSO₄, CaCl₂, MgCl₂) | Cannot be removed by boiling. Treatments: washing soda (Na₂CO₃), ion exchange resin (zeolite/permutit) |
Water Treatment
- Boiling: Removes temporary hardness; kills microbes; doesn't remove dissolved salts.
- Filtration: Removes suspended particles. Sand filters, candle filters.
- Sedimentation: Letting fine particles settle.
- Chlorination: Adding chlorine (Cl₂) or bleaching powder kills microbes. Urban water treatment standard.
- Ozonisation: Ozone gas O₃ — stronger disinfectant than chlorine; doesn't leave taste.
- UV treatment: Ultraviolet light kills microbes.
- Reverse Osmosis (RO): Pressure forces water through a semi-permeable membrane that blocks dissolved salts. Common in domestic purifiers.
- Distillation: Boiling and condensing. Produces very pure water.
- Ion exchange resin (Permutit / Zeolite): Replaces Ca²⁺/Mg²⁺ with Na⁺. Used in water softeners.
NDA PYQ Examples
Q: Permanent hardness of water is due to:
(a) Bicarbonates of Ca and Mg (b) Sulphates and chlorides of Ca and Mg (c) Nitrates of Na and K (d) Dust particles
Answer: (b) Sulphates and chlorides of Ca and Mg.
Q: Temporary hardness can be removed by:
(a) Filtration (b) Boiling (c) Adding salt (d) Distillation
Answer: (b) Boiling — decomposes bicarbonates.
Q: Water has maximum density at:
(a) 0°C (b) 4°C (c) 25°C (d) 100°C
Answer: (b) 4°C — anomalous behaviour.
Q: Which is used in RO water purifiers?
(a) Cation exchanger only (b) Semi-permeable membrane under pressure (c) Boiling (d) UV only
Answer: (b) Semi-permeable membrane under pressure.
Drill Water Chemistry and Hardness for NDA
NDA-pattern items on Water Chemistry and Hardness with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does ice float on water?
Water expands when freezing (anomalous expansion). Ice is less dense than liquid water (~0.92 vs 1.00 g/cm³). Float. Critical for aquatic life — ice on top insulates the liquid water below, allowing fish and plants to survive winter.
Why is rainwater soft?
It is essentially distilled water — evaporated from oceans, condensed, and fallen as rain. No time for it to dissolve Ca/Mg salts (which require ground contact). Pure water naturally lathers easily with soap.
How does soap react with hard water?
Soap (sodium stearate) reacts with Ca/Mg ions in hard water to form insoluble calcium/magnesium stearate (scum). This wastes soap and leaves residue. Detergents (sulphonate-based) are not affected — why they replaced soap for laundry.
Why is reverse osmosis called 'reverse'?
Normal osmosis: water flows from low solute concentration to high concentration through a semipermeable membrane. In RO, pressure forces water in the opposite direction — from high to low concentration — leaving salts behind. Reverses natural flow.
How does chlorination kill germs?
Chlorine in water forms hypochlorous acid (HOCl), which damages cell membranes and oxidises essential enzymes of bacteria/viruses. Standard treatment for safe drinking water worldwide.