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Chemistry · Class 10 · Chapter 14: Environmental Chemistry II: Water

8 min

Key points

Water is a universal solvent and essential for all living organisms — covers 71% of Earth's surface
Hard water contains dissolved calcium and magnesium salts, while soft water contains negligible amounts of these salts
Temporary hardness is caused by bicarbonates of calcium and magnesium and can be removed by boiling
Permanent hardness is caused by chlorides and sulfates of calcium and magnesium and cannot be removed by boiling
Water treatment involves screening, sedimentation, filtration, and chlorination to make it safe for drinking
Water pollution in Pakistan occurs due to industrial waste, sewage discharge, and agricultural runoff
Biological Oxygen Demand (BOD) is a measure of water pollution — higher BOD indicates more pollution
Distillation produces the purest form of water by removing all dissolved impurities
pH of pure water is 7 at 25°C, making it neutral in nature

Definitions

Hard Water: Water which does not readily form lather with soap due to the presence of dissolved salts of calcium and magnesium
Soft Water: Water which readily forms lather with soap and contains negligible amounts of calcium and magnesium salts
Temporary Hardness: Hardness of water caused by the presence of bicarbonates of calcium and magnesium which can be removed by boiling
Permanent Hardness: Hardness of water caused by the presence of chlorides and sulfates of calcium and magnesium which cannot be removed by boiling
BOD: Biological Oxygen Demand is the amount of oxygen required by bacteria to decompose organic matter in water at 20°C for 5 days

Worked example

Define hard water and soft water. Explain any two methods used to remove hardness of water. (6 marks)

Hard water is water that contains dissolved salts of calcium and magnesium, mainly bicarbonates, chlorides and sulphates. Soft water is water that does not contain dissolved salts of calcium and magnesium or contains them in very small amounts. Methods to remove hardness: (1) Boiling - removes temporary hardness by converting bicarbonates to insoluble carbonates: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. (2) Ion exchange method - hard water is passed through ion exchange resins that replace Ca²⁺ and Mg²⁺ ions with Na⁺ ions.

Common mistakes

Students confuse temporary and permanent hardness causes — remember bicarbonates vs chlorides/sulfates
Writing CaCO₃ instead of Ca(HCO₃)₂ for temporary hardness
Forgetting to mention time period (5 days) and temperature (20°C) in BOD definition
Not drawing the water treatment flow diagram in proper sequence
Confusing water purification methods with hardness removal methods

Quick recap

Water pollution is contamination by harmful substances affecting aquatic life and human health. Hard water contains Ca²⁺ and Mg²⁺ salts causing soap wastage, while soft water doesn't. Water treatment involves coagulation with alum, sedimentation, filtration and chlorination. BOD measures oxygen demand of organic pollutants in water.

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