Environmental chemistry: water: Class 10 Chemistry

Key points

Water covers about 71% of the Earth's surface, but only about 3% of it is fresh, and most of that is frozen in glaciers and ice caps. The rest is in rivers, lakes and groundwater. Pakistan depends on the Indus river system and on groundwater, so clean water is a major concern.

Water (H₂O) is a polar molecule with a bent shape and a bond angle of about 104.5°. Hydrogen bonding between molecules gives it a high boiling point (100 °C), high specific heat capacity and high surface tension. Ice is less dense than liquid water, with maximum density at 4 °C, so ice floats and aquatic life survives under frozen lakes. Water is called the universal solvent because it dissolves many ionic and polar substances.

Hardness of water is its failure to form lather easily with soap. It is caused by dissolved calcium and magnesium salts. Soap, a sodium salt of a fatty acid, forms an insoluble scum with these ions: 2C₁₇H₃₅COONa + Ca²⁺ → (C₁₇H₃₅COO)₂Ca + 2Na⁺. Soft water has no or few such ions and lathers easily.

Two kinds of hardness:

• Temporary hardness: due to bicarbonates of calcium and magnesium, Ca(HCO₃)₂ and Mg(HCO₃)₂. It is removed by boiling.
• Permanent hardness: due to chlorides and sulphates of calcium and magnesium, such as CaCl₂, MgCl₂, CaSO₄ and MgSO₄. Boiling does not remove it.

Hard water causes waste of soap, a scale (boiler scale) in kettles and boilers that wastes fuel and can lead to explosions, and blocked pipes. It leaves a stain on clothes and utensils. Its advantages are a pleasant taste, and calcium that is good for bones and teeth.

Removal of hardness:

• Boiling (temporary only): Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. The insoluble carbonate settles.
• Clark's method (temporary only): add the exact amount of slaked lime, Ca(OH)₂ + Ca(HCO₃)₂ → 2CaCO₃ + 2H₂O.
• Washing soda (both kinds): Na₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl. The Ca²⁺ and Mg²⁺ ions precipitate as carbonates.
• Ion exchange (both kinds): water passes through a resin that swaps Ca²⁺ and Mg²⁺ for Na⁺ (or H⁺), and gives soft water.
• Distillation gives the purest water but is costly.

Water pollution is the addition of harmful substances to water that make it unfit for use. Sources are domestic sewage, industrial waste, farm run-off with fertilizers and pesticides, oil spills, and dumping of solid waste. Industrial wastes may contain heavy metals such as lead, mercury and chromium, which are toxic and build up in the food chain. High arsenic in groundwater is a known problem in parts of Punjab and Sindh.

Water-borne diseases come from germs in sewage: cholera, typhoid, dysentery, hepatitis A and diarrhoea. Drinking contaminated water is a major cause of illness, especially among children. Chemical pollutants can cause cancer, kidney damage and bone problems.

Dissolved oxygen (DO) is the oxygen dissolved in water, needed by fish and other aquatic life. Clean water has a high DO, around 8 to 9 mg per litre at room temperature, and fish need above about 5 mg per litre. Biochemical oxygen demand (BOD) is the amount of oxygen that bacteria need to break down the organic matter in a sample. A high BOD means heavy pollution and low DO.

Eutrophication is the over-enrichment of a water body with nutrients such as nitrates and phosphates from fertilizers and detergents. Algae grow in large amounts (algal bloom). When they die, bacteria decompose them and use up the oxygen, so fish die and the water smells.

Treatment of water for drinking:

• Screening and sedimentation: large objects are removed and heavy particles settle in tanks.
• Coagulation: alum, Al₂(SO₄)₃, is added so that tiny particles stick into flocs and settle.
• Filtration through sand and gravel beds removes the remaining solids.
• Disinfection: chlorine, ozone or UV light kills germs. Chlorine is commonly used because a little remains to protect water in pipes.
• At home: boiling for a few minutes, chlorine tablets and candle or ceramic filters.

Safe drinking water should be clear, colourless, odourless, free from germs and toxic chemicals, with a pH of about 6.5 to 8.5. Conservation includes not wasting water, repairing leaks, rain water harvesting, treating waste water before release, and re-using treated water for farms and gardens.

Study tip: Prepare a table for temporary and permanent hardness with cause, effect and removal methods, with an equation for each method. In mole calculations, find the molar masses first and show the mole ratio from the balanced equation.

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Common mistakes

• Saying boiling removes all hardness. It removes only temporary hardness.
• Mixing up the causes: bicarbonates cause temporary hardness and chlorides and sulphates cause permanent hardness.
• Thinking high BOD means clean water. A high BOD means more pollution.
• Writing that ice sinks in water, or forgetting that maximum density of water is at 4 °C.
• Leaving out the balanced equation in softening calculations.

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