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Loading home…Water is a polar molecule with a bent shape and partial charges on oxygen and hydrogen. It forms hydrogen bonds with many solutes and hydrates ions strongly. These properties allow water to dissolve a wide range of ionic and polar compounds. It is not truly universal—non-polar substances dissolve poorly—but the phrase reflects its exceptional solvent power. Dissolved substances enable biological transport and geochemical cycles. Polarity and hydrogen bonding are the structural reasons for this behaviour.
Biochemical oxygen demand (BOD) measures the oxygen consumed by microorganisms while decomposing organic matter in water over a standard period. Chemical oxygen demand (COD) measures oxygen required to chemically oxidise organic (and some inorganic) pollutants. High BOD or COD indicates heavy organic pollution and potential oxygen depletion. Fish kills occur when dissolved oxygen falls too low. Wastewater treatment aims to reduce BOD/COD before discharge. They are key indicators of water quality.
Eutrophication is nutrient enrichment of water bodies leading to excessive algal growth. Nitrates and phosphates from fertilisers, sewage and detergents are major nutrients. Algal blooms block light and, when they die, bacterial decay consumes dissolved oxygen. Hypoxia harms fish and biodiversity. Control includes better fertiliser practice, phosphate-free detergents and wastewater nutrient removal. It is a major freshwater and coastal environmental problem.
Hard water contains substantial dissolved calcium and magnesium ions. Soft water has low concentrations of these ions. Temporary hardness is due mainly to hydrogencarbonates and can be removed by boiling. Permanent hardness is due to sulfates and chlorides of Ca and Mg and needs chemical softening. Hardness wastes soap by forming scum and scales boilers. Ion-exchange resins and washing soda are common softening methods.
Raw water is screened to remove large debris. Coagulation and flocculation with alum or iron salts aggregate fine particles. Sedimentation allows flocs to settle; filtration removes remaining solids. Disinfection with chlorine, ozone or UV kills pathogens. Sometimes fluoridation or pH adjustment is applied for health and corrosion control. Treated water is then stored and distributed safely.
Ice is less dense than liquid water because of its open hydrogen-bonded crystal structure. Floating ice insulates the water below, allowing aquatic life to survive winter. If ice sank, water bodies could freeze solid from the bottom up. Seasonal ice cover also influences climate and ocean circulation at large scales. This anomaly is a direct consequence of hydrogen bonding in water. It is one of water’s most environmentally significant physical properties.
Water pollution is contamination that degrades water quality for humans or ecosystems. Pathogenic pollution from sewage causes cholera, typhoid and other diseases. Organic pollution raises BOD and depletes oxygen. Nutrient pollution drives eutrophication. Heavy metals such as Pb, Hg and Cd are toxic and may bioaccumulate. Pesticides and industrial organics can be persistent and carcinogenic.
Oil spills harm marine life and coasts. Thermal pollution from power-plant cooling water reduces dissolved oxygen and stresses organisms. Plastic debris damages wildlife and fragments into microplastics.
Prevention is cheaper than remediation: treat wastewater, control runoff, regulate industry, and protect catchments. Regular testing of pH, DO, BOD, turbidity and microbes guides management.
Hardness is caused chiefly by Ca²⁺ and Mg²⁺. Temporary hardness from Ca(HCO₃)₂ and Mg(HCO₃)₂ decomposes on boiling to carbonate precipitates. Permanent hardness from CaSO₄, MgSO₄ and chlorides survives boiling.
Disadvantages include soap scum, reduced cleaning efficiency, scale in kettles and boilers, and increased fuel costs. Scale can also cause local overheating in industrial boilers.
Removal methods: boiling (temporary), Clark’s method (lime), washing soda, ion-exchange zeolite or resin softeners, and demineralisation for high-purity needs. Choice depends on hardness type, scale and cost. Understanding carbonate chemistry is central to hardness treatment.
Preliminary treatment removes grit and large solids. Primary sedimentation settles suspended solids. Secondary (biological) treatment uses aerobic microorganisms to oxidise dissolved organic matter, sharply cutting BOD. Activated sludge and trickling filters are common processes. Tertiary treatment may remove nutrients, pathogens or residual organics.
Sludge is digested, dewatered and disposed of or reused carefully. Disinfection precedes discharge or reuse. Lowering BOD protects receiving waters from oxygen collapse.
Modern plants also target nitrogen and phosphorus to limit eutrophication. Water chemistry, microbiology and engineering combine in effective sanitation.
Eutrophication follows excess N and P inputs, algal blooms, decay and hypoxia. Dead zones in coastal seas illustrate the scale of the problem. Controls: precision agriculture, buffer strips, wetland restoration, phosphate bans in detergents, and nutrient removal in sewage plants.
Heavy metals enter from mining, plating, batteries and improper waste disposal. They are not biodegradable and may biomagnify in food chains; mercury in fish is a classic example. Controls include effluent limits, safer materials, recycling, and remediation such as precipitation or adsorption.
Integrated catchment management addresses both nutrient and metal pathways together.
Polarity and hydrogen bonding make water an excellent solvent for nutrients and pollutants alike, enabling transport through soils and organisms. High specific heat and latent heats moderate climate and weather. High surface tension supports some aquatic organisms and influences droplets and aerosols.
The low density of ice due to open hydrogen-bonded structure allows ice to float, insulating lakes in winter. Maximum density near 4 °C drives seasonal turnover that redistributes oxygen and nutrients in temperate lakes.
These same properties mean pollutants dissolve and migrate widely, so protection of water resources is globally critical. Environmental chemistry of water begins with recognising why water is such an unusual liquid.