# 14.8 Properties of Water Related to Pollution Water is a unique substance whose chemical and physical properties make it an excellent solvent, earning it the title of "universal solvent." These same properties, however, also make water susceptible to pollution. Understanding these characteristics is crucial for assessing water quality and developing strategies to combat pollution. --- ## Water as a Universal Solvent Water's ability to dissolve a wide range of substances is due to several key properties: **Polarity:** The water molecule ($H_2O$) has a bent shape with an uneven distribution of charge, creating a positive end (hydrogen atoms) and a negative end (oxygen atom). **High Dielectric Constant:** This property allows water to weaken the electrostatic forces between ions in a crystal lattice, enabling ionic compounds to dissolve. **Hydrogen Bonding:** Water molecules form strong intermolecular hydrogen bonds with each other and with other polar molecules (such as alcohols, amines, and carboxylic acids), facilitating their dissolution. **Ion-Dipole Forces:** The polar nature of water allows it to surround and stabilize ions, pulling them into the solution. This solvent ability is why rainwater can dissolve atmospheric carbon dioxide ($CO_2$), forming a weak acid (carbonic acid, $H_2CO_3$). This acidic rainwater can then dissolve carbonate rocks (such as limestone) by converting insoluble carbonates into soluble bicarbonates. $$ H_2O + CO_2 \rightleftharpoons H_2CO_3 $$ $$ H_2CO_3 \rightleftharpoons H^+_{(aq)} + HCO^-_{3(aq)} $$ --- ### Properties and Their Relation to Pollution **1. High Surface Tension** *Property:* Water molecules cohere strongly at the surface, creating high surface tension. *Pollution Impact:* This allows many microorganisms, including disease-causing microbes, to live and move on the water's surface, making water a habitat for pathogens. It also affects how oil and detergents spread over water bodies. **2. High Heat Capacity** *Property:* Water can absorb a large amount of heat with only a small change in its own temperature. *Pollution Impact:* Industries often use water as a coolant, discharging heated water back into rivers and lakes. This *thermal pollution* raises the water temperature, reducing dissolved oxygen levels and making the water uninhabitable for many aquatic species. **3. Solubility of Surfactants** *Property:* Water's solvent ability extends to soaps and detergents (surfactants). *Pollution Impact:* When surfactants enter waterways, they can leach out toxic heavy metal ions from soil and minerals, introducing them into the water supply. **4. Capillary Action and Osmosis** *Property:* Water moves through narrow spaces and across membranes. *Pollution Impact:* This property is responsible for the swelling of organic matter and dead bodies in water due to the movement of water into the tissues. --- ### Sources and Impacts of Water Pollution **Sources of Pollution:** * **Point Sources:** Pollutants discharged from a single, identifiable location (such as a factory pipe). * **Non-Point Sources:** Pollutants from diffuse sources over a large area (such as agricultural runoff). For more details, refer to . **Major Pollutants and Their Effects:** * **Agricultural Runoff:** Fertilizers (nitrates, phosphates), pesticides, and herbicides can cause *eutrophication*—an overgrowth of algae that depletes oxygen and kills aquatic life. * **Industrial Effluents:** Heavy metals (such as lead, $Pb^{2+}$, and mercury, $Hg^{2+}$) and toxic chemicals are highly dangerous to human health, potentially causing neurological damage, organ failure, and cancer. * **Health Problems:** Contaminated water can lead to epidemics of diseases such as cholera and typhoid, as well as chronic stomach and liver problems. For more information, see and . --- ### Water Treatment To ensure water is safe for consumption and to protect ecosystems, treatment is essential. * **Raw Water Treatment:** Cleaning water from reservoirs (lakes and dams) before it is supplied to municipalities. This involves steps such as coagulation, sedimentation, filtration, and disinfection. * **Wastewater Treatment:** Cleaning used water (sewage and industrial discharge) before it is released back into the environment to prevent pollution of larger water bodies. For detailed procedures, refer to . --- ## Possible Questions/Answers **Q: Why is water called a "universal solvent"?** **A:** Due to its polarity, high dielectric constant, and ability to form hydrogen bonds and ion-dipole forces, water can dissolve a vast number of ionic, polar, and organic substances. **Q: What type of pollutants are the primary cause of eutrophication?** **A:** Agricultural pollutants, specifically fertilizers rich in nitrates and phosphates. **Q: What is the process of clumping small suspended particles into larger ones for removal called?** **A:** Coagulation. **Q: Which type of pollution is most commonly associated with causing cancer?** **A:** Heavy metals from industrial effluents. **Q: Name three methods for disinfecting water.** **A:** Chlorination, ozonation, and UV radiation.