Pollution of Air and Water — CBSE Class 8 Science Chapter 18 Notes

Air Pollution: Causes, Smog, and Acid Rain

Air pollution occurs when unwanted substances (air pollutants) contaminate the atmosphere, causing harmful effects on both living organisms and non-living things. Primary pollutants include carbon monoxide (CO), carbon dioxide ($CO_2$), nitrogen oxides, and sulphur dioxide ($SO_2$). When these pollutants, specifically nitrogen oxides, combine with fog and dust in the cold winter air, they form Smog (smoke + fog). Smog causes severe breathing difficulties such as asthma, bronchitis, and wheezing in children. Another devastating effect of gaseous air pollutants ($SO_2$ and $NO_2$) is Acid Rain. These gases react with water vapour in the clouds to form sulphuric acid and nitric acid, which fall with rain. Acid rain corrodes historic monuments made of marble (like the Taj Mahal, causing 'marble cancer') and turns soil acidic, destroying crops and aquatic life.

Understanding Eutrophication: How Water Bodies Suffocate

1. Runoff of Fertilizers — Rain washes chemical fertilizers (rich in nitrates and phosphates) from agricultural fields into nearby ponds and lakes.
2. Algal Bloom — These dissolved chemicals act as nutrients, causing an explosive growth of algae over the water's surface, turning it completely green.
3. Death of Algae — When this huge population of algae dies, decomposers (like bacteria) feed on the dead matter.
4. Oxygen Depletion — The bacteria consume massive amounts of oxygen dissolved in the water during decomposition, suffocating and killing fish and other aquatic organisms.

Avoid Common Mistakes in Board Exams

1. Greenhouse Effect vs. Global Warming: Do not confuse these two! The greenhouse effect is a natural, beneficial process that traps heat to keep the Earth warm enough to sustain life. Global Warming is the harmful acceleration of this effect due to excess $CO_2$ and methane emissions.
2. Potable Water definition: Potable water is water that is safe and fit for human consumption (drinking). Water that looks crystal clear might still contain disease-causing microorganisms or dissolved chemical impurities, making it unsafe until purified.
3. Chemicals in Water Purification: Remember that chlorine tablets or bleaching powder are added to kill harmful germs (chlorination). Never add excess chlorine as it can be toxic.

Practice Questions with Solutions

• Q: Explain how acid rain is formed and write its harmful effects on historical monuments. A: Step 1: Understand the sources. Industrial activities and vehicle exhaust release sulphur dioxide ($SO_2$) and nitrogen dioxide ($NO_2$) into the atmosphere. Step 2: Chemical reaction. These gases react with water vapor present in the atmosphere to form sulphuric acid ($H_2SO_4$) and nitric acid ($HNO_3$). Step 3: Precipitation. These acids fall down with rain, making the rain acidic (pH below 5.6). Step 4: Impact on monuments. Acid rain reacts with the calcium carbonate (marble) of monuments like the Taj Mahal: $CaCO_3 + Acid \rightarrow Corrosion$. This discoloration and corrosion of marble is called 'Marble Cancer'. Final answer: Acid rain forms when sulfur and nitrogen oxides react with water vapour to create acids. This rain corrodes the marble of historical monuments, turning them yellow.
• Q: What is the Greenhouse Effect? Name two greenhouse gases and explain how global warming is caused. A: Step 1: Define the effect. The trapping of solar radiation (infrared rays) by the Earth's atmosphere, preventing it from escaping into space, is called the Greenhouse Effect. Step 2: Identify gases. Carbon dioxide ($CO_2$), methane ($CH_4$), water vapour, and nitrous oxide are key greenhouse gases. Step 3: Explain global warming. When human activities (deforestation, burning fossil fuels) release excess carbon dioxide, more heat is trapped in the atmosphere. This gradual, steady increase in the Earth's average temperature is called Global Warming. Final answer: The greenhouse effect is the natural trapping of heat by atmospheric gases. Excess greenhouse gases like carbon dioxide and methane trap too much heat, resulting in global warming.
• Q: Explain the difference between pure water and potable water. A: Step 1: Analyze pure water. Pure water consists only of hydrogen and oxygen molecules ($H_2O$) with no dissolved salts, minerals, or impurities. It is chemically pure but does not contain beneficial minerals. Step 2: Analyze potable water. Potable water is water that is clean, free of disease-causing pathogens, and safe for drinking. It contains essential dissolved minerals required by the human body. Step 3: Safety check. Water can look clear (physically pure) but still carry invisible bacteria, meaning it is not automatically potable until boiled or chemically treated. Final answer: Pure water contains absolutely no solutes or minerals, whereas potable water is chemically and biologically safe for drinking and contains essential minerals.
• Q: Suggest three individual actions you can take to reduce air and water pollution in your locality. A: Step 1: Transport action. Use public transport, carpool, or ride a bicycle for short distances instead of individual fossil-fuel cars to reduce carbon emissions. Step 2: Domestic waste action. Never throw household garbage, plastics, or chemical detergents directly into open drains or water bodies; practice waste segregation at home. Step 3: Resource conservation. Save water by turning off the tap while brushing, watering plants with kitchen wastewater (greywater), and planting more trees (afforestation) to purify the air. Final answer: To reduce pollution, individuals can plant more trees (afforestation), use public transport/bicycles to minimize emissions, and never dump domestic waste in water bodies.

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