Biology Class 12 NCERT Solutions: Chapter 16 Environmental Issues Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q. 1 What are the various constituents of domestic sewage? Discuss the effects of sewage discharge on a river.


Domestic Sewage
  • Domestic sewage is the waste originating from the kitchen, toilet, laundry, and other sources. It contains impurities such as suspended solid (sand, salt, clay) , colloidal material (fecal matter, bacteria, plastic and cloth fiber) , dissolved materials (nitrate, phosphate, calcium, sodium, ammonia) , and disease-causing microbes.
  • When organic wastes from the sewage enter the water bodies, it serves as a food source for microorganisms such as algae and bacteria. As a result, the population of these microorganisms in the water body increases.
  • Here, they utilize most of the dissolved oxygen for their metabolism. This results in an increase in the levels of Biological oxygen demand (BOD) in river water and results in the death of aquatic organisms.
  • Also, the nutrients in the water lead to the growth of planktonic algal, causing algal bloom. This causes deterioration of water quality and fish mortality.

Q. 2 List all the wastes that you generate, at home, school or during your Trips to other places, could you very easily reduce? Which would be Difficult or rather impossible to reduce?


  • Wastes generated at home include plastic bags, paper napkin, toiletries, kitchen wastes (such as peelings of vegetables and fruits, tea leaves) , domestic sewage, glass, etc.
  • Wastes generated at schools include waste paper, plastics, vegetable and fruit peels, food wrappings, sewage etc.
  • Wastes generated at trips or picnics include plastic, paper, vegetable and fruit peels, disposable cups, plates, spoons etc.
  • Yes, wastes can be easily reduced by the judicious use of the above materials. Wastage of paper can be minimized by writing on both sides of the paper and by using recycled paper.
  • Plastic and glass waste can also be reduced by recycling and re-using. Also, substituting plastics bags with biodegradable jute bags can reduce wastes generated at home, school, or during trips.
  • Domestic sewage can be reduced by optimizing the use of water while bathing, cooking, and other household activities.
  • Non-biodegradable wastes such as plastic, metal, broken glass, etc. are difficult to decompose because micro-organisms do not have the ability to decompose them.

Q. 3 Discuss the causes and effects of global warming. What measures need to be taken to control global warming?


  • Global warming is defined as an increase in the average temperature of the Earth՚s surface.
  1. Causes of global warming:
  2. Global warming occurs as a result of the increased concentration of greenhouse gases in the atmosphere. Greenhouse gases include carbon dioxide, methane, and water vapour.
  3. These gases trap solar radiations released back by the Earth. This helps in keeping our planet warm and thus, helps in human survival.
  4. However, an increase in the amount of greenhouse gases can lead to an excessive increase in the Earth՚s temperature, leading to global warming. Global warming is a result of industrialization, burning of fossil fuels, and deforestation.
  5. Effects of global warming:
  6. Global warming is defined as an increase in the average temperature of the Earth՚s surface. It has been observed that in the past three decades, the average temperature of the Earth has increased by .
  7. As a result, the natural water cycle has been disturbed resulting in changes in the pattern of rainfall. It also changes the amount of rain water. Also, it results in the melting of Polar ice caps and mountain glaciers, which has caused a rise in the sea level, leading to the inundation of coastal regions.
  8. Control measures for preventing global warming:
  9. Reducing the use of fossil fuels
  10. Use of bio-fuels
  11. Improving energy efficiency
  12. Use of renewable source of energy such as CNG etc.
  13. Reforestation.
  14. Recycling of materials

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