Chemistry Class 11 NCERT Solutions: Chapter 14 Environmental Chemistry Part 3

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Q: 11. What are the major causes of water pollution? Explain.


Water pollution arises as a result of several human activities, which leads to the presence of several undesirable substances in water.

Major water pollutants with their sources have been tabulated as follows:

Water Pollutants and Their Source
Sources of water pollutants




Domestic sewage

Organic wastes

Domestic sewage, decaying animals and plants, animal excreta and waste, discharge from food processing industries

Plant nutrients

Chemical fertilizers

Toxic heavy metals

Chemical factories and industries


Strip mining and soil erosion


Chemicals used for killing fungi, weed, insects

Radioactive substances

Mining of uranium-containing minerals


Water used for cooling in industries

Roles played by major pollutants are:

1. Pathogens: These water pollutants include bacteria and other organisms. They enter water from animal excreta and domestic sewage. Bacteria present in human excreta (for example, Escherichia coli, and Streptococcus faecalis) cause gastrointestinal diseases.

2. Organic wastes: These are biodegradable wastes that pollute water as a result of run off. The presence of excess organic wastes in water decreases the amount of oxygen held by water. This decrease in the amount of dissolved oxygen inhibits aquatic life.

3. Chemical pollutants: These are water-soluble chemicals like heavy metals such as cadmium, mercury, nickel, etc. The presence of these chemicals (above the tolerance limit) can damage the kidneys, central nervous system, and liver.

Q: 12. Have you ever observed any water pollution in your area? What measures would you suggest to control it?


Water pollution arises as a result of various human activities. This includes discharges from wastewater treatment plants, run-off from agricultural fields, storm-water drainage, etc. Pollutants from these sources enter the water bodies, thereby contaminating the water and rendering it impure.

Industries and chemical factories discharge toxic, heavy metals such as Fe, Mn, Al, etc., along with organic wastes into water. Domestic sewage and animal excreta are also responsible for pathogenic contamination of water. These pollutants make water unfit for drinking.

Therefore, all industrial and chemical discharges should be made free from toxic metals before allowing them to enter a water body. The concentration of these pollutants should be checked regularly. Compost should be preferred over chemical fertilizers in garden and agricultural fields to avoid harmful chemicals from entering ground water.

Q: 13. What do you mean by Biochemical Oxygen Demand


Biochemical oxygen demand is the amount of oxygen required by bacteria to decompose organic matter in a certain volume of sample of water. Clean water would have a BO value of less than 5 ppm, whereas highly polluted water has a BOD value of 17 ppm or more.

Q: 14. Do you observe any soil pollution in your Neighbourhood? What efforts will you make for controlling the soil pollution?


Major sources of soil pollution are industrial wastes and agricultural pollutants such as pesticides, fertilizers, etc.

It is very important to maintain the quality and fertility of soil to ensure and sustain the growth of plants and food crops.

Insecticides like DDT are not soluble in water. For this reason, they remain in soil for a long time, contaminating the root crops. Pesticides like Aldrin and Dieldrin are non- biodegradable and highly toxic in nature. They can enter the higher trophic levels through food chains, causing metabolic and physiological disorders. The same is true for industrial wastes that comprises of several toxic metals like , etc.

Hence, the best way to check soil pollution is to avoid direct addition of pollutants to the soil. Also, wastes should undergo proper treatment. They should be recycled and only then, allowed to be dumped.

Q: 15. What are pesticides and herbicides? Explain giving examples.


Pesticides are a mixture of two or more substances. They are used for killing pests. Pest include insects, plant pathogens, weeds, mollusks, etc., that destroy the plant crop an spread diseases. Aldrin and Dieldrin are the names of some common pesticides Herbicides are pesticides specially meant for killing weeds. For example, sodium chlorate sodium arsenite , etc.

Q: 16. What do you mean by green chemistry? How will it help decrease environmental pollution?


Green chemistry is a production process that aims at using the existing knowledge and principles of chemistry for developing and implementing chemical products and processes to reduce the use and generation of substances hazardous to the environmental, The release of different harmful chemicals (particulates, gases, organic and inorganic wastes) causes environmental pollution. In green chemistry, the reactants to be used in chemical reactions are chosen in such a way that the yield of the end products is up to . This prevents or limits chemical pollutants from being introduced into the environment. Through the efforts of green chemists, has replaced tetrachlorethane and chlorine gas in drying and bleaching of paper.

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