NCERT Solutions for class 12th Biology Chapter 16 Environmental Issues


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

Sol. Domestic sewage is composed of human body waste and sullage which is the waste water resulting from kitchen, bathroom and laundry sources.

  • Constitute of domestic sewage
    • Domestic sewage contains four kinds of impurities:
    • Suspended solids: They are soil particles such as sand and silt.
    • Colloidal particles: They are inorganic and organic materials such as faecal matter, bacteria, paper and cloth.
    • Dissolved solids: They are nitrates, phosphates, ammonia, sodium, calcium and other nutrients.
    • Pathogens (disease causing microbes): Domestic sewage has pathogens of various diseases such as typhoid, cholera, dysentery, diarrhoea, etc.
  • Effects of sewage discharge on a river: Sewage may affect the aquatic environment by increasing the level of biological oxygen demand (BOD) and formation of algal bloom. Effect of sewage discharge on river are as follow:
    • Eutrophication.
    • Growth of pathogenic bacteria.
    • Ageing of river where slit and decaying matters start accumulating and filling river.
  • Increase in BOD. When organic wastes from the sewage enter the water bodies, they become the food source for micro-organisms (such as algae and bacteria) and thereby increase their population. Here, they utilise most of the dissolved oxygen for their metabolism. This results in an increase in the levels ofbiological oxygen demand (BOD) in river water and results in the death of aquatic organisms.
  • Destruction of flora and fauna of that river.
  • The nutrients in the water lead to the growth of planktonic algal, causing algal bloom. This causes deterioration of water quality and fish mortality.

Question 2. List all the wastes that you generate, at home, school or during your trips to other places. Could you very easily reduce the generation of these wastes? 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.
    • schools include waste paper, plastics, vegetable and fruit peels, food wrappings, sewage etc.
    • trips or picnics include plastic, paper, vegetable and fruit peels, disposable cups, plates, spoons etc.
  • Yes, we can easily reduce these generated waste through judicious use of material by changing our habits & life styles. It can also be reduced if we carefully use or recycle the above-mentioned waste materials generated at home, school or trips.
  • Waste materials which can be easily reduced and recycled are paper, plastic and glass. Wastage of paper can be minimised by writing on its both sides also. Also, substituting plastics bags with biodegradable jute bags can reduce wastes generated at home, school, or during trips. Domestic sewage can be reduced by optimising the use of water while bathing, cooking, and other household activities.
  • Waste material which is difficult to reduce are non biodegradable waste. 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.

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


  • Global warming is defined as a gradual rise in the average temperature of the Earth’s surface. Increase in atmospheric concentration of green house gases has resulted in rise of atmospheric temperature byO.6°C (global warming) in the 20th century. This has been confirmed by intergovernmental panel on climatic change (IPCC) in its reports of 1991 and 1992. This predictable change in near future may affect climate, sea level, range of species distribution, food production as well as fisheries resources in the oceans.
  • Causes of global warming
    • Increase in concentration of greenhouse gases.
    • Increase of automobile and use of fossil fuel.
    • Deforestation and change in land use.
    • CFC and aerosol emission from refrigerator and aeroplane.
    • Increased particulate matter in lower atmosphere.
  • Effects of global warming
    • CO2 fertilisation effect.
    • Many species of plants, being sensitive to temperature will die with sudden rise in temperature and their place will be taken over by scrub vegetation.
    • Loss of biodiversity.
    • Rise in sea level and change of precepitation and local climate conditions.
    • Possibilities of drought and floods.
    • Erruption of plant disease and pests.
    • Change in rainfall pattern.
  • Methods that can reduce the atmospheric concentration of greenhouses gases are as follows:
    • Reducing the greenhouse gas emission by limiting the use of fossil fuels, and by developing alternative renewable sources of energy (wind energy, solar energy etc.)
    • Increasing the vegetation cover, mainly the forests, for photosynthetic utilisation of CO2.
    • Minimising the use of nitrogen fertilisers in agriculture for reducing N2O emissions.
    • Developing substitutes for chlorofluorocarbons.

Question 4. Match the items given in column A and B.

Sol. (a)-(ii) Catalytic converter is a device on a car that reduces the amount of poisonous gas (as carbon monoxide, unburned hydrocarbons, and oxides of nitrogen) that is released from the exhaust. It converts harmful pollutants into less harmful emissions by catalysing a redox reaction (an oxidation and a reduction reaction) before they ever leave the car’s exhaust system.

(b)-(i) Electrostatic converter is a filtration device. It removes suspended dust particles from a gas or exhaust by applying a high-voltage electrostatic charge and collecting the particles on charged plates. It can remove over 99 per cent particulate matter present in the exhaust from a thermal power plant.

(c)-(iii) Earmuffs are a pair of ear covering which protects it from loud noise. They consist of a thermoplastic or metal head-band that fits over the top or back of the head, and a cushion or cup at each end, to cover the external ears.

(d)-(iv) Landfills may be defined as an area which built up from deposits of solid refuse in layers covered by soil.

Question 5. Write critical notes on the following:

(a) Eutrophication                 (b)  Biological magnification

(c) Groundwater depletion and ways for its replenishment

Sol. (a) Eutrophication

  • It is an natural process in lakes, occurring as they age through geological time.
  • Eutrophication is the process by which a body of water becomes enriched in dissolved nutrients (such as nitrogen and phosphates) that stimulate the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen leading to the death of animal life.
  • As a result, it causes a tremendous increase in the primary productivity of the ecosystem. This leads to an increased growth of algae, resulting into algal blooms (caused by the phosphates in detergents, fertilisers, and sewage). Later, the decomposition of these algae depletes the supply of oxygen, leading to the death of another aquatic animal life.

(b) Biological magnification

  • Biomagnifications is defined as the increase in concentration of pollutants or harmful substances that occur in the food chain due to the following consequences :
  • Persistence (means it can’t be broken down by environmental process).Bioenergetic in the food chain. Low rate of internal degradation or excretion of the substances. The maximum concentration is accumulated at the top carnivore’s level of food chain.
  • Biological magnification depends on two factors: chemicals that are soluble in fat, but insoluble in water; and, chemicals that either are not broken down or are broken down slowly in the environment.
  • This phenomenon can be seen in the case of mercury or DDT. The concentration of mercury/DDT increases at the successive trophic levels.
  • To illustrate the process of biological magnification, the example of the pesticide DDT moving through an aquatic food chain can be given. DDT is insoluble in water, soluble in fat and breaks down slowly in the environment.
  • High concentration of DDT (dichlorodiphenyltrichloroethane) was found in a pond. DDT enters the pond as runoff from farmland which has been sprayed with the insecticides.
  • DDT increases or biomagnifies ifwe look at a food chain. DDT is fat-soluble, meaning animals store it in fatty tissues. It also takes a relatively long amount of time to break down.
  • Thus, if a predator consumes multiple organisms and each organism has a small amount , the amount of DDT in the predator is going to be the result of all of those small amounts combined.
  • In the given figure, we see that the concentration of DDT is increased at successive trophic levels, say if it starts at 0.003 ppb (ppb = parts per billion) in water, it can ultimately reach 25 ppm (ppm= parts per million) in fish eating birds, through biomagnifications.
  • High concentrations of DDT disturb calcium metabolism in birds, which causes thinning of eggshell and their premature breaking, eventually causing decline in bird populations.

Note: Organisms at the top of the food chain are at a greater risk of damage due to biomagnifications. It is due to the reason that toxic elements cannot be absorbed and simultaneously they get transferred to the successive higher trophic levels resulting in more concentration of toxicants at the higher trophic level.

(c) Ground water depletion and replacement

  • Ground water is generally considered to be safe and useful for drinking, agricultural and industrial purposes. Ground water is also less prone to pollution.
    • However, in the recent years ground water is depleting due to the following reasons:
    • Its injudicious use both for agricultural and domestic purposes.
    • Deforestation also affects the level of ground water.
    • Lack of vegetation cover also results in very small amounts of water seeping through the ground.
    • An increase in water pollution is another factor that has reduced the availability of ground water.
  • Measures for replenishing ground water:
    • Preventing over-exploitation of ground water.
    • Optimising water use and reducing water demand.
    • Rain water harvesting (RWH).
    • Preventing deforestation and plantation of more trees.
    • Rain water can be collected on the rooftops of buildings and can be stored underground for future use.

Question 6. Why does ozone hole forms over Antarctica? How will enhanced ultraviolet radiation affect us?


  • Joseph Farman, Brian Gardiner, and Jonathan Shanklin discovered a recurring springtime Antarctic ozone hole in 1984.
  • The ozone hole is formed due to an excessive use of increased concentration of chlorine in the atmosphere. A large amount of ODS (Ozone Depleting Substances) like CFCs (chlorofluorocarbons), N2O,halons, SO2, CH4, c1- are released by advanced countries like USA, Japan, European countries. These are released in stratosphere, drift towards poles and reach there before the coming of winter. During winter (temp. 85°C) ice clouds are formed over Antarctica and no sunrise is received in polar areas. It catalyses release of Cl from CFCs. With the coming of spring season, the release of chlorine atoms causes the conversion of ozone into molecular oxygen. One atom of chlorine can destroy around 10,000 molecules of ozone and causes ozone depletion. All this leads to thinning of ozone layer which is more prominent over Antarctica region and called as the ozone hole. This hole disappears in summer due to free mixing of air of Antarctica with the rest of the global air.
  • The formation of the ozone hole will result on an increased concentration of UV-B radiations on the Earth’s surface.
  • Effects of enhanced UV Radiation are as follows:
    • Snow blindness or inflammation of cornea.
    • Damage of skin cells and development of skin cancer.
    • Damage to nucleic acids and proteins.
    • Reduced immunity.
    • Higher number of cataracts in humans.

Question 7. Discuss the role of women and communities in protection and conservation of forests.

Sol. Women and communities have played a major role in environmental conservation movements. It includes the following:

  • Amrita Bishnoi’s wildlife protection project
    • This community strictly believes in the concept of simple living peacefully with nature.
    • In 1731, to fulfill the order of king of Jodhpur for arranging wood for the construction of his new palace, minister and the workers went to Bishnoi village. Amrita Devi protested against king’s men’s attempt to cut trees as it was prohibited in Bishnoi religion. It was a party of Maharaja Abhay Singhji, Rular of Marwar (Jodhpur) state who wanted to fell green khejdali trees. Amrita Devi & her three daughter & more than 360 of other Bishnois lost their lives in saving trees & became martyers. This resistance by the people of the village forced the king to give up the idea of cutting trees.
  • Chipko movement
    • This movement was started in 1974 in the village of Garhwal region of the Himalayas. In this movement, the women stopped the contractors from cutting forest trees by embracing them.Gaura Devi of village Gopeshwar in Chamoli district had led a movement, where in village people hugged the trees to save them from being cut.
    • Many other people in different villages under the leadership of Chandi Prasad and Sunder Lal Bahuguna participated in similar protests.

Question 8. What measures, as an individual, would you take to reduce environmental pollution?

Sol. Pollution prevention is any practice that reduces, eliminates, or prevents pollution at its source. It is also known as source reduction. It is a main global concern because of their harmful effects on a person’s health and on the environment. Environmental pollution occurs in various forms, such as air pollution, water pollution, soil pollution, etc.

To reduce environmental pollution we should take following measures :

  • Use of clean and renewable energy sources such as CNG and bio-fuels.
  • Reducing the use of fossil fuels and using catalytic converters in automobiles.
  • Optimising the use of water.
  • Using kitchen waste water in gardening and other household purposes.
  • Composting of biodegradable kitchen waste.
  • Segregation of waste and composting of biodegradable kitchen waste.
  • Reducing the use of plastics and recycling and reuse of plastic and paper.
  • Disposing off waste safely.
  • Reducing use of polythene.
  • Not disposing off waste in water bodies.
  • Making automobiles pollution free.
  • Prevention of noise pollution by using fire crackers/TV/musical instruments at permissible limits.
  • Tree plantation in school, around residence.

Question 9. Discuss briefly the following:

  • Radioactive wastes
  • Defunct ships and e-wastes
  • Municipal solid wastes

Sol. (a) Radioactive waste

  • Radioactive waste is usually a by-product of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine.
  • Nuclear waste is rich in radioactive materials that generate large quantities of ionising radiations such as gamma rays. These rays cause mutation in organisms, which often results in skin cancer. At high dosage, these rays can be lethal.
  • Increased risk of cancer, birth defects & infertility are few harmful effects caused by nuclear waste. So, nuclear waste is an extremely potent pollutant.
  • It was realised that the use of nuclear energy has two very serious inherent problems. The. first is accidental leakage, as occurred in the Three Mile Island and Chernobyl incidents and the second is safe disposal of radioactive wastes. Safe disposal of radioactive wastes is a big challenge. It is recommended that nuclear wastes should be stored after pre-treatment in suitable shielded containers, which should then be buried in rocks.

(b) Defunct ships & e-wastes

  • Defunct ships
    • These are dead ships of no longer in use. Such ships are broken down for scrap metal in countries such as India and Pakistan. Dismantling of defunct ship is a technically complex process, which is potentially harmful to the environment & human health.
    • These ships are a source of various toxicants such as asbestos, lead, mercury etc. Thus, they contribute to solid wastes that are hazardous to health.
    • The workers breaking the ships are not suitably protected and are exposed to toxic chemicals. The coastal areas in the vicinity of the ship-breaking yard also becomes polluted. At the international level, it is accepted that there is uncertainty about the relevant controls for the dismantling of such vessels & there is an urgent need to establish a specific enforceable control framework.

E-wastes or electronic wastes

  • Electronic waste comprised of irrepairable computer and other electronic goods, generated by developed countries.
  • It is valuable source of secondary raw materials, if treated properly, however if not treated properly it is the major source of toxins.
  • These are rich in metals such as copper, iron, silicon, gold etc. These metals are highly toxic and pose serious health hazards. People of developing countries are involved in the recycling process of these metals and therefore, get exposed to toxic substances present in these wastes.
  • Eventually recycling is the only solution for the treatment of e-wastes provided it is carried out in an environment friendly manner.

Municipal solid wastes

  • These are commonly known as trash or garbage. It consists of everyday items such as product packaging, furniture, clothing, bottles, food scraps, newspapers. appliances, paints, batteries etc.
  • The open dumps of municipal wastes serve as a breeding ground for flies, mosquitoes, and other disease-causing microbes. Hence, it is necessary to dispose municipal solid waste properly to prevent the spreading of diseases.
  • Sanitary landfills and incineration are the methods for the safe disposal of solid wastes. Source reduction, recycling and compositing are several municipal social waste management practices.

Question 10. What initiatives were taken for reducing vehicular air pollution in Delhi? Has air quality improved in Delhi?

Sol. Delhi has been categorised as the fourth most polluted city of the world in a list of 41 cities. Burning of fossil fuels has added to the pollution of air in Delhi.

  • Various steps which have been taken to improve the quality of air in Delhi are as follows:
    • Introduction of CNG (Compressed Natural Gas): By the order of the supreme court of India, CNG-powered vehicles were introduced at the end of year 2006 to reduce the levels of pollution in Delhi. CNG is a clean fuel that produces very little unburnt particles.
    • Phasing out of old vehicles.
    • Introduction of green zones.
    • Use of unleaded and low-sulphur petrol, and diesel. Use of catalytic converters in vehicles.
    • Application of stringent pollution-level norms for vehicles.
    • Implementation of Bharat stage I, which is equivalent to euro II norms in vehicles of major Indian cities.
  • Yes, air quality has been improved in Delhi. The introduction of CNG­ powered vehicles led to a substantial fall in the level of CO and S0 . However, the problem of suspended particulate matter (SPM) and respiratory suspended particulate matter (RSPM) still persists.

Question 11. Discuss briefly the following :

  • Greenhouse gases
  • Catalytic converter
  • Ultraviolet B

Sol. (a)  Greenhouse gases

  • The greenhouse effect refers to an overall increase in the average temperature of the Earth due to the presence of greenhouse gases.
  • Many greenhouse gases occur naturally in the atmosphere, such as carbon dioxide, methane, water vapour, and nitrous oxide, while others are synthetic. Those that are man-made include the chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and
  • Perfluorocarbons (PFCs), as well as sulfur hexafluoride (SF6).
  • These gases has the property of absorbing infrared radiation (net heat energy) emitted from Earth’s surface and re-radiating it back to Earth’s surface, thus contributing to the greenhouse effect. When solar radiations reach the Earth, some of these radiations are absorbed. These absorbed radiations are released back into the atmosphere which is then trapped by the greenhouse gases present in the atmosphere. This helps in keeping our planet warm and thus, helps in human survival.
  • An increase in the amount of greenhouse gases can lead to an excessive increase in the Earth’s temperature, thereby causing global warming.

Note: Carbon dioxide, methane, and water vapour are the most important greenhouse gases.

  • Catalytic converter
    • Catalytic converter are used to reduce emission of poisonous gases like nitrogen oxides, carbon monoxide & unreacted hydrocarbon in automotive emission.
    • It is made of platinum, palladium and rhodium and is used as catalyst.
    • As the vehicular discharge passes through the catalytic converter, the unburnt hydrocarbons present in it get converted into carbon dioxide and water. Carbon monoxide and nitric oxide released by catalytic converters are converted into carbon dioxide and nitrogen gas (respectively).
    • The only precaution required is not to use gasoline having lead as lead inactivates the catalysts of the converter.
  • Ultraviolet B
    • Ultraviolet B is one of the three types of invisible light rays given off by the sun. Ultraviolet B penetrates the ozone layer in attenuated form & reaches Earth.
    • It induces many health hazards in humans. UV-B damages DNA and activates the process of skin aging. It also causes skin darkening and skin cancer. High levels of UV-B cause corneal cataracts in human beings. It also reduces the rate of photosynthesis in phytoplankton, the diversity of aquatic ecosystems.

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