NCERT Solutions for Class 12 Biology Chapter 16 Environmental Issues – Free PDF

NCERT Solutions for Class 12 Biology Chapter 16 – Environmental Issues – FREE PDF Download

On seeking the answers from our Chapter 16 Biology Class 12 NCERT Solutions, you can be assured that your preparation for the CBSE board exams will be ideal. Our team of experts in CoolGyan has formulated the best solutions for Environmental Issues Class 12 NCERT that will help you find out the precise answers of all the questions mentioned in the exercises. Do not forget to download Class 12 Biology Chapter 16 – Environmental Issues NCERT Solutions PDF from our website so that you can refer to it as per your convenience while studying offline.

Revision Notes Class 12 Biology
NCERT Solutions Class 12 Biology

Answers of Biology NCERT Chapter 16 – Environmental Issues

Exercise Page No.286

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


The wastes generated from households – from toilet, kitchen, laundry and other related sources is termed as Domestic sewage. It contains contaminants in the form of disease-causing microbes, suspended solid (clay, salt, sand), colloidal matter (bacteria, plastic, cloth fiber, fecal matter), dissolved matter (phosphate, nitrate, ammonia, sodium, calcium). The organic wastes from the sewage entering the water bodies serve as a source of food for microbes such as bacteria and algae causing these microbes to multiply, hence their population increases. Almost all of the dissolved oxygen is utilized by them for their metabolism leading to the Biological Oxygen Demand (BOD) levels in river water to increase, causing the death of aquatic entities. Additionally, the nutrients present in water cause the planktonic algal growth, hence the algal bloom which leads to the decline in the quality of water and fish mortality.

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?


Some of the wastes generated at different places are as given below:

At HomeAt schoolsAt trips
Paper napkinPlasticsWaste paper
Plastic coversWaste paperPlastic bags
ToiletriesFruits & vegetable peelsFood wrappers
Wastes from kitchen – fruits & vegetable peels, unwanted grains, perished food, glass, domestic sewageWrappersFruits & vegetable peels
SewageDisposable plates and cups
Dust from chalk, crayons, pencil sharpeners

Yes, the use of these wastes can certainly be reduced with ease. This can be achieved by cutting down the use of plastics and switching to other alternatives. Wastes generated from paper can be greatly minimized by using both sides of paper, by recycling papers. Re-use and recycling of glass waste and plastics can greatly help as well. Substituting the use of plastic covers with biodegradable bags (jute) can cause less waste to be generated at schools, home or even trips. Optimized use of water during cooking, bathing and other household activities can greatly reduce domestic sewage. It is difficult to decompose non-biodegradable wastes such as metal, plastic, glass shards as microbes cannot decompose them.

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


An increase in the average temperature of the surface of earth is termed as Global warming. It can be caused due to the following reasons:

  • Increased concentration of the greenhouse gases in the atmosphere of the earth causes the phenomena of global warming. These gases are methane, carbon dioxide and water vapor which trap solar rays that are given out by the Earth helping to keep the planet warm, thus aiding human survival. Rise in the level of these greenhouse gases can cause a huge increase in the temperature of the Earth resulting in Global warming
  • Global warming can also be caused due to burning of fossil fuels, industrialization and the act of deforestation

Impact of Global warming:

  • It has been observed over the past few decades that global warming has led the average temperature of the Earth to increase by 0.6 ˚C, causing a disturbance in the natural water cycle, hence fluctuations in the pattern of rainfall. These changes also have an impact on the rainfall.
  • Global warming has also caused the polar ice caps and mountain glaciers to melt, leading to an increase in the sea level, hence the inundation of coastal areas.

Preventive measures:

  • Use of fossil fuels must be reduced
  • Increase the usage of bio-fuels
  • Switch to renewable source of energy such as CNG
  • Promote reforestation
  • Encourage and carry out recycling of materials
  • Energy must be efficiently used
4. Match the items given in column A and B: 

Column AColumn B
(a) Catalytic converter(i) Particulate matter
(b) Electrostatic precipitator(ii) Carbon monoxide and nitrogen oxides
(c) Earmuffs(iii) High noise level
(d) Landfills(iv) Solid wastes



Column AColumn B
(a) Catalytic converter(ii) Carbon monoxide and nitrogen oxides
(b) Electrostatic precipitator(i) Particulate matter
(c) Earmuffs(iii) High noise level
(d) Landfills(iv) Solid wastes

5. Write critical notes on the following:

(a) Eutrophication

(b) Biological magnification

(c) Groundwater depletion and ways for its replenishment


(a) Eutrophication –

It is referred to the natural ageing process of a lake as a result of nutrient enrichment caused by the overflow of nutrients such as fertilizers, animal wastes and sewage from land leading to an increased fertility of the lake. Consequently, there is a huge increase in the primary productivity of the ecosystem bringing about an increased growth of algae, hence in algal blooms. After sometime, the decomposition of these algae reduces oxygen supply, resulting in the death of aquatic animal life.

(b) Biological magnification

Pesticides are used on a large scale to protect crops from various diseases and pests. When these pesticides enter the soil, they are absorbed by plant parts with minerals and water from the soil. These chemicals can enter water sources as a result of rains, furthermore into the bodies of aquatic animals and plants, hence into the food chain. As these chemicals cannot be disintegrated, at each of the tropic levels, they keep getting accumulated, wherein most of the accumulation occurs at the top carnivore’s level. This is termed as biological magnification where there is increase in the concentration of harmful pollutants with each increasing tropic level.

For instance, high DDT concentration in a pond saw the producers (phytoplankton) to have a DDT concentration of 0.04 ppm. As these phytoplankton were consumed by zooplankton – the consumers, the concentration of DDT in the bodies of zooplankton was noted to be 0.23 ppm. When this in turn is consumed by small fish, more of DDT gets collected in their bodies. Consequently, the large fish (top carnivore) feeding on different small fish tend to have the maximum concentration of DDT.

NCERT solutions class 12 biology chapter 16 - 1

(c) Groundwater depletion and ways for its replenishment

Since the past years, the level of ground water has decreased. Water supply from sources is declining rapidly with each passing year due to an increase in water pollution and population explosion. In order to meet the demand of water, water is being taken out from water bodies such as rivers, ponds etc. hence, the source of ground water is fast depleting as the quantity of groundwater that is withdrawn for human utilization is more than the quantity that is being replaced by rainfall. Small quantities of water penetrates through the ground due to lack of vegetation cover. Another factor that has caused a decline in the availability of ground water is an increase in water pollution.

Measures to replenish ground water are as follows:

  • Over-exploitation of ground water must be prevented
  • Water use must be optimized and water demand must be reduced
  • Promote and encourage rain water harvesting
  • Prevent deforestation
  • Promote afforestation, plant more trees

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


Ozone holes are more apparent in the region of Antarctica. They are formed as a result of an increased concentration of chlorine in the atmosphere. The release of chlorine is mainly from Chlorofluorocarbons (CFCs) which is extensively used as a refrigerant. The CFCs migrate from different layers of the atmosphere – troposphere to stratosphere, where chlorine atoms are released by the action of UV radiations on them. Liberation of chlorine atoms leads to the conversion of ozone into molecular oxygen. One chlorine atom can destruct 10,000 ozone molecules, causing ozone depletion. Ozone hole formation will result in an increased concentration of UV-B radiations on the surface of earth. The UV – B is known to damage DNA, activating the process of skin ageing. Also, it causes darkening of skin and skin cancer. In humans, high levels of UV-B causes corneal cataract.

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


The role of communities and women in protecting and conserving forests has been significant.

  • The Chipko Movement

This movement was started in the Garhwal region of the Himalayas in 1974 wherein the women of the village opposed the contractors from chopping forest trees. They did so by embracing them.

  • Case study of the Bishnoi community

In Rajasthan, the Bishnoi community has a strict belief in the concept of peacefully co-existing with nature. The king of Jodhpur in 1731 ordered his ministers to organize wood to construct his new palace for which the king and his people visited the Bishnoi village where a Bishnoi woman known as Amrita Devi along with her daughter and hundreds of other Bishnois were courageous enough to take a step and stop them from chopping trees. They embraced the trees, losing their lives at the hands of soldiers. This conflict by the villagers forced the King to give up on the idea of chopping trees.

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


Listed below are some measures that can be taken to prevent environmental pollution:

To prevent air pollution

  • Clean and renewable energy sources to be used such as bio-fuels and CNG
  • Afforestation – promote planting
  • Limit usage of fossil fuels
  • Catalytic converters to be used in automobiles

To prevent water pollution

  • Use of water to be optimized
  • For gardening and other related chores, switch to using kitchen waste water

To prevent noise pollution

  • Plant more trees
  • Reduce/avoid bursting of crackers

Measures to decrease solid waste generation

  • Different types of wastes to be separated
  • Reuse and recycle paper and plastic
  • Plastic usage must be reduced and eventually avoided completely
  • Biodegradable kitchen waste must be composted

9. Discuss briefly the following:

(a) Radioactive wastes

(b) Defunct ships and e-wastes

(c) Municipal solid wastes


(a) Radioactive wastes

These are the wastes produced when nuclear energy is generated from radioactive materials. This nuclear waste is richly supplied with radioactive material which generate large quantities of ionizing radiations like the gamma rays causing mutation in entities resulting in ailments such as skin cancer. These rays can be lethal at high dosages. Harmless disposal of radioactive wastes is a huge challenge. Ideally, nuclear wastes should be stored after pre-treatment in appropriate shielded containers and later buried in rocks.

(b) Defunct ships and e-wastes

The dead ships that are no more used as referred to as Defunct ships. These ships are dismantled for scrap materials in Pakistan and India. There are different toxicants in such ships such as lead, mercury, asbestos etc. which lead to solid wastes that can be dangerous to health. E-wastes or electronic wastes include electronic goods such as mobiles, computers and other gadgets that are rich in metals like iron, copper, gold, silicon etc. Such metals are extremely toxic, posing severe health hazards. Hence people of developing countries who participate in the recycling process of such metals are debunked to toxic matter found in the wastes.

(c) Municipal solid wastes

These are the wastes that is produced from offices, schools, stores and homes which is typically rich in metal, glass, food, rubber, leather, paper waste and textiles. Municipal wastes are dumped in landfills and open dumps which serve as a breeding area for mosquitoes, flies and other microbes who have the potential to cause diseases. Therefore, it is crucial to dispose municipal wastes appropriately in order to prevent the diseases from spreading. Some measures for safe disposal of solid wastes are incineration and sanitary landfills.

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


In a list of 41 cities, Delhi has been characterized as the fourth most polluted city of the world. The pollution of air in Delhi is added by burning of fossil fuels. Several measures haven been to improve the air quality in Delhi, they are:

  • Introduction of CNG (Compressed Natural Gas) – CNG is a clean fuel producing very little unburnt substances. The Supreme Court of India has ordered the use of CNG-powered vehicles that were introduced at the end of the year 2006 in order to reduce the pollution levels in Delhi.
  • Use of old vehicles is discontinued
  • Use of catalytic converters
  • Use of Unleaded petrol
  • Use of diesel and low-sulphur petrol
  • Applying strict pollution-level norms for vehicles
  • Vehicles of major Indian cities have been implemented with Bharat Stage I, similar to Euro II norms

Air quality of Delhi has shown improvement due to the introduction of CNG-powered vehicles leading to a considerable dip in the level of SO2 and CO2. But, the issue of suspended particulate matter (SPM) and respiratory suspended particulate matter (RSPM) still exists.

11. Discuss briefly the following:

(a) Greenhouse gases

(b) Catalytic converter

(c) Ultraviolet B


(a) Greenhouse gases

This effect refers to the global increase in the average temperature of the Earth as a result of greenhouse gases. These gases are methane, carbon dioxide and water vapor. When solar rays are incident on Earth, some of these are absorbed which are released back into the atmosphere. The greenhouse gases present in the atmosphere trap these radiations which helps in keeping Earth warm, hence suitable for humans to survive. Therefore, increase in the level of greenhouse gases can lead to an increase in the temperature of Earth overall resulting in Global warming.

(b) Catalytic converter

These are devices fixed in automobiles to lessen the vehicular pollution and contain expensive metals such as palladium, platinum and rhodium that serve as catalysts. When the vehicular discharge passes through the catalytic converter, the unburnt hydrocarbons found in it is converted into water and carbon dioxide. Nitric oxide and carbon monoxide liberated by catalytic converters are converted into nitrogen gas and carbon dioxide respectively.

(c) Ultraviolet-B

It is an electromagnetic radiation possessing a short wavelength compared to the visible light. They are produced by Sun, and are harmful rays which penetrate through the ozone hole on the surface of the Earth. UV-B poses many health hazards in humans, it damages DNA, activating the skin ageing process. These rays are also known to cause skin cancer and skin darkening. High levels of UV-B causes corneal cataract in humans.