Environmental Issues Chapter 16 Class 12 Biology NCERT Textbook PDF

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NCERT Class 12 Biology Textbook Chapter 16 With Answer PDF Free Download

Environmental Issues

Chapter 16: Environmental Issues

The human population size has grown enormously over the last hundred years. This means an increase in demand for food, water, home, electricity, roads, automobiles, and numerous other commodities.

These demands are exerting tremendous pressure on our natural resources, and are also contributing to the pollution of air, water, and soil.

The need of the hour is to check the degradation and depletion of our precious natural resources and pollution without halting the process of development.

Pollution is any undesirable change in physical, chemical, or biological characteristics of air, land, water, or soil.

Agents that bring about such an undesirable change are called pollutants. In order to control environmental pollution, the Government of India has passed the Environment (Protection) Act, 1986 to protect and improve the quality of our environment (air, water, and soil).


We are dependent on air for our respiratory needs. Air pollutants cause injury to all living organisms.

They reduce the growth and yield of crops and cause the premature death of plants. Air pollutants also deleteriously affect the respiratory system of humans and of animals. Harmful effects depend on the concentration of pollutants, duration of exposure, and the organism.

Smokestacks of thermal power plants, smelters, and other industries release particulate and gaseous air pollutants together with harmless gases, such as nitrogen, oxygen, etc.

These pollutants must be separated/ filtered out before releasing the harmless gases into the atmosphere.

There are several ways of removing particulate matter; the most widely used of which is the electrostatic precipitator (Figure 16.1), which can remove over 99 percent of particulate matter present in the exhaust from a thermal power plant.

It has electrode wires that are maintained at several thousand volts, which produce a corona that releases electrons.

These electrons attach to dust particles giving them a net negative charge. The collecting plates are grounded and attract charged dust particles.

The velocity of air between the plates must be low enough to allow the dust to fall. A scrubber (Figure 16.1) can remove gases like sulfur dioxide.

In a scrubber, the exhaust is passed through a spray of water or lime. Recently we have realized the dangers of particulate matter that are very very small and are not removed by these precipitators.

According to Central Pollution Control Board (CPCB), particulate sizes 2.5 micrometers or less in diameter (PM 2.5) are responsible for causing the greatest harm to human health.

These fine particulates can be inhaled deep into the lungs and can cause breathing and respiratory symptoms, irritation, inflammations and damage to the lungs, and premature deaths. Automobiles are a major cause of atmospheric pollution at least in the metro cities.

As the number of vehicles increases on the streets, this problem is now shifting to the other cities too. Proper maintenance of automobiles along with the use of lead-free petrol or diesel can reduce the pollutants they emit.

Catalytic converters, having expensive metals namely platinum-palladium and rhodium as the catalysts, are fitted into automobiles for reducing the emission of poisonous gases.

As the exhaust passes through the catalytic converter, unburnt hydrocarbons are converted into carbon dioxide and water, and carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas, respectively.

Motor vehicles equipped with catalytic converters should use unleaded petrol because lead in the petrol inactivates the catalyst.

Language English
No. of Pages17
PDF Size1330 KB

NCERT Solutions Class 12 Biology Chapter 16 Environmental Issues

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


The wastes generated from households – from the toilet, kitchen, laundry, and other related sources are termed Domestic sewage.

It contains contaminants in the form of disease-causing microbes, suspended solid (clay, silt, sand), colloidal matter (bacteria, plastic, cloth fiber, fecal matter), and 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 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.

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 the earth is termed Global warming. It can be caused due to the following reasons:

(i) The increased concentration of greenhouse gases in the atmosphere of the earth causes the phenomena of global warming.

These gases are methane, carbon dioxide, and water vapour which trap solar rays that are given out by the Earth helping to keep the planet warm, thus aiding human survival. The rise in the level of these greenhouse gases can cause a huge increase in the temperature of the Earth resulting in Global warming

(ii) Global warming can also be caused due to burning of fossil fuels, industrialization, and the act of deforestation

Impact of Global warming:

(i) 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.

(ii) 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:

(i) Use of fossil fuels must be reduced

(ii) Increase the usage of bio-fuels

(iii) Switch to a renewable source of energy such as CNG

(iv) Promote reforestation

(v) Encourage and carry out recycling of materials

(vi) Energy must be efficiently used

6. Why does an 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 are extensively used as a refrigerant.

The CFCs migrate from different layers of the atmosphere – troposphere to the stratosphere, where chlorine atoms are released by the action of UV radiations on them. The 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 radiation on the surface of the earth.

The UV – B is known to damage DNA, activating the process of skin aging. Also, it causes darkening of the skin and skin cancer. In humans, high levels of UV-B causes corneal cataract.

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


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

(i) 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 down forest trees. They did so by embracing them.

(ii) 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 among the villagers forced the King to give up on the idea of chopping trees.

Environmental Issues NCERT Textbook With Solutions PDF Free Download

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