NCERT Solutions for Class 12-science Biology Chapter 16 - Environmental Issues

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Chapter 16 - Environmental Issues Exercise 286

Solution 1

Constituents of domestic sewage:

i. Faecal matter, bacteria and paper fibres.

ii. Nutrients such as nitrate, ammonia, phosphate, sodium and calcium.

iii. Toxic metal ions and organic compounds.

Effects of sewage discharge on a river:

i. Sewage promotes the growth of phytoplankton. Their excessive growth depletes the oxygen in water. The reduction of oxygen and the presence of poisonous wastes affect the fish population and are unsafe for human consumption.

ii. Because sewage water contains disease-causing germs, detergents, other decomposing substances and excreta, they may cause severe diseases such as typhoid, cholera etc. in human population.

Solution 2

The State Government of Delhi took the following measures to improve the quality of air:

(i) Use of CNG buses as a public transport system.

(ii) Use of unleaded petrol.

(iii) Phasing out of old vehicles.

(iv) Use of low sulphur diesel.

(v) Use of catalytic converters in vehicles.

(vi) Application of Euro-II norms for vehicles.

Yes, the air quality has certainly improved with a substantial fall in the concentration of carbon monoxide, oxides of sulphur and nitrogen.


Solution 3

(a) Greenhouse gases: Greenhouse gases play an important role in trapping heat in the atmosphere. This prevents radiation from the Sun from going back to space. Due to this property, greenhouse gases help in maintaining the earth's temperature at an optimum level. Examples of greenhouse gases are carbon dioxide, methane and chlorofluorocarbon (CFC).

Because of higher emissions, the greenhouse gas concentration has increased in the atmosphere. This has led to a general rise in global temperature creating major climatic changes.

(b) Catalytic converters: Catalytic converters are fitted into automobiles for reducing emission of poisonous gases. They have expensive metals, namely platinum, palladium and rhodium, as the catalysts. Because 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. This helps to reduce the emission of polluting gases from automobiles.

(c) Ultraviolet B: UV-B damages DNA and mutation may occur. It causes ageing of skin, damage to skin cells and various types of skin cancers. In the human eye, the cornea absorbs UV-B radiation, and a high dose of UV-B causes inflammation of cornea called snow-blindness cataract. Such exposure may permanently damage the cornea. It also diminishes the functioning of the immune system.

Solution 4

Waste generated at home: Paper, broken glass, cloth, plastic, cans, carton box, crockery and thermocol.

Waste generated at school: Paper, chalk, plastic envelops etc.

Waste generated during trips: Disposable cups and glasses, spoons, discarded food and polythene bags.

Yes, wastes can easily be reduced by us through the judicious use of materials by changing our habits and life styles.

There are two types of wastes-biodegradable and non-biodegradable.

Wastes which can be reduced are called biodegradable wastes as these are broken down naturally by the activity of microorganisms. They include all articles of organic origin, i.e. food waste, paper, wood, rags etc.

Wastes which cannot be reduced by the action of microorganisms are called non-biodegradable wastes. Broken glass, metal cans and plastic and polythene articles are non-biodegradable wastes.

It is difficult or rather impossible to degrade non-biodegradable waste because microorganisms cannot decompose these.


Solution 5

The increase in the atmospheric temperature of the earth due to the increase in the concentration of greenhouse gases (CO2, CH4, CFC) is called global warming.

Causes of global warming:

(i) Burning of fossil fuel in automobiles and industries

(ii) Deforestation

Effects of global warming:

(i) Due to the rise in temperature, the polar ice caps will melt resulting in the rise of the sea water level.

(ii) Global warming will lead to the explosive growth of weeds and increased incidence of plant diseases and pests. All these factors will decrease crop production in tropical and subtropical regions.

(iii) Effects on weather and climate lead to increased chances of cyclones and floods.

Control measures:

(i) Deforestation should be reduced.

(ii) Plantation of trees should be enhanced.

(iii) Limited use of fossil fuels by developing and using alternative sources of energy.

(iv) Reduction in human population.

Solution 6

(a) - (ii)

(b) - (i)

(c) - (iii)

(d) - (iv)


Solution 7

(a) Eutrophication: Fertilisers are washed out with rain water from the fields into the rivers, lakes and ponds and promotes the profuse growth of algae (algal bloom), especially the blue-green algae. Such algal blooms may totally cover the water surface and consume excess quantity of dissolved oxygen, causing depletion of oxygen in water for other organisms. The algae may release toxins in water to inhibit the growth of other algae and aquatic animals (fish). Thus, reduction of oxygen and toxin released in water affects the fish population and other aquatic life. This process of nutrient enrichment of water and consequent loss of species diversity is referred to as eutrophication.

(b) Biological magnification: It is the process of concentration of harmful chemicals at each successive trophic level in a food chain. The harmful chemicals are used to destroy pests in the field, but these are absorbed by plants along with other nutrients from the soil. These pesticides are then transferred to the higher trophic level through the food chain.

Example: Dichlorodiphenyltrichloroethane (DDT) is an insecticide which is sprayed on water to kill mosquitoes. In the US, after regular DDT spraying for few years, the population of fish-eating birds began to decline. Later, it was found that the concentration of DDT had increased to about 800 times in phytoplankton relative to the concentration in water. DDT was about five times more in zooplankton than in phytoplankton. The DDT concentration in birds was 25 times greater than that in fish. So, in this food chain, the concentration of pesticides was gradually increasing, and it showed biological magnification.

(c) Groundwater depletion and ways for its replenishment: The level of groundwater is falling day by day caused by a high demand in urban areas as well as in agriculture. Due to the over use of surface water, people rely on ground water for irrigation, drinking and industrial use. About 85% of rural water supply and more than 50% of urban and industrial supply is mined. This results in depletion of ground water.

Groundwater can be replenished by the following ways:

i. Water should be used judiciously.

ii. Avoid using more water than what is replenished by rain water.

iii. Flow of water into drains, streams, rivers and lakes should be minimised.

iv. Maximum amount of rain water should be collected in catchment areas for direct utilisation.

Solution 8

The decline in the thickness of the ozone layer over a restricted area is called the ozone hole. It was first discovered over Antarctica. The Antarctic air is completely isolated from the rest of the world by the natural circulation of wind called as Polar vertex. CFCs released in the atmosphere slowly enter the stratosphere, and the wind pushes these CFCs towards the poles. Environmental conditions prevailing in Antarctica during the winter months are conducive for the formation of the ozone hole. During winter months, the lack of sunlight and low temperature facilitate the formation of ice clouds, which provide the catalytic surface for the reaction of chlorine.

Effects of ultraviolet rays:

i. Skin cancer

ii. Blindness and increased chances of cataract in eyes

iii. Malfunctioning of the immune system


Solution 9

Women and tribal communities have played a significant role in the protection and conservation of forests. The Amrita Devi Bishnoi Wildlife Protection Award has been instituted by the Government of India for individuals and rural communities for their contribution in the protection of wildlife.

In 1731, a woman Amrita Devi showed exemplary courage by hugging a tree to prevent its cutting. Her three daughters and hundreds of other Bishnoi followed her. They were killed by the soldiers of the king of Jodhpur. Later, the Chipko movement of Garhwal was started by the local women to prevent the cutting of trees.

Solution 10

i. I will use only those articles which are either disposable or recyclable.

ii. I would help in tree plantation in my school and surroundings.

iii. I will minimise the use of fossil fuels.


Solution 11

(a) Radioactive wastes: They are wastes generated from nuclear plants which release radioactivity (emission of alpha and beta particles or gamma rays) from nuclides of their elements.

Depending on the amount of radioactivity, there are three types of radioactive wastes-low level, intermediate level and high level. High-level radiations are highly destructive and are emitted due to accidental leakage of atomic reactors. These radiations cause tumours, cancers and genetic disorders. High-level wastes require special protective shields during handling and transport besides cooling.

(b) Defunct ships and e-wastes: Old defunct ships are broken down in developing countries such as India, Pakistan and Bangladesh because of easy and cheap labour and the demand for scrap material/metal. These ships possess several toxic materials such as asbestos, lead, mercury, tributylin and polychlorinated biphenyls. The labour engaged in ship breaking is exposed to these toxic materials. The coastal areas where ship breaking is undertaken also become polluted.

E-wastes are the irreparable computers and other electronic goods which are imported to developing countries for the extraction of metals through recycling.

(c) Municipal solid wastes: These are the waste materials from homes, offices, stores, hospitals and small-scale industries settled in residential areas. These wastes comprise clothes, broken glass, bottles, polythene bags, leather, metals and electronic items.

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