NCERT Solutions For Class 8 Science Chapter 18
Pollution of Air and Water

The correct options are  (d).

When water bodies are nutrient enriched, then a dense growth of algae occurs. When these algae die, the decomposers decompose them by using a lot of oxygen. It results in decrease in concentration of oxygen in water resulting in death of aquatic organisms. 

No, clear and transparent water is not always fit for drinking. Water which looks clean may still have disease carrying microorganisms and dissolved impurities. Therefore, it is essential to purify water before drinking.

Sulphur dioxide and nitrogen dioxide produced from the industries like rubber processing industry, Mathura oil refinery etc. located in and around Agra are responsible for the discolouring of white marble of Taj Mahal. Sulphur dioxide and nitrogen dioxide gases released from the industries, react with water vapour present in the air to form sulphuric acid and nitric acid, respectively. The acids drop down with rain, making rain acidic. Acid rain corrodes the marble of the Taj Mahal. The phenomenon is also known as ‘Marble Cancer’. Suspended particulate matters emitted by ‘Mathura oil refinery’ have also contributed towards the yellowing of the marble.

Gases like carbon dioxide, methane, water vapour etc. present in the atmosphere are called green house gases. These gases trap the heat from Sun and prevent it from escaping into the outer space. It helps in keeping earth warm and is essential for life on earth. But increase in the concentration of these gases leads to excessive increase in Earth’s temperature leading to global warming.

Sulphur dioxide and oxides of nitrogen are produced from various industries. These gases react with water vapour present in the air to form sulphuric acid and nitric acid, respectively. These acids come down with the rain water and this process is called acid rain.

Acid rain destroys plants and vegetation. The water of the rivers and the lakes becomes acidic and this causes the destruction of the aquatic life. Acid rain also destroys the ancient buildings, monuments and sculptures due to the continuous erosion of the marble.

As a member of the municipal body of my town, I would take certain necessary actions to ensure the supply of clean water.

1. Water must be treated properly to make it fit for use.
2. Chlorination must be done to disinfect the drinking water supplies.
3. Rain water harvesting will be made mandatory in every house.

Water gets contaminated by following ways:

1. Excessive amounts of pesticides and fertilisers are being used to increase crop production. These chemicals are carried away to the water bodies by rains and floods which contaminate water.
2. Harmful chemicals discharged from industries pollute water bodies.
3. Water gets contaminated by the discharge of sewage into rivers.

Global warming is an increase in the average temperature of Earth’s atmosphere. It is caused due to increased levels of green house gases like carbon dioxide, oxides of nitrogen, water vapour, methane, etc. These gases absorb solar radiation and prevent it from escaping into the outer space thereby, increasing Earth’s average temperature.

This continuous warming has lead to an increase in the temperature of Earth’s surface by 0.6°C approx. in the last three decades. Due to a rise in the temperature, the polar ice is melting and is threatening the survival of the polar bears.

The melting of the polar ice causes a rise in the sea level. It harms the coastal life and vegetation.

An individual can reduce air pollution by:

1. Using public transport and encouraging car pooling.
2. Using bicycle for short distances.
3. Using CNG and unleaded petrol instead of leaded petrol and diesel in automobiles.
4. Using alternative fuels like solar energy, wind energy and hydropower instead of fossil fuels.
5. Planting trees and nurturing the ones already present.