DAV Class 8 Science Chapter 19 Question Answers- Pollution of Air
We breathe about twenty thousand times a day, and every one of those breaths depends on the quality of the air around us. Chapter 19 of the DAV Class 8 Science book explains what spoils that air, what the polluted air does to people, buildings and the planet, and what we can do about it. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the idea and write it in your own words.
The chapter in one minute
A pollutant is any substance that causes harm when it is present in the environment in a larger amount than normal. In air, the main sources are burning fuels such as coal, petrol and diesel, and the smoke from factories and vehicles.
Several problems come out of this. Acid rain forms when gases like sulphur dioxide and nitrogen dioxide mix with water vapour in the air and fall as rain. Smog is a thick layer of smoke mixed with fog, common in winter. Global warming is the slow rise in the Earth’s average temperature, caused by greenhouse gases such as carbon dioxide and methane that trap heat. Ozone depletion is the thinning of the ozone layer, which protects us from the sun’s harmful ultraviolet rays, and the main culprits are chlorofluorocarbons (CFCs).
A. Fill in the blanks
1. Nitrogen dioxide and sulphur dioxide gases dissolve in water to form acid rain.
2. The damage that acid rain does to the marble of monuments is called marble cancer.
3. Carbon dioxide and methane are two of the main greenhouse gases.
4. The likely rise in the Earth’s average temperature due to more greenhouse gases is called global warming.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. Burning fossil fuels is one cause of air pollution. | True | Burning coal, petrol and diesel releases smoke and harmful gases. |
| 2. A high level of nitrogen dioxide in the air can cause breathing problems. | True | It irritates the lungs and airways. |
| 3. Ultraviolet rays are mainly absorbed by the oxygen in the atmosphere. | False | They are mainly absorbed by the ozone layer. |
| 4. Acid rain does not corrode metals. | False | The acids in it eat away metals, as well as stone. |
| 5. Sulphur dioxide is a greenhouse gas. | False | It causes acid rain. The greenhouse gases are carbon dioxide, methane and water vapour. |
C. Choose the correct option
1. The ozone layer is depleted by chlorofluorocarbons (CFCs).
2. More carbon dioxide and methane in the air is likely to lead to global warming.
3. The main gases that trap heat sent back from the Earth are carbon dioxide, methane and water vapour.
4. The Taj Mahal at Agra can be damaged if the air around it has more sulphur dioxide, because this gas leads to acid rain, which eats into marble.
5. Smoke that combines with water vapour and dust particles, in the presence of sunlight, forms smog.
D. Short answer questions
1. What is a pollutant?
A pollutant is a substance that harms the environment when it is present in a larger amount than it normally is in nature.
2. State two causes of air pollution.
First, burning too much fossil fuel such as coal, petrol, diesel and kerosene. Second, smoke released from factories and vehicles.
3. How is carboxyhaemoglobin formed?
When we breathe in carbon monoxide, it passes from the lungs into the blood and attaches itself to haemoglobin, the substance that carries oxygen. This forms carboxyhaemoglobin. Because haemoglobin is now busy with carbon monoxide, the blood can carry less oxygen to the body.
4. State the harmful effect of chlorofluorocarbons.
CFCs break down ozone into oxygen, and this thins the ozone layer that protects us from the sun’s ultraviolet rays.
5. Suggest three measures to check global warming.
Stop cutting down forests, plant more trees, and reduce the use of CFCs. Using less fossil fuel also helps.
E. Long answer questions
1. What is smog? State its harmful effects.
Smog is a thick, fog-like layer in the air that forms when smoke mixes with fog and other pollutants. The name comes from joining the words “smoke” and “fog”. It is seen mostly in cold weather, when smoke from vehicles and factories gets trapped near the ground.
Its harmful effects are:
- It causes lung problems such as bronchitis and asthma, and it makes breathing difficult.
- It blocks sunlight from reaching the ground, which harms the normal growth of plants.
2. Write a short note on acid rain.
Acid rain is rain that is more acidic than normal. Factories, vehicles and power plants release gases like sulphur dioxide and nitrogen dioxide into the air. These gases mix with water vapour and form sulphuric acid and nitric acid. When it rains, these acids fall to the ground with the water. Acid rain damages monuments, especially marble ones like the Taj Mahal, corrodes metals, harms plants and makes lakes and rivers unsafe for water life.
3. What are the main causes of the rise in greenhouse gases?
The main causes are:
- Burning coal and wood in homes for cooking and heating releases carbon dioxide.
- Thermal power plants burn large amounts of coal to produce electricity, and this releases a lot of carbon dioxide and carbon monoxide.
- Cutting down trees on a large scale means less carbon dioxide is absorbed from the air, so its level keeps rising.
4. What are the likely results of an increase in the greenhouse effect?
The Earth’s average temperature rises gradually, which is called global warming. The warmer air makes forest fires more common. Glaciers melt faster, which raises the sea level and can lead to floods and loss of land along the coast.
5. Write five ways to control air pollution.
- Use CNG in vehicles instead of petrol or diesel.
- Plant more trees, because they take in carbon dioxide and clean the air.
- Follow strict anti-pollution rules for factories and vehicles.
- Do not burn dry leaves and farm waste.
- Use smokeless chulhas, solar cookers and biogas for cooking.
Mistakes to avoid in the exam
Many students think the ozone layer is a greenhouse gas problem. They are two different problems. Ozone depletion is caused mainly by CFCs, while global warming is caused by carbon dioxide and methane.
Another slip is saying that sulphur dioxide causes global warming. It causes acid rain.
When writing about acid rain, name both acids. Sulphur dioxide leads to sulphuric acid and nitrogen dioxide leads to nitric acid.
Finally, in the “five methods” question, avoid vague answers such as “stop pollution”. Give specific, practical steps to get full marks.
Frequently asked questions
What is the greenhouse effect?
It is the trapping of heat in the Earth’s atmosphere by gases such as carbon dioxide, methane and water vapour. A small amount of it keeps the planet warm, but too much of it causes global warming.
Why is the ozone layer important?
It absorbs most of the sun’s harmful ultraviolet rays and protects living things from them.
Why is carbon monoxide dangerous?
It attaches to haemoglobin in the blood and stops it from carrying oxygen, so the body does not get enough oxygen.
How does acid rain damage the Taj Mahal?
The acids in acid rain react with the marble and slowly eat it away. This damage is called marble cancer.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.