CG Board Class 8 Science Chapter 3 Solutions: Air (वायु) – 2026-27

Air is a mixture of gases. What are they?

Air is a mixture of several gases. By volume, it mainly consists of:

1. Nitrogen (N₂): ~78%
2. Oxygen (O₂): ~21%
3. Argon (Ar): ~0.9%
4. Water Vapour: ~0.04%
5. Carbon Dioxide (CO₂): ~0.03%
6. Traces of Sulphur dioxide, other gases, and dust particles.

What is ozone layer? What is its importance?

The ozone layer is a protective layer of ozone gas (O₃) present in the atmosphere at a height of about 16 to 23 kilometres above the Earth’s surface. Three atoms of oxygen combine to form one molecule of ozone (O₃).

Importance: It blocks harmful ultraviolet (UV) radiation coming from the Sun, thereby protecting living organisms on Earth from skin cancer and other dangerous health hazards.

Which are the gases, which have the maximum percentage in the air?

Nitrogen gas (N₂) has the highest percentage in the air, making up about 78% of the atmospheric volume, followed by Oxygen gas (O₂) which constitutes about 21%.

Explain the reaction, which takes place when potassium permanganate is heated?

When solid potassium permanganate (KMnO₄) is heated in a hard glass test tube, it undergoes thermal decomposition to produce potassium manganate (K_2MnO₄), manganese dioxide (MnO₂), and liberates oxygen gas (O₂).

Balanced Chemical Equation:

2KMnO₄ → [Heat] K_2MnO₄ + MnO₂ + O₂↑

(Potassium permanganate → Potassium manganate + Manganese dioxide + Oxygen)

What happens when red hot coal is brought near a test tube filled with oxygen?

When a piece of red hot coal (carbon) is brought near a test tube filled with oxygen, it burns brilliantly with a bright flame forming Carbon Dioxide gas (CO₂). When lime water is poured into the test tube, it turns milky, confirming the formation of carbon dioxide.

Balanced Chemical Equation:

C + O₂ → CO₂

(Carbon + Oxygen → Carbon dioxide)

What are the uses of oxygen in our daily life?

1. Respiration: Oxygen is indispensable for the respiration of living beings. Cylinders of compressed oxygen are used by mountaineers, astronauts, and scuba divers. In hospitals, oxygen masks are provided to patients suffering from asthma, heart disease, or respiratory distress.
2. Welding and Cutting: A mixture of oxygen with other flammable gases is burned to generate an extremely hot flame used to weld and cut metals.

What are the main gases that pollute the air?

The primary gases that pollute the atmosphere are Carbon Monoxide (CO), Sulphur Dioxide (SO₂), Oxides of Nitrogen (NO_x), and Chlorofluorocarbons (CFCs).

What difference do you notice on the layers of deposits on the leaves of a tree near a road and a tree inside a garden?

1. Tree near a road: The leaves have a thick, dark layer of dust, soot, and unburnt carbon particles deposited on them due to continuous heavy vehicular smoke and dust.
2. Tree inside a garden: The leaves are significantly cleaner with only a very thin or negligible layer of dust because of lower vehicular emissions.

What do you understand by atmospheric pressure?

The pressure exerted by the weight of the air column present in the atmosphere on the Earth’s surface is called atmospheric pressure. At sea level, standard atmospheric pressure equals 1.013 × 10⁵ Pascal (N/m²) or a column of 76 cm of mercury.

What will happen to the human body if the air pressure of an area suddenly decreases?

The internal cellular fluids and blood vessels in the human body exert an outward pressure equal to normal atmospheric pressure. If the external atmospheric pressure drops suddenly, the higher internal body pressure forces the blood vessels to swell up or even rupture, leading to nosebleeds or internal bleeding.

Which gas has the highest percentage in air?

(a) Oxygen

(b) Nitrogen

(c) Helium

(d) Carbon dioxide

(b) Nitrogen / (b) नाइट्रोजन

Nitrogen is used in the storage of food items because:

(a) it provides oxygen to the food items

(b) it provides carbon-dioxide to the food items

(c) it provides protein to the food items

(d) it keeps the food fresh

(d) it keeps the food fresh / (d) यह भोजन को ताजा रखती है

Unit measure of atmospheric pressure is equal to the column of mercury pressure of:

(a) 76 cm

(b) 76 mm

(c) 1.013 mm

(d) 10.13 mm

(a) 76 cm / (a) 76 सेमी

Answer: (b) lighter / (b) हल्की**

The cause of heating up of our atmosphere is:

(a) Ozone layer

(b) greenhouse effect

(c) nitrogen

(d) oxygen

(b) greenhouse effect / (b) ग्रीनहाउस प्रभाव

#### Statement 1:

Sentence: The percentage of oxygen in air is 10 percent.

Evaluation: Incorrect statement.

Corrected Sentence (English): The percentage of oxygen in air is 21 percent.

Corrected Sentence (Hindi): वायु में ऑक्सीजन की मात्रा 21 प्रतिशत है।

What are the components of air?

Air is a mixture composed of gases, water vapour, and particulate matter. Its main components by volume are:

• Nitrogen (N₂): 78%
• Oxygen (O₂): 21%
• Argon (Ar): 0.9%
• Water Vapour: 0.04%
• Carbon Dioxide (CO₂): 0.03%
• Other gases & Dust particles: Sulphur dioxide, oxides of nitrogen, and suspended particulate matter.

Explain with a well-labelled diagram the preparation of oxygen.

Laboratory Method:

Oxygen gas is prepared in the laboratory by heating potassium permanganate (KMnO₄).

Procedure:

1. Take about 2 to 3 grams of potassium permanganate in a hard glass test tube and fit a cotton plug near its mouth to prevent solid particles from entering the gas collection assembly.
2. Close the test tube with a cork fitted with a glass delivery tube and mount it on a stand over a burner.
3. Lead the delivery tube into a trough filled with water under an inverted test tube filled with water.
4. Upon heating, potassium permanganate decomposes to release oxygen gas, which displaces water downwards and gets collected at the top of the inverted test tube.

Balanced Chemical Equation:

2KMnO₄ → [Heat] K_2MnO₄ + MnO₂ + O₂↑

(Potassium permanganate → Potassium manganate + Manganese dioxide + Oxygen)

Diagrammatic Description (Fig 3.1):

• Hard glass test tube: Held by a stand containing potassium permanganate and a cotton plug.
• Heating source: Burner or candle placed below the test tube.
• Delivery tube: Connects the test tube to the water trough.
• Water trough & Inverted test tube: Oxygen gas collects at the top by downward displacement of water.

What are the causes of air pollution? What measures would you adopt to stop it?

Causes of Air Pollution:

1. Vehicular Exhaust: Excessive use of petrol and diesel vehicles releases poisonous gases like carbon monoxide, nitrogen oxides, and lead due to incomplete fuel combustion.
2. Industrial Chimneys: Smoke, ash, and toxic gases (Sulphur dioxide) from steel, cement, and chemical factories pollute surrounding air.
3. Thermal Power Stations: Coal combustion in power plants generates massive amounts of smoke and fly ash.
4. Open Garbage Burning: Burning dry leaves, plastics, and agricultural waste in open fields releases hazardous pollutants.

Measures to Control Air Pollution:

1. Adopt eco-friendly, non-polluting renewable energy sources such as solar and wind energy.
2. Perform regular vehicular emission checks (PUC) and maintain engines properly.
3. Increase green cover by planting more trees (trees act as nature’s lungs).
4. Fit high-altitude factory chimneys with electrostatic precipitators and gas filters.
5. Ban open burning of leaves, paper, and solid wastes.

What are the main uses of nitrogen gas?

1. Growth of Living Organisms: Essential for synthesized proteins necessary for the growth of plants and animals.
2. Fertilizer Manufacture: Reacts with hydrogen to form ammonia, which is used to manufacture urea and other nitrogenous fertilizers.
3. Electric Bulbs: Filled in electric lamps due to its chemically inert nature to prevent filament oxidation.
4. Food Packaging: Packed into sealed bags of potato chips and fried snacks to displace oxygen and maintain food freshness.
5. Industrial Coolant: Liquid nitrogen serves as an industrial coolant and is used in medical facilities for preserving organ and blood samples for transplantation.

What is acid rain? How is it harmful for us?

Acid Rain:

Pollutant gases present in the air such as Sulphur dioxide (SO₂) and Nitrogen dioxide (NO₂) react with atmospheric moisture and rainwater to form acids like sulphurous acid, sulphuric acid, and nitric acid. When rain falls containing these dissolved acids, it is known as Acid Rain.

Balanced Chemical Equations:

1. SO₂ + H_2O → H_2SO₃ (Sulphurous acid)
2. 2SO₂ + 2H_2O + O₂ → 2H_2SO₄ (Sulphuric acid)
3. 2NO₂ + H_2O → HNO₃ + HNO₂ (Nitric acid + Nitrous acid)

Harmful Effects:

1. Flora Damage: Causes plant leaves to turn yellow and drop off prematurely.
2. Aquatic Life Extinction: Acidifies water bodies, disrupting aquatic ecosystems and causing extinction of fish species.
3. Monument Corrosion: Corrodes historical stone structures and marble monuments (e.g., discoloration and corrosion of the Taj Mahal at Agra).
4. Soil & Health Hazards: Degrades soil fertility and causes skin/respiratory diseases in human beings.

Find out how many planets in the solar system has atmosphere in it.

In our solar system, seven planets have substantial atmospheres: Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune (Mercury has only an extremely tenuous exosphere).

Will you propagate celebrating ‘no crackers Diwali’? Give reason to support your answer.

Yes, I will propagate celebrating a ‘no crackers Diwali’ because burning firecrackers releases huge amounts of poisonous gases (SO₂, CO), toxic metal particulates, and heavy smoke into the atmosphere, leading to hazardous smog and severe respiratory issues for children, elderly, and asthmatic patients.

Name the following:

a. It is used as rocket fuel.

b. An eco-friendly gas that does not emit pollution.

c. The gas responsible for greenhouse effect.

d. The gas that forms protective shield around earth.

e. Nitric acid & sulphuric acid with rain water.

a. Liquid Oxygen / Liquid Hydrogen

b. Compressed Natural Gas (CNG) / Hydrogen gas

c. Carbon Dioxide (CO₂) / Methane (CH₄)

d. Ozone gas (O₃)

e. Acid Rain