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Chapter: 8th Science : Chapter 11 : Air

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8th Science : Chapter 11 : Air : Text Book Back Questions Answers, Solution



I. Choose the best answer.


1. Which of the following is true about oxygen?

a) Completely burning gas

b) Partially burning gas

c) Doesn’t support burning

d) Supports burning

[Answer: (d) Supports burning]


2. Aerated water contains

a) air

b) oxygen

c) carbon dioxide

d) nitrogen

[Answer: (c) carbon dioxide]


3. Solvay process is a method to manufacture

a) lime water

b) aerated water

c) distilled water

d) sodium carbonate

 [Answer: (d) sodium carbonate]


4. Carbon dioxide with water changes

a) blue litmus to red

b) red litmus to blue

c) blue litmus to yellow

d) doesn’t react with litmus

 [Answer: (a) blue litmus to red]


5. Which of the following is known as azote?

a) Oxygen

b) Nitrogen

c) Sulpher

d) Carbon dioxide

[Answer: (d) Carbon dioxide]


II. Fill in the blanks.


1. Oxygen is called as vital life.

2. Nitrogen is lighter than air.

3. Nitrogen is used as a fertilizer.

4. Dry ice is used as a refrigerant.

5. The process of conversion of iron into hydrated form of oxides is called rusting.


III. Match the following.


1. Nitrogen - Respiration in living animals

2. Oxygen - Fertilizer

3. Carbon dioxide - Refrigerator

 4. Dry ice - Fire extinguisher


1. Nitrogen - Fertilizer

2. Oxygen - Respiration in living animals

3. Carbon‘dioxide - Fire extinguisher

4. Dry ice - Refrigerator


IV. Answer briefly.


1. Mention the physical properties of oxygen.

Answer:  Physical properties of oxygen.

(i) Oxygen is a colourless, odourless and tasteless gas.

(ii) It is a poor conductor of heat and electricity

(iii) Oxygen dissolves readily in cold water.

(iv) It is denser than air.

(v) It can be made into liquid (liquified) at high pressure and low temperature.

(vi) It supports combustion.

2. List out the uses of nitrogen.

Answer: Uses of nitrogen:

(i) Liquid nitrogen is used as a refrigerant.

(ii) It provides an inert atmosphere for conducting certain chemical reactions.

(iii) It is used to prepare ammonia (by Haber’s process) which is then converted into fertilizers and nitric acid.

(iv) It is used for inflating tyres of vehicles.

3. Write about the reaction of nitrogen with non metals.

Answer: Nitrogen reacts with non-metals like hydrogen, oxygen etc., at high temperature to form their corresponding nitrogen compounds.

Non-metal + Nitrogen  —___ Nitrogen compound

Example :

3H2 + N2___ 2NH3

Hydrogen + Nitrogen  —___Ammonia

4. What is global warming?

Answer: The increased green house effect is caused due to increase in the air pollutants and it results in the average increase of temperature of the atmosphere. This is called as Global warming.

5. What is dry ice? What are its uses?


(i) Solid carbon dioxide, called as dry ice is used as a refrigerant.

(ii) The gas is so cold that moisture in the air condenses on it, creating a dense fog which is used in stage shows and movie effects.


V. Answer in detail.


1. What happens when carbon dioxide is passed through lime water? Write the equation for this reaction.

Answer: When a limited amount of CO2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.

Ca(OH)2+ CO2 → CaCO3 (Calcium carbonate) + H2O

When an excess amount of CO2 is passed through lime water, it first turns milky and the milkyness disappears due to the formation of soluble calcium hydrogen carbonate, Ca(HCO3)2.

2. Name the compounds produced when the following substances burn in oxygen.

a) Carbon

b) Sulphur

c) Phosphorous

d) Magnesium

e) Iron

f) Sodium


(a) Carbon dioxide (CO2)

(b) Sulphur dioxide (SO2)

(c) Phosphorus trioxide (P2O3 ) (or) Phosphorus pentoxide (P2O5)

(d) Magnesium Oxide (MgO)

(e) Iron Oxide (Fe2O3)

(f) Sodium Oxide (Na2O)

3. How does carbon dioxide react with the following?

a) Potassium

b) Lime water

c) Sodium hydroxide

(a) Potassium

Lighter metals like sodium, potassium and calcium, combine with CO2 to form corresponding carbonates whereas magnesium gives its oxide and carbon.

(b) When a limited amount of CO2 is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.

Ca (OH)2 + CO2 → CaCO3 (Calcium carbonate) + H2O

(c) Sodium hydroxide (base) is neutralized by carbon dioxide (acidic) to form sodium carbonate (salt) and water.

Base + Acid → Salt + Water

2NaOH + CO2 → Na2CO3 (Sodium carbonate) + H2O

4. What are the effects of acid rain? How can we prevent them?

Answer: Acid rain affects us in many ways. Some of the consequences are given below.

(i) It irritates eyes and skin of human beings.

(ii) It inhibits germination and growth of seedlings.

(iii) It changes the fertility of the soil, destroys plants and aquatic life.

(iv) It causes corrosion of many buildings, bridges, etc.

Preventive measures :

Acid rain and its effects can be controlled by the following ways.

(i) Minimizing the usage of fossil fuel such as petrol, diesel etc.

(ii) Using CNG (Compressed Natural Gas).

(iii) Using non-conventional source of energy.

(iv) Proper disposal of the industrial wastes.


VI. Higher Order Thinking Questions.


1. Soda bottle bursts sometimes when it is opened during summer. Why?

(i) In soda bottle carbon dioxide gas is dissolved in water under pressure.

(ii) The gas in the bottle expands during summer.

(iii) Hence, the pressure inside the bottle increases.

(iv) Thus the bottle may burst in hot summer.

2. It is said that sleeping beneath the tree during night is not good for health. What is the reason?

Answer: During night trees absorb oxygen and release carbon dioxide. Therefore, anyone who sleeps under, tree, will not get oxygen, which can cause breathing problems, suffocation etc.

3. Why does the fish die when it is taken out of water?


(i) Gills are richly supplied with blood capillaries and can readily absorb the oxygen dissolved in water.

(ii) When fishes are taken out of water, the supply of oxygen to the fishes is cut as the fishes cannot absorb and breathe using the oxygen present in the atmosphere.

(iii) Hence they die, when it is taken out of water.

4. How do astronauts breathe when they go beyond earth’s atmosphere?


(i) Astronauts cannot breathe in space unless they carry their own oxygen with them.

(ii) They can make their own oxygen by using energy from the solar arrays to split hydrogen and oxygen from water.

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