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8th Science Guide Unit 11 Air

8th Science Guide Unit 11 Air

8th Std Science Guide Lesson  11 Air | English Medium

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8th Science Guide Unit 11 Air

I. Choose the correct answers:

1. Which of the following is true about oxygen?

  1. Completely burning gas
  2. Partially burning gas
  3. Doesn’t support burning
  4. Supports burning

Ans : Supports burning

2. Aerated water contains

  1. air
  2. oxygen
  3. carbon dioxide
  4. nitrogen

Ans : carbon dioxide

3. Solvay process is a method to manufacture

  1. lime water
  2. aerated water
  3. distilled water
  4. sodium carbonate

Ans : sodium carbonate

4. Carbon dioxide with water changes

  1. blue litmus to red
  2. red litmus to blue
  3. blue litmus to yellow
  4. doesn’t react with litmus

Ans : blue litmus to red

5. Which of the following is known as azote?

  1. Oxygen
  2. Nitrogen
  3. Sulpher
  4. Carbon dioxide

Ans : Nitrogen

II. Fill in the blanks.

  1. ________ is called as vital life Ans : oxygen
  2. Nitrogen is ___________ than air. Ans : lighter
  3. ________ is used as a fertilizer Ans : nitrogen
  4. Dry ice is used as a ___________. Ans : refrigerant
  5. The process of conversion of iron into hydrated form of oxides is called ___________.

Ans : rusting

III.Match the following

  1. Nitrogen – Respiration in living animals
  2. Oxygen – Fertilizer
  3. Carbon dioxide – Refrigerator
  4. Dry ice – Fire extinguisher

Ans : 1 – b, 2 – a, 3 – d, 4 – c

IV. Answer briefly.

1. What are the sources of oxygen?.

i) Atmospheric air

Oxygen in free state

ii) water


iii) plants and animals

Oxygenin combined state

iv) Minerals in the form of silicates, carbonates and oxides

2. Mention the physical properties of oxygen.

  • Oxygen is a colourless, odourless and tasteless gas.
  • It is a poor conductor of heat and electricity
  • Oxygen dissolves readily in cold water
  • It supports combustion.

3. List out the uses of nitrogen.

  • Liquid nitrogen is used as a refrigerant.
  • It provides an inert atmosphere for conducting certain chemical reactions.
  • It is used for inflating tyres of vehicles.

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

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

on-metal + Nitrogen



Nitrogen compound



3H2       +       N2

Hydrogen      Niterogen






5. What is global warming?

  • Global warming refers to an average increase in the temperature of the atmosphere or simply it is the warming of the earth.

6. What is dry Ice? What are its uses?

  • Solid carbon dioxide is called dry ice
  • Dry ice is used as a refrigerant.

V. Answer In detall.

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

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   +   H2O

Calclum carbonate

When an excess amount of CO, 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.

  1. a) Carbon b) Sulphur c) Phosphorous d) Magnesium e) Iron f) Sodium

Substances burn in oxygen

Compounds produced

a) Carbon

Carbon dioxide (CO2)

b) Sulphur

Sulphur dioxide(SO2)

c) Phosphorous

Phosphorous trioxide (P2O3) or Phosphorous pentoxide (P2O5)

d) Magnesium

Magnesium oxide (MgO)

e) lron

Iron oxide (Fe3O4)

f) Sodium

Sodium oxide (Na2O)

3. How does carbon dioxide react with the following?

  1. a) Potasslum:

When carbon dioxide react with potassium it gives potassium carbonate.

4K   +   3CO2




2K2CO3  +  C

Potasslum carbonate

  1. b) Lime water:

When alimited amounted of CO, is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.

Ca(OH)2  +   CO2




CaCO3  + H2O

Calcium carbonate

  1. c) Sodium hydroxide

Sodium hydroxide is neutralized by carbon dioxide to form sodium carbonate and water

2NaOH +   CO2




Na2CO3 + H2O

Sodium carbonate


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

Effects of Acid rain:

  • It irritates eyes and skin of human beings.
  • It inhibits germination and growth of seedlings.
  • It changes the fertility of the soil, destroys plants and aquatic life.
  • It causes corrosion of many buildings, bridges, etc.

Preventive measures :

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

  • Minimizing the usage of fossil fuel such as petrol, diesel etc.,
  • Using CNG (Compressed Natural Gas).
  • Using non-conventional source of energy.
  • Proper disposal of the industrial wastes.

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