Worksheet Solutions: Air and Atmosphere | Chemistry Class 7 ICSE PDF Download

Part A: Multiple Choice Questions 

Q1. Which gas makes up the largest proportion of air by volume?
a) Oxygen
b) Carbon dioxide
c) Nitrogen
d) Argon

Answer: c) Nitrogen
Explanation: About 78% of air is nitrogen, which is the highest. Oxygen is only 21%, and other gases are present in much smaller amounts.

Q2. Why is oxygen called the “breath of life”?
a) Because it is the lightest gas in air
b) Because it supports burning and respiration
c) Because it is used in making proteins
d) Because it is heavier than air

Answer: b) Because it supports burning and respiration
Explanation: Oxygen is essential for respiration in living beings and for combustion of fuels.

Q3. Which of the following proves that air is a mixture and not a compound?
a) Air has weight
b) Air contains dust particles
c) Air composition can vary from place to place
d) Air can dissolve in water

Answer: c) Air composition can vary from place to place
Explanation: A compound has a fixed composition, but air’s composition changes with location and time (e.g., more water vapour in rainy season). This makes it a mixture.

Q4. Which scientist discovered oxygen as a component of air?
a) Rutherford
b) Lavoisier
c) Black
d) Van Helmont

Answer: b) Lavoisier
Explanation: In 1776, Antoine Lavoisier proved that air contains both oxygen and nitrogen in about a 1:4 ratio.

Q5. Which inert gas is filled in bulbs to prevent filament from burning?
a) Helium
b) Neon
c) Argon
d) Xenon

Answer: c) Argon
Explanation: Argon, an inert gas, prevents the hot tungsten filament from reacting with oxygen, increasing bulb life.

Part B: Short Answer Questions 

Q6. Why is air considered matter?
Answer (Stepwise):

1. Matter has mass and occupies space.

2. When we fill air in a balloon or football, it becomes heavier.

3. This proves that air has weight and volume, so it is matter.

Q7. State two differences between air as a mixture and a compound.
Answer (Stepwise):

1. In mixture (air), gases are present in variable ratios; in compound, ratio is fixed.

2. In air, gases retain their properties (e.g., oxygen supports burning); in a compound, new properties appear.

Q8. Why is nitrogen important for plants?
Answer (Stepwise):

1. Plants need nitrogen to make proteins and amino acids, essential for growth.

2. Nitrogen from the air is fixed into nitrates by bacteria or lightning and absorbed by plants.

Q9. Why is oxygen collected by downward displacement of water in laboratories?
Answer (Stepwise):

1. Oxygen is only slightly soluble in water.

2. It is heavier than air, so it cannot be collected over air.

3. Hence, oxygen is collected over water without much loss.

Q10. How does carbon dioxide help in extinguishing fire?
Answer (Stepwise):

1. Carbon dioxide is heavier than air and forms a blanket over the fire.

2. It cuts off oxygen supply, which is necessary for burning.

3. Thus, the fire goes out.

Part C: Long Answer Questions 

Q11. Explain with steps why air is called a mixture and not a compound.
Answer (Stepwise):

1. Variable composition: Oxygen is 21%, nitrogen 78%, CO₂ and others vary with place and time.

2. No energy change: Mixing gases does not release or absorb heat.

3. Separate identities: Each gas retains its own property (oxygen supports burning, nitrogen does not).

4. Physical separation possible: Components can be separated by methods like liquefaction and fractional distillation.
   → Therefore, air is a mixture, not a compound.

Q12. What are inert gases? Mention their properties and uses.
Answer (Stepwise):

1. Definition: Inert gases are gases present in air in very small amounts (like helium, neon, argon, krypton, xenon).

2. Properties: They are chemically inactive, do not react with other substances easily.

3. Uses:

   • Argon is used in electric bulbs.

   • Helium is used in balloons.

   • Neon is used in signboards.

Explanation:
Inert gases help in practical applications but remain unchanged in chemical reactions, which is why they are special.

Q13. Explain the oxygen cycle in nature.
Answer (Stepwise):

1. Use of oxygen: Animals use oxygen for respiration, and fuels burn in oxygen.

2. Release of CO₂: Both respiration and combustion release carbon dioxide.

3. Photosynthesis: Plants take CO₂ and release oxygen in sunlight.

4. Balance: This continuous exchange keeps the proportion of oxygen and carbon dioxide in the atmosphere stable.