Air Exercise Solutions | KalviGavani

How to Use This Page

This page covers all exercise questions from the Samacheer Kalvi Class 8 Science chapter on Air, with detailed answers for every section. Work through the MCQs and short answers first to check your basics, then move to the detailed answers and Common Mistakes to deepen your understanding before exams.

Key Concepts (Quick Recap)

I. Choose the Best Answer (MCQs)

Q1. The percentage of nitrogen in air is approximately

• (a) 21% (b) 78% (c) 0.03% (d) 1%

Q2. Which gas supports combustion?

• (a) Nitrogen (b) Carbon dioxide (c) Oxygen (d) Argon

Q3. The gas that turns limewater milky is

• (a) Oxygen (b) Nitrogen (c) Carbon dioxide (d) Water vapour

Q4. Which of the following processes releases oxygen into the atmosphere?

• (a) Respiration (b) Photosynthesis (c) Combustion (d) Decomposition

Q5. The layer of atmosphere closest to the Earth is

• (a) Stratosphere (b) Mesosphere (c) Troposphere (d) Thermosphere

Q6. Ozone layer is found in the

• (a) Troposphere (b) Stratosphere (c) Mesosphere (d) Thermosphere

Q7. The process of converting atmospheric nitrogen into usable forms is called

• (a) Nitrification (b) Nitrogen fixation (c) Denitrification (d) Ammonification

Q8. Which of the following is NOT a stage in the nitrogen cycle?

• (a) Nitrogen fixation (b) Nitrification (c) Photosynthesis (d) Denitrification

Q9. Water vapour in the air is responsible for

• (a) Rusting of iron (b) Humidity and rainfall (c) Ozone formation (d) Nitrogen fixation

Q10. The mesosphere is the layer of the atmosphere that

• (a) Contains the ozone layer (b) Is closest to Earth (c) Burns up most meteoroids (d) Contains ions

Q11. Which bacteria convert nitrates in the soil back into nitrogen gas?

• (a) Rhizobium (b) Nitrosomonas (c) Pseudomonas (denitrifying) (d) Azotobacter

II. Fill in the Blanks

1. Air contains approximately 78% nitrogen and 21% oxygen. 2. The process of burning is called combustion. 3. The gas that turns limewater milky is carbon dioxide. 4. The layer of atmosphere where weather changes occur is the troposphere. 5. The ozone layer protects us from ultraviolet (UV) rays from the Sun. 6. Plants release oxygen during photosynthesis. 7. Animals release carbon dioxide during respiration. 8. Nitrogen-fixing bacteria in root nodules of legumes are called Rhizobium. 9. The process of conversion of ammonia into nitrites and nitrates is called nitrification. 10. The gas needed for respiration is oxygen. 11. The amount of water vapour in air varies from 0 to 4% depending on temperature and location. 12. The process by which denitrifying bacteria return nitrogen to the atmosphere is called denitrification.

III. True or False

IV. Match the Following

V. Short Answer Questions

Q1. What is the composition of air? Answer: Air is a mixture of gases, not a compound. Its composition is approximately:

• Nitrogen: 78%
• Oxygen: 21%
• Argon: 0.93%
• Carbon dioxide: 0.03%
• Other gases (neon, helium, methane, etc.): trace amounts
• Water vapour: varies (0-4%)
• Dust particles: variable

The proportions of the main gases (nitrogen and oxygen) remain fairly constant, but water vapour and dust content vary with location, temperature, and season.

Q2. How do you show that air contains water vapour? Answer: Place a clean, dry glass tumbler in a refrigerator for a few minutes. Take it out and keep it on a table. After some time, water droplets appear on the outer surface of the glass. This is because the water vapour in the surrounding air condenses on the cold surface of the glass, proving that air contains water vapour. The colder the glass, the more droplets form, showing that water vapour content varies with temperature and humidity.

Q3. How is oxygen gas tested in a laboratory? Answer: Oxygen can be tested using a glowing splinter test. Bring a glowing wooden splinter near the gas. If the splinter bursts into flame (rekindles), the gas is oxygen. This happens because oxygen supports combustion and intensifies burning. This is a simple and reliable test because no other common gas has this property of rekindling a glowing splinter.

Q4. How is carbon dioxide gas tested in a laboratory? Answer: Pass the gas through limewater (calcium hydroxide solution). If the limewater turns milky, the gas is carbon dioxide. This is because CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate: CO₂ + Ca(OH)₂ → CaCO₃↓ (milky) + H₂O If excess CO₂ is passed, the milky solution turns clear again because calcium carbonate reacts with more CO₂ and water to form soluble calcium bicarbonate.

Q5. What is the importance of the ozone layer? Answer: The ozone layer in the stratosphere absorbs most of the harmful ultraviolet (UV) radiation from the Sun. UV rays can cause skin cancer, eye damage (cataracts), and harm to plants and marine life. Without the ozone layer, life on Earth would be severely affected. Ozone depletion, caused by chlorofluorocarbons (CFCs) and other chemicals, increases UV radiation reaching the Earth's surface, making the ozone layer's preservation critical for all life forms.

Q6. What are the causes of air pollution? Answer: 1. Vehicle exhaust (carbon monoxide, nitrogen oxides, particulate matter) 2. Industrial emissions (sulphur dioxide, carbon dioxide, smoke) 3. Burning of fossil fuels (coal, petroleum) 4. Agricultural activities (crop burning, use of chemical fertilisers) 5. Deforestation (reducing CO₂ absorption) 6. Construction activities (dust and particulate matter) 7. Natural causes (volcanic eruptions, forest fires)

Air pollution is worsened in cities due to higher concentration of vehicles and industries, and it has both immediate health effects and long-term environmental consequences like global warming and acid rain.

Q7. What is the nitrogen cycle? Answer: The nitrogen cycle is the continuous process by which nitrogen is exchanged between the atmosphere, soil, and living organisms. It involves: 1. Nitrogen fixation: Converting atmospheric N₂ into usable forms (ammonia) by Rhizobium bacteria and lightning. 2. Nitrification: Converting ammonia into nitrites and then nitrates by soil bacteria. 3. Assimilation: Plants absorb nitrates from soil; animals get nitrogen by eating plants. 4. Ammonification: Decomposers break down dead organic matter into ammonia. 5. Denitrification: Denitrifying bacteria convert nitrates back into N₂ gas, returning it to the atmosphere.

The nitrogen cycle ensures a continuous supply of usable nitrogen to organisms and maintains the balance of nitrogen in nature.

VI. Detailed Answer Questions

Q1. Explain the oxygen cycle in nature. Answer: The oxygen cycle is the continuous movement of oxygen through the atmosphere, biosphere, and lithosphere. It is a biogeochemical cycle that maintains the balance of oxygen in nature.

Oxygen enters the atmosphere through: 1. Photosynthesis: Green plants, algae, and cyanobacteria release oxygen as a by-product during photosynthesis. This is the primary source of atmospheric oxygen. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

2. Photodissociation: Sunlight breaks down water vapour in the upper atmosphere into hydrogen and oxygen, contributing a small amount of oxygen.

Oxygen is consumed through: 1. Respiration: All living organisms use oxygen to break down glucose and release energy. This is the largest consumer of atmospheric oxygen. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

2. Combustion (burning): Burning of fuels (wood, coal, petroleum) consumes oxygen and releases CO₂. C + O₂ → CO₂

3. Decomposition: Decomposition of organic matter by bacteria uses oxygen.

4. Rusting: Iron reacts with oxygen and moisture to form iron oxide (rust). 4Fe + 3O₂ + 2H₂O → 2Fe₂O₃·H₂O

Balance of the oxygen cycle:

• The rate of oxygen production (mainly through photosynthesis) and consumption (mainly through respiration and combustion) must remain balanced.
• Deforestation reduces the number of plants producing oxygen, while increased burning of fossil fuels consumes more oxygen and releases CO₂.
• This imbalance contributes to global warming and threatens the natural oxygen cycle.

Q2. Describe the preparation of oxygen in the laboratory. Answer: Oxygen can be prepared in the laboratory using two common methods:

Method 1: Decomposition of potassium chlorate

• Heat potassium chlorate (KClO₃) in the presence of manganese dioxide (MnO₂) as a catalyst.
• 2KClO₃ →(MnO₂, heat)→ 2KCl + 3O₂
• Manganese dioxide lowers the decomposition temperature of potassium chlorate from about 400°C to about 250°C, making the reaction easier and safer.
• Oxygen is collected by downward displacement of water (since oxygen is only slightly soluble in water).

Method 2: Decomposition of hydrogen peroxide

• Add manganese dioxide to hydrogen peroxide (H₂O₂).
• 2H₂O₂ →(MnO₂)→ 2H₂O + O₂
• Manganese dioxide acts as a catalyst (it speeds up the reaction without being consumed).
• This method is quicker and safer as it does not require heating.

Collection method: Oxygen is collected over water in a gas jar by downward displacement of water because oxygen is only slightly soluble in water and is slightly heavier than air. The gas jar is filled with water and inverted over the delivery tube in a water trough. As oxygen is produced, it displaces the water in the jar.

Test for oxygen: A glowing wooden splinter bursts into flame when introduced into the gas jar containing oxygen. This is the standard test for oxygen gas and works because oxygen actively supports combustion.

Precautions:

• Always check that the apparatus is airtight before starting.
• Ensure the delivery tube does not dip into the reaction mixture.
• Collect the gas only after all air has been expelled from the apparatus.

Q3. Describe the preparation of carbon dioxide in the laboratory and its properties. Answer: Preparation: Carbon dioxide is prepared by the action of dilute hydrochloric acid on marble chips (calcium carbonate). CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑

Apparatus: The reaction is carried out in a Woulf's bottle. The gas is collected by upward displacement of air (since CO₂ is heavier than air, about 1.5 times the density of air). Unlike oxygen, CO₂ cannot be collected over water because it is moderately soluble in water.

Physical properties: 1. Colourless and odourless gas 2. Heavier than air (about 1.5 times the density of air) 3. Slightly soluble in water 4. Can be liquefied under high pressure and solidified into dry ice at -78°C 5. Does not support combustion (non-flammable)

Chemical properties: 1. Does not support combustion (non-flammable) — a burning candle is extinguished in CO₂ 2. Turns limewater milky: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O 3. Dissolves in water to form carbonic acid: CO₂ + H₂O → H₂CO₃ (this is why rainwater is slightly acidic) 4. Reacts with sodium hydroxide: CO₂ + 2NaOH → Na₂CO₃ + H₂O 5. With excess CO₂ in limewater, the milky solution clears: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble calcium bicarbonate)

Uses of carbon dioxide: 1. In fire extinguishers (because it does not support combustion and is heavier than air) 2. In carbonated drinks (soda water, soft drinks) 3. In photosynthesis by plants 4. As dry ice for refrigeration and cold storage 5. In the manufacture of urea and washing soda

Q4. Explain the carbon cycle in nature. Answer: The carbon cycle is the continuous exchange of carbon between the atmosphere, organisms, and the Earth. Carbon is a fundamental element found in all living organisms and is exchanged through various natural processes.

Carbon enters the atmosphere through: 1. Respiration: All living organisms release CO₂ during respiration. This is a continuous process happening in all living cells. 2. Combustion: Burning of fossil fuels (coal, petroleum, natural gas) releases CO₂. This is a major anthropogenic source of atmospheric carbon. 3. Decomposition: Breakdown of dead organisms by decomposers releases CO₂ back into the atmosphere. 4. Volcanic activity: Volcanic eruptions release CO₂ trapped within the Earth's interior.

Carbon is removed from the atmosphere through: 1. Photosynthesis: Green plants absorb CO₂ and convert it to glucose. This is the primary mechanism for removing atmospheric CO₂. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ 2. Dissolution in oceans: CO₂ dissolves in seawater to form carbonic acid, and marine organisms use it to form shells (CaCO₃). The oceans absorb roughly one-quarter of human CO₂ emissions. 3. Fossilisation: Dead organisms buried under pressure for millions of years form fossil fuels (coal, petroleum). This stores carbon underground for very long periods.

Impact of human activities:

• Deforestation reduces the number of trees absorbing CO₂ through photosynthesis, increasing atmospheric CO₂.
• Excessive burning of fossil fuels releases stored carbon back into the atmosphere far faster than it can be naturally absorbed.
• These activities are increasing atmospheric CO₂ levels, leading to global warming, climate change, and ocean acidification.
• The carbon cycle is being disrupted because the rate of CO₂ addition to the atmosphere exceeds the rate of CO₂ removal by natural processes.

Common Mistakes

✅ Correct approach: Water vapour is invisible. It becomes visible only after condensation. The droplets on a cold glass prove water vapour was in the air, but the vapour itself cannot be seen.

✅ Correct approach: Nitrogen fixation adds usable nitrogen to the soil (N₂ → NH₃), while denitrification removes it (NO₃⁻ → N₂). Think: fixation = fixing nitrogen in soil, denitrification = de-removing nitrogen from soil.

• Calling air a compound: Air is a mixture, not a compound. Its components can be separated by physical methods like fractional distillation, and its composition varies slightly from place to place.
• Confusing nitrogen fixation and nitrification: Nitrogen fixation converts N₂ to NH₃; nitrification converts NH₃ to nitrites (NO₂⁻) and then nitrates (NO₃⁻). These are two distinct steps in the nitrogen cycle performed by different bacteria.
• Saying oxygen is the most abundant gas in air: Nitrogen (78%) is the most abundant gas in air, not oxygen (21%). Oxygen is the second most abundant.
• Confusing troposphere and stratosphere: Weather occurs in the troposphere; the ozone layer is in the stratosphere. Temperature decreases with altitude in the troposphere but increases in the stratosphere due to ozone absorbing UV radiation.
• Forgetting that CO₂ turns limewater milky, not cloudy: The correct observation is that limewater turns milky due to the formation of insoluble calcium carbonate.
• Confusing oxygen and carbon dioxide tests: Oxygen rekindles a glowing splinter; carbon dioxide turns limewater milky. Do not mix up these tests.
• ❌ Confusing water vapour with water droplets: Water vapour is an invisible gas present in air; it becomes visible only when it condenses into liquid water droplets (as on a cold glass surface). Saying "water vapour is visible as clouds" is incorrect — clouds are formed from tiny liquid water droplets or ice crystals, not water vapour.
• ❌ Confusing denitrification with nitrogen fixation: Students sometimes write that denitrification adds nitrogen to the soil. In fact, denitrification removes nitrogen from the soil by converting nitrates back into N₂ gas, which escapes to the atmosphere.

Exam Checklist

• [ ] Can you state the composition of air with percentages?
• [ ] Can you describe the oxygen and carbon cycles?
• [ ] Can you describe the nitrogen cycle with all its stages?
• [ ] Can you explain the laboratory preparation and tests for oxygen and carbon dioxide?
• [ ] Can you describe the layers of the atmosphere and their features?
• [ ] Can you explain the importance of the ozone layer and the causes of ozone depletion?
• [ ] Can you list the causes and effects of air pollution?
• [ ] Can you write balanced equations for photosynthesis, respiration, and combustion?
• [ ] Can you explain how water vapour in air can be demonstrated?
• [ ] Can you distinguish between physical and chemical properties of oxygen and carbon dioxide?
• [ ] Can you differentiate between nitrogen fixation, nitrification, and denitrification?
• [ ] Can you explain why air is a mixture and not a compound, with supporting reasons?