Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Air — Exercise Solutions Guide
Use this page alongside the official Samacheer Kalvi textbook. Solve every exercise question on your own first, then check the methods and answers below.
I. Multiple Choice Questions (MCQs)
Q1. What is the percentage of nitrogen in air?
(a) 78% (b) 21% (c) 0.03% (d) 1%
Q2. Gas exchange in plants takes place using:
(a) Stomata (b) Chlorophyll (c) Leaves (d) Flowers
Q3. The air constituent that supports combustion is:
(a) Nitrogen (b) Carbon dioxide (c) Oxygen (d) Water vapour
Q4. Nitrogen is used in food packaging because it:
(a) Provides colour to food (b) Provides oxygen to food (c) Adds proteins and minerals to food (d) Keeps the food fresh
Q5. Which two gases together make up about 99% of air?
I. Nitrogen II. Carbon dioxide III. Noble gases IV. Oxygen
(a) I and II (b) I and III (c) I and II (d) I and IV
Additional MCQs
Q6. Movement of wind takes place in which layer?
(a) Troposphere (b) Ozone (c) Stratosphere (d) Ionosphere
Q7. A component of air used by green plants to prepare food is:
(a) N₂ (b) O₂ (c) He (d) CO₂
Q8. Which layer contains the ozone layer?
(a) Troposphere (b) Stratosphere (c) Mesosphere (d) Exosphere
Q9. Percentage of oxygen in exhaled air is:
(a) 78% (b) 16% (c) 4% (d) 21%
Q10. During respiration, carbon dioxide is exhaled through the:
(a) Lungs (b) Heart (c) Kidney (d) Skin
Q11. Which organism respires using its skin?
(a) Fish (b) Frogs (c) Rats (d) Human beings
Q12. Which gas cylinders are used by divers going deep into the sea?
(a) Hydrogen (b) Carbon dioxide (c) Oxygen (d) Nitrogen
II. Fill in the Blanks
1. ______ is the active component of air.
Oxygen is called the active component because it supports life (respiration) and combustion (burning), while nitrogen is the inactive component.
2. The gas released during photosynthesis is ______
During photosynthesis, plants take in carbon dioxide and water, use sunlight and chlorophyll, and produce glucose (food) along with oxygen as a by-product.
3. ______ gas is given to patients with breathing problems.
Patients with breathing difficulties are given oxygen from cylinders to help them breathe more easily and get sufficient oxygen into their blood.
4. ______ can be seen moving in a beam of sunlight in a dark room.
When a beam of sunlight enters a dark room through a small opening, tiny dust particles become visible as they scatter the light. This demonstrates that air contains suspended particles.
5. ______ gas turns lime water milky.
When carbon dioxide is passed through lime water (calcium hydroxide solution), it forms calcium carbonate, which is insoluble and makes the solution appear milky white.
Additional Fill in the Blanks
6. The atmosphere is held in place by Earth's ______
Earth's gravitational force pulls the gases of the atmosphere towards its surface, preventing them from escaping into space.
7. The atmosphere is made of ______ different layers.
The five layers of the atmosphere (from lowest to highest) are: troposphere, stratosphere, mesosphere, thermosphere (ionosphere), and exosphere.
8. A ______ shows the direction of air movement at a particular place.
A weathercock (or wind vane) is an instrument that shows the direction from which the wind is blowing.
9. The ozone layer protects life from harmful ______ rays of the sun.
The ozone layer in the stratosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation, preventing it from reaching Earth's surface.
10. The highly reactive gas was later named 'oxygen' by ______
Antoine Lavoisier named the gas "oxygen" (meaning "acid-former") after discovering its role in combustion and respiration. Joseph Priestley had earlier isolated the gas but called it "dephlogisticated air."
11. ______ discovered nitrogen.
Daniel Rutherford discovered nitrogen in 1772 by removing oxygen and carbon dioxide from air, leaving behind an inert gas.
12. Air contains small amounts of CO₂, water vapour, and gases like ______ etc.
Besides nitrogen and oxygen, air contains small quantities of noble gases such as argon, helium, neon, and krypton.
13. Air contains more ______ in the rainy season.
During the rainy season, increased evaporation from water bodies adds more water vapour to the air, raising humidity levels.
III. True or False (with Corrections)
Q1. Inhaled air contains a large amount of carbon dioxide.
Correction: Inhaled air contains a large amount of oxygen (about 21%). Exhaled air contains more carbon dioxide (about 4%) than inhaled air.
Q2. Planting trees helps decrease global warming.
Plants absorb carbon dioxide during photosynthesis, reducing the amount of CO₂ in the atmosphere. Since CO₂ is a major greenhouse gas, planting trees helps reduce global warming.
Q3. The composition of air is always exactly the same.
Correction: The composition of air changes slightly from place to place and from time to time. For example, industrial cities have more CO₂, coastal areas have more water vapour, and windy places have more dust particles.
Q4. Whales come up to the water surface to breathe in oxygen.
Whales are mammals that breathe air using lungs. They cannot extract dissolved oxygen from water like fish do with gills, so they must surface to breathe.
Q5. The oxygen balance in the atmosphere is maintained through photosynthesis in animals and respiration in plants.
Correction: The oxygen balance is maintained through photosynthesis in plants (which release oxygen) and respiration in animals (which consume oxygen). Animals cannot perform photosynthesis.
Additional True or False
Q6. Air moving cool and soothing is called a cyclone.
Correction: Cool and soothing air movement is called a breeze. A cyclone is a violent, destructive windstorm.
Q7. Exosphere has a high temperature.
Correction: The exosphere has a low temperature. Although individual gas molecules can have high kinetic energy, the overall temperature is low because there are very few molecules to transfer heat.
Q8. Jan Ingenhousz showed that chlorophyll is essential for photosynthesis.
Correction: Jan Ingenhousz showed that sunlight is essential for photosynthesis. It was later scientists who demonstrated the role of chlorophyll.
Q9. The second major component of air is oxygen.
After nitrogen (78%), oxygen (21%) is the second most abundant gas in air.
Q10. Air contains small amounts of carbon dioxide, water vapour, and gases like argon, helium.
Besides nitrogen and oxygen, air contains traces of CO₂ (0.04%), water vapour (variable), and noble gases like argon (0.93%) and helium.
Q11. The amount of water vapour is more in windy places than other areas.
Correction: Windy places have more dust particles in the air. Water vapour is higher in coastal areas and during the rainy season.
IV. Match the Following
Match the Following — Set 1
| # | Column A | Column B | Match |
|---|---|---|---|
| 1 | Moving Air | Photosynthesis | Wind |
| 2 | Layer in which we live | Troposphere | Troposphere |
| 3 | Stratosphere | Wind | Ozone layer |
| 4 | Oxygen | Ozone layer | Combustion |
| 5 | Carbon dioxide | Combustion | Photosynthesis |
Match the Following — Set 2
| # | Column A | Column B | Match |
|---|---|---|---|
| 1 | More industrial cities | (a) Dust particles | (c) Carbon dioxide |
| 2 | Coastal areas | (b) Humidity | (d) Water vapour |
| 3 | Rainy season | (c) Carbon dioxide | (b) Humidity |
| 4 | Windy places | (d) Water vapour | (a) Dust particles |
V. Arrange in Correct Sequence
Arrange the following statements logically:
1. Plants manufacture food by photosynthesis. 2. Plants require energy for growth. 3. Plants take in oxygen and release carbon dioxide, like animals. 4. Plants take carbon dioxide from air, use chlorophyll and sunlight, and prepare food. 5. Such oxygen is available to animals and humans for breathing. 6. During this process, oxygen is released by plants.
VI. Analogy
Q1. Photosynthesis : ______ :: Respiration : Oxygen
Photosynthesis uses carbon dioxide as input (and releases oxygen). Respiration uses oxygen as input (and releases carbon dioxide). The analogy pairs each process with its primary input gas.
Q2. 78% of air : Does not support combustion :: ______ : Supports combustion
Nitrogen (78%) does not support combustion. Oxygen (21%) supports combustion. The analogy pairs each gas with its percentage and property.
Additional Analogies
Q3. Cool and soothing wind : Breeze :: Uprooting wind : ______
A breeze is a gentle, pleasant wind. A cyclone is a powerful, destructive wind. The analogy pairs wind type with its classification.
Q4. Weather changes : Troposphere :: Ozone layer : ______
Weather changes occur in the troposphere. The ozone layer is found in the stratosphere. The analogy pairs a phenomenon with the atmospheric layer where it occurs.
Q5. Oxygen : Joseph Priestley :: Nitrogen : ______
Joseph Priestley discovered oxygen. Daniel Rutherford discovered nitrogen. The analogy pairs the gas with its discoverer.
Q6. Respire through skin : ______ :: Respire through gills : Fish
Fish respire through gills. Frogs respire through their skin (cutaneous respiration). The analogy pairs the organism with its respiratory organ.
VII. Observe the Figure and Answer
Q1. What happens if plants are removed from the aquarium?
Q2. What happens if the aquarium (with green plants but without fish) is kept in a dark place?
VIII. Very Short Answer Questions
Q1. What is the atmosphere? Name its five layers.
1. Troposphere 2. Stratosphere 3. Mesosphere 4. Thermosphere (Ionosphere) 5. Exosphere
Q2. How do roots of land plants get oxygen for breathing?
Q3. What should be done if a person's clothes catch fire? Why?
- STOP — Don't run; running fans the flames and supplies more oxygen.
- DROP — Drop to the ground and cover the face with hands.
- ROLL — Roll on the ground to cut off the oxygen supply to the flames.
- COOL — Cool the burns with water for first and second-degree burns.
Running increases air flow and adds more oxygen, making the fire worse. Rolling smothers the fire by cutting off its oxygen supply.
Q4. What happens if you breathe through your mouth?
IX. Short Answer Questions
Q1. Why do biscuits lose crispness when kept open during monsoon days?
Q2. Why do traffic assistants wear masks on duty?
Additional Short Answer Questions
Q3. Define atmosphere.
Q4. Why does air become less dense at higher altitudes?
Q5. Define troposphere.
Q6. Why do aircraft fly above the troposphere?
Q7. How can you show the presence of carbon dioxide in air?
Q8. Write the composition of air.
| Component | Percentage |
|---|---|
| Nitrogen | 78% |
| Oxygen | 21% |
| Carbon dioxide | 0.04% |
| Argon and other noble gases | ~0.93% |
| Water vapour | Variable |
| Dust particles | Variable |
Nitrogen is the most abundant gas. Oxygen is the second most abundant and the active component that supports life and combustion.
Q9. Give examples showing air composition varies by place.
1. Industrial cities — have higher carbon dioxide levels due to factory emissions. 2. Coastal areas — have more water vapour in the air compared to inland areas. 3. Rainy season — the air contains more water vapour, increasing humidity. 4. Windy places — have more dust particles suspended in the air.
This variation shows that air is not a compound with a fixed composition but a mixture whose proportions can change.
Q10. What is meant by burning?
Q11. Why do rockets carry oxygen along with fuel?
Q12. Write the equation of photosynthesis.
Carbon dioxide + Water → (Sunlight + Chlorophyll) → Glucose (Food) + Oxygen
Plants take in CO₂ from air and water from soil, use sunlight energy captured by chlorophyll, and produce glucose as food while releasing oxygen as a by-product.
Q13. What is dry ice and what is its use?
Q14. What are the uses of windmills?
1. Draw water by running pumps 2. Run flour mills for grinding grain 3. Generate electricity (wind farms)
Q15. Give the components of air with percentages.
| Component | Percentage |
|---|---|
| Nitrogen | 78% |
| Oxygen | 21% |
| Carbon dioxide, Argon, Water vapour and other gases | 1% |
X. Detailed / Long Answer Questions
Q1. How do plants and animals maintain the balance of oxygen and carbon dioxide in the air?
The oxygen-carbon dioxide balance in the atmosphere is maintained through the interdependent processes of photosynthesis and respiration:
Respiration in plants: Plants take in oxygen and release carbon dioxide, similar to animals. This gaseous exchange occurs through stomata on the leaves. Respiration happens continuously, day and night.
Photosynthesis in plants: During the day, plants take in carbon dioxide and water, use sunlight energy captured by chlorophyll, and produce glucose (food). Oxygen is released as a by-product through stomata. The oxygen released during photosynthesis exceeds the oxygen consumed during respiration.
Respiration in animals: Animals inhale oxygen, which reacts with digested food in cells to release energy. Carbon dioxide is produced as a waste product and exhaled. Exhaled air contains more carbon dioxide (about 4%) than inhaled air (about 0.04%).
The balance: Plants release more oxygen during photosynthesis; animals release more carbon dioxide during respiration. Plants consume carbon dioxide for photosynthesis, while animals consume oxygen for respiration. This mutual dependence between plants and animals maintains the balance of oxygen and carbon dioxide in the atmosphere.
This is why deforestation is dangerous — removing trees disrupts this balance, leading to increased CO₂ levels and global warming.
Q2. Why is the atmosphere essential for life on Earth?
The atmosphere is a huge envelope of air extending over 800 km above the Earth, held by gravity. It is essential for life for the following reasons:
1. Protection from harmful rays: The atmosphere shields us from harmful solar radiation. The stratosphere contains the ozone layer, which blocks dangerous ultraviolet (UV) rays from reaching the Earth's surface. Without it, life would be exposed to lethal radiation.
2. Breathing and respiration: Air provides oxygen — the gas essential for respiration in all living organisms. Humans, animals, and even plant roots need oxygen to survive.
3. Fuel combustion: Air supports the burning of fuels like wood, coal, kerosene, and LPG, which are needed for cooking, heating, and industry. Oxygen in air is essential for combustion.
4. Weather and climate: The troposphere (up to ~16 km) contains wind and water vapour needed for cloud formation, rain, and weather patterns. Without the atmosphere, there would be no rain or weather cycles.
5. Water cycle: Air plays an important role in maintaining the water cycle in nature — evaporation, condensation, and precipitation all depend on atmospheric processes.
6. Temperature regulation: The atmosphere traps some heat (greenhouse effect), keeping Earth warm enough for life. Without it, temperatures would drop to extreme lows at night.
7. Density variation: The atmosphere is denser near the surface due to gravity, providing enough air pressure for breathing. Density decreases with altitude.
Additional Detailed Answers
Q3. Prove by experiment that oxygen is released during photosynthesis.
Experiment:
1. Place a healthy Hydrilla (aquatic plant) branch in a funnel. 2. Invert the funnel in a beaker filled with water. 3. Invert a test tube over the funnel stem, keeping it filled with water and immersed. 4. Leave the setup in bright sunlight for a few hours.
Observation: Bubbles of gas rise from the plant and collect in the test tube, displacing the water.
Conclusion: When a glowing splinter is inserted into the collected gas, it burns brightly. This confirms the gas is oxygen, proving that oxygen is released during photosynthesis.
Q4. Prove by experiment that oxygen is necessary for burning.
Experiment:
1. Place two identical candles on a table. 2. Light both candles. 3. Cover one candle with an inverted glass tumbler.
Observation: The uncovered candle continues burning because it has a constant supply of fresh oxygen. The covered candle initially burns using the limited oxygen inside the tumbler, glows briefly, and then extinguishes once all the oxygen inside is consumed.
Conclusion: This demonstrates that oxygen is essential for combustion. When oxygen is cut off, burning stops.
Q5. Compare inhaled and exhaled air composition.
| Component | Inhaled Air | Exhaled Air |
|---|---|---|
| Nitrogen | 78% | 78% |
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
| Water vapour | Variable | Increases |
| Noble gases | 0.95% | 0.95% |
| Dust particles | Variable | None |
| Temperature | Room temperature | Body temperature (37°C) |
The key differences: exhaled air has less oxygen (16% vs 21%), much more CO₂ (4% vs 0.04%), more water vapour, no dust, and is at body temperature.
Q6. Write the uses of air.
1. Breathing: Air is used by all plants and animals for respiration. Oxygen in air is essential for life.
2. Burning fuels: Air supports the combustion of fuels like wood, coal, kerosene, and LPG used in cooking and industry.
3. Tyre inflation: Compressed air fills the tyres of cars, bicycles, and other vehicles, providing cushioning and support.
4. Ozone protection: The ozone layer in the atmosphere prevents harmful solar UV radiation from reaching Earth's surface.
5. Medical use: Oxygen cylinders are provided to patients with breathing difficulties, mountaineers at high altitudes, and deep-sea divers.
6. Wind energy: Wind turns windmill blades, which are used to pump water, run flour mills, and generate electricity.
7. Weather and water cycle: Air plays an important role in cloud formation, rain, and the water cycle in nature.
XI. Higher Order Thinking Skills (HOTS)
Q1. Why do fire extinguishers release a stream of carbon dioxide to put out fires?
Fire extinguishers use CO₂ for three key reasons:
1. Non-reactive and safe: CO₂ is colourless, odourless, and does not react with burning materials or create toxic by-products. It is safe to use on most types of fires.
2. Non-conductive: CO₂ does not conduct electricity, making it ideal for putting out electrical fires where water would be dangerous.
3. Dual action — cooling and smothering: When released, the pressurised CO₂ expands rapidly, creating a cooling effect (visible as mist or ice crystals). The gas also displaces oxygen around the fire, cutting off the oxygen supply needed for combustion. Without oxygen, the fire is extinguished.
Common Mistakes to Avoid
1. Saying "air is only oxygen" — Air is a mixture of nitrogen (78%), oxygen (21%), CO₂, water vapour, noble gases, and dust. Oxygen is just one component, though it is the most active one.
2. Confusing photosynthesis and respiration inputs/outputs — Photosynthesis takes in CO₂ and releases O₂. Respiration takes in O₂ and releases CO₂. Mixing these up is a very common exam error.
3. Writing "exhaled air has no oxygen" — Exhaled air still contains about 16% oxygen (down from 21%). The body does not use up all the oxygen in one breath. Only the difference is consumed.
4. Saying "air composition is always the same everywhere" — Air composition varies by place and season. Industrial areas have more CO₂, coastal areas have more water vapour, and windy places have more dust. The percentages (78% N₂, 21% O₂) are approximate averages.
5. Confusing troposphere and stratosphere functions — The troposphere is where weather occurs and we live; the stratosphere contains the ozone layer. Do not write "weather happens in the stratosphere."
6. Writing "cold air contains more oxygen" — Cold air is denser but the percentage of oxygen remains the same (~21%). Density changes, not composition.
7. Confusing breeze and cyclone — A breeze is a gentle, pleasant movement of air. A cyclone is a violent, destructive windstorm. They are opposite in nature, not similar.
Exam Checklist
Before your exam, make sure you can answer yes to every item below:
- [ ] I can state the composition of air with approximate percentages (N₂ 78%, O₂ 21%, CO₂ 0.04%).
- [ ] I can explain why air is called a mixture (components can be separated by physical methods, each retains its properties).
- [ ] I can name all five layers of the atmosphere in order and state what happens in each.
- [ ] I can distinguish between the troposphere and stratosphere (weather vs ozone layer).
- [ ] I can define photosynthesis and write its word equation.
- [ ] I can define respiration and explain how it differs from photosynthesis.
- [ ] I can explain the oxygen-carbon dioxide balance between plants and animals.
- [ ] I can compare inhaled and exhaled air in a table (oxygen, CO₂, water vapour, temperature).
- [ ] I can describe the experiment showing oxygen is released during photosynthesis (Hydrilla experiment).
- [ ] I can describe the experiment showing oxygen is necessary for burning (candle under a glass tumbler).
- [ ] I can explain why nitrogen is used in food packaging (inert gas, prevents spoilage).
- [ ] I can define dry ice, state its temperature (−57°C), and explain its use.
- [ ] I can list uses of air (breathing, burning, tyre inflation, ozone protection, medical oxygen, wind energy, water cycle).
- [ ] I can explain why biscuits lose crispness in monsoon (higher humidity, moisture absorption).
- [ ] I can explain the STOP-DROP-ROLL method for clothes catching fire.
- [ ] I can state who discovered oxygen (Joseph Priestley) and nitrogen (Daniel Rutherford).
- [ ] I can explain why rockets carry their own oxygen (no atmospheric oxygen at high altitudes/in space).
- [ ] I can give examples of how air composition varies by place (industrial, coastal, rainy, windy).
- [ ] I can explain why CO₂ fire extinguishers are effective (non-reactive, non-conductive, displaces oxygen, cooling effect).