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 by volume?
(a) 21% (b) 78% (c) 0.04% (d) 1%
Q2. The gas that supports combustion is:
(a) Nitrogen (b) Carbon dioxide (c) Oxygen (d) Argon
Q3. Nitrogen in air helps to:
(a) Support burning (b) Control the rate of burning (c) Increase combustion (d) Produce fire
Q4. Carbon dioxide is used in fire extinguishers because it:
(a) Supports combustion (b) Is lighter than air (c) Displaces oxygen and does not support burning (d) Is a noble gas
Q5. Rusting of iron requires:
(a) Only oxygen (b) Only water (c) Both oxygen and moisture (d) Only carbon dioxide
Q6. The ozone layer protects us from:
(a) Infrared rays (b) Visible light (c) Ultraviolet rays (d) X-rays
Q7. During photosynthesis, plants take in:
(a) Oxygen (b) Nitrogen (c) Carbon dioxide (d) Hydrogen
Q8. During respiration, living organisms take in:
(a) Carbon dioxide (b) Nitrogen (c) Oxygen (d) Argon
Q9. Which of the following is an inert (noble) gas found in air?
(a) Oxygen (b) Carbon dioxide (c) Argon (d) Hydrogen
Q10. Air pressure decreases with:
(a) Increase in altitude (b) Decrease in altitude (c) Increase in temperature at sea level (d) Increase in humidity
II. Fill in the Blanks
1. Air is a ______ of gases.
Air is a mixture, not a compound. Its components — nitrogen, oxygen, carbon dioxide, water vapour, and noble gases — can be separated by physical methods, and each retains its own properties.
2. The percentage of nitrogen in air by volume is ______
Nitrogen is the most abundant gas in air, making up approximately 78% by volume. Oxygen comes next at 21%.
3. ______ gas supports burning.
Oxygen is the active component of air that supports combustion. A substance burns only in the presence of oxygen.
4. Carbon dioxide turns lime water ______
When CO₂ is passed through lime water (calcium hydroxide solution), it forms insoluble calcium carbonate, which makes the solution appear milky white. This is a standard test for carbon dioxide.
5. The ozone layer is found in the ______
The ozone layer is located in the stratosphere, approximately 15–35 km above Earth's surface. It absorbs harmful UV radiation from the Sun.
6. An example of an inert gas found in air is ______
Argon is the most abundant noble gas in air (~0.93%). Other inert gases include helium, neon, and krypton.
7. Air pressure ______ with increase in altitude.
Gravity pulls most air molecules close to Earth's surface. As altitude increases, fewer molecules are present, reducing density and pressure. This is why mountaineers need oxygen cylinders.
8. The gas released during photosynthesis is ______
During photosynthesis, plants take in CO₂ and water, use sunlight and chlorophyll, and produce glucose. Oxygen is released as a by-product through stomata.
9. Solid carbon dioxide is called ______
When CO₂ is cooled to −78°C, it changes directly from gas to solid without becoming liquid (sublimation). This solid CO₂ is called dry ice and is used for refrigerating perishable items.
10. The component of air that varies most from place to place is ______
Water vapour content changes with location (coastal areas have more) and season (rainy season has more). The other gases remain relatively constant in proportion.
III. True or False (with Corrections)
Q1. Air is a single compound.
Correction: Air is a mixture of gases — nitrogen, oxygen, carbon dioxide, water vapour, noble gases, and dust particles. Its components can be separated by physical methods and each retains its own properties, which is not possible for a compound.
Q2. Oxygen is the most abundant gas in air.
Correction: Nitrogen is the most abundant gas in air at approximately 78%. Oxygen is the second most abundant at 21%.
Q3. Plants use carbon dioxide for photosynthesis.
Green plants take in CO₂ from air through stomata, combine it with water from soil, use sunlight energy captured by chlorophyll, and produce glucose (food). Oxygen is released as a by-product.
Q4. Nitrogen helps control the rate of combustion.
Nitrogen dilutes oxygen in the air. If air were pure oxygen, even a small spark would cause violent, uncontrollable combustion. Nitrogen acts as a moderator, slowing down the rate of burning.
Q5. The ozone layer is found in the troposphere.
Correction: The ozone layer is found in the stratosphere, not the troposphere. The troposphere is the layer where weather occurs and where we live. The stratosphere lies above it and contains the protective ozone layer.
Q6. Carbon dioxide is heavier than air.
CO₂ has a molecular weight of 44, while the average molecular weight of air is about 29. Being heavier, CO₂ tends to settle near the ground, which is why it can form a blanket over a fire and displace oxygen — making it effective in fire extinguishers.
Q7. Dry ice is solid carbon dioxide.
Dry ice is the solid form of carbon dioxide (CO₂), formed at −78°C. It sublimes directly from solid to gas without passing through the liquid state, leaving no residue — which is why it is called "dry" ice.
Q8. Exhaled air contains no oxygen.
Correction: Exhaled air still contains about 16% oxygen (down from 21% in inhaled air). The body does not consume all the oxygen in a single breath. Only about 5% of the oxygen in each breath is absorbed; the rest is exhaled.
Q9. The composition of air is exactly the same everywhere.
Correction: Air composition varies slightly by place and season. Industrial areas have higher CO₂ levels. Coastal regions have more water vapour. Windy places carry more dust particles. The standard percentages (N₂ 78%, O₂ 21%) are averages.
Q10. Rusting requires both oxygen and moisture.
For iron to rust, both oxygen and water (moisture) must be present. Iron in dry air does not rust, and iron in oxygen-free water does not rust. This can be demonstrated using three test tubes with different conditions.
IV. Match the Following
Match the Following — Set 1
| # | Column A | Column B | Match |
|---|---|---|---|
| 1 | Nitrogen | 21% | 78% |
| 2 | Oxygen | Noble gas | 21% |
| 3 | Carbon dioxide | 78% | 0.04% |
| 4 | Ozone | 0.04% | Stratosphere |
| 5 | Argon | Stratosphere | Noble gas |
Match the Following — Set 2
| # | Column A | Column B | Match |
|---|---|---|---|
| 1 | Rusting | Combustion | Iron + Oxygen + Moisture |
| 2 | Photosynthesis | Iron + Oxygen + Moisture | CO₂ → Glucose + O₂ |
| 3 | Respiration | CO₂ → Glucose + O₂ | O₂ → Energy + CO₂ |
| 4 | Nitrogen in food packaging | O₂ → Energy + CO₂ | Prevents spoilage |
| 5 | CO₂ in fire extinguisher | Prevents spoilage | Displaces oxygen |
V. Very Short Answers
Q1. What is air?
Q2. Which is the most abundant gas in air?
Q3. Which gas supports combustion?
Q4. What is dry ice?
Q5. What is the ozone layer?
Q6. Which gas is used in food packaging and why?
Q7. Which component of air varies most from place to place?
Q8. Who discovered oxygen?
VI. Short Answers
Q1. Why is air called a mixture and not a compound?
1. Its components (nitrogen, oxygen, CO₂, water vapour, noble gases) can be separated by physical methods like fractional distillation of liquid air. 2. Each component retains its own properties — oxygen still supports combustion, nitrogen still does not. 3. The proportions of air can vary slightly (e.g., more water vapour near the coast, more CO₂ in industrial areas). 4. No chemical reaction occurs when the components of air mix.
These properties are characteristic of mixtures, not compounds.
Q2. What are the uses of nitrogen?
1. Food packaging: Nitrogen is filled in food packets (like chips) to displace oxygen and prevent spoilage, keeping food fresh. 2. Fertilizer manufacturing: Nitrogen is a key ingredient in making fertilizers like ammonium nitrate and urea, which are essential for agriculture. 3. Controlling combustion: Nitrogen dilutes oxygen in the air and prevents things from burning too fast. 4. Cryogenics: Liquid nitrogen is used for rapid freezing in medical and industrial applications. 5. Inert atmosphere: Nitrogen provides an oxygen-free environment in industries where oxidation must be avoided.
Q3. How do photosynthesis and respiration differ in terms of gas exchange?
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Gas taken in | Carbon dioxide (CO₂) | Oxygen (O₂) |
| Gas released | Oxygen (O₂) | Carbon dioxide (CO₂) |
| When it occurs | During the day (needs sunlight) | Continuously, day and night |
| Where it occurs | In plants (chloroplasts) | In all living organisms (mitochondria) |
| Energy | Absorbs solar energy | Releases energy from food |
Q4. Why do biscuits lose crispness during the monsoon?
Q5. How does carbon dioxide extinguish fire?
1. Displaces oxygen: CO₂ is heavier than air. When released, it forms a blanket over the fire, cutting off the oxygen supply that is essential for combustion. 2. Non-reactive: CO₂ does not support combustion itself and does not react with burning materials, so it does not fuel the fire. 3. Cooling effect: When pressurised CO₂ expands, it cools the surrounding area, further helping to lower the fire's temperature.
These properties make CO₂ ideal for use in fire extinguishers, especially for electrical fires where water would be dangerous.
Q6. Why do mountaineers carry oxygen cylinders?
VII. Detailed Answers
Q1. Describe the composition of air with percentages and the significance of each component.
Air is a mixture of the following components:
| Component | Percentage (by volume) | Significance |
|---|---|---|
| Nitrogen (N₂) | 78% | Dilutes oxygen and controls combustion rate; used in food packaging and fertilizer manufacturing |
| Oxygen (O₂) | 21% | Supports respiration in all living organisms; supports combustion (burning); essential for rusting |
| Carbon dioxide (CO₂) | 0.04% | Used by plants for photosynthesis; turns lime water milky (test for CO₂); used in fire extinguishers; greenhouse gas |
| Noble gases (Argon, Helium, Neon, etc.) | ~0.93% | Chemically inert; argon is used in welding and light bulbs; helium is used in balloons and deep-sea diving |
| Water vapour | Variable | Causes humidity; essential for the water cycle; varies most by place and season |
| Dust particles | Variable | Act as nuclei for cloud formation; scatter sunlight (making the sky appear blue); visible in sunbeams |
The two most abundant gases — nitrogen and oxygen — together make up about 99% of air. The remaining 1% includes CO₂, noble gases, water vapour, and dust particles. This composition can vary slightly by location and season, proving that air is a mixture, not a compound.
Q2. Explain the role of oxygen in burning and rusting with experiments.
Role of oxygen in burning:
Experiment: Place two identical candles on a table. Light both. Cover one with an inverted glass tumbler.
Observation: The uncovered candle continues burning. The covered candle uses up the limited oxygen inside the tumbler, glows briefly, and then extinguishes.
Conclusion: Oxygen is essential for combustion. When the oxygen supply is cut off, burning stops.
Role of oxygen in rusting:
Experiment: Take three test tubes. In the first, place an iron nail in dry air (with calcium chloride to absorb moisture). In the second, place an iron nail in boiled water (oil layer on top prevents air contact). In the third, place an iron nail in tap water with the nail partially exposed to air.
Observation: After a few days, the nail in the first test tube (dry air only) does not rust. The nail in the second test tube (water only, no air) does not rust. The nail in the third test tube (both air and water) develops rust.
Conclusion: Rusting requires both oxygen and moisture. Neither oxygen alone nor water alone is sufficient for rust to form.
Q3. Compare the composition of inhaled and exhaled air.
| Component | Inhaled Air | Exhaled Air | Change |
|---|---|---|---|
| Nitrogen | 78% | 78% | No change |
| Oxygen | 21% | 16% | Decreases by ~5% |
| Carbon dioxide | 0.04% | 4% | Increases significantly |
| Water vapour | Variable | Increases | More moisture added |
| Noble gases | 0.93% | 0.93% | No change |
| Dust particles | Variable | None | Filtered out |
| Temperature | Room temperature | Body temperature (37°C) | Warmer |
Key observations:
- Oxygen decreases from 21% to 16% because the body uses it for respiration.
- Carbon dioxide increases from 0.04% to 4% because it is produced as a waste product of respiration.
- Water vapour increases because air is moistened in the respiratory tract.
- Dust particles are removed by nasal hairs and mucus.
- Temperature rises to body temperature as air is warmed in the lungs.
Q4. Describe the layers of the atmosphere and the importance of each.
The atmosphere has five layers, from lowest to highest:
1. Troposphere (0–16 km): This is the layer we live in. All weather phenomena — wind, clouds, rain, storms — occur here. It contains most of the air's water vapour and dust particles. Temperature decreases with altitude in this layer.
2. Stratosphere (16–50 km): This layer contains the ozone layer, which absorbs harmful ultraviolet (UV) radiation from the Sun. Aircraft fly in the lower stratosphere to avoid the turbulent weather of the troposphere. Temperature increases with altitude here due to ozone absorbing UV radiation.
3. Mesosphere (50–80 km): This is the coldest layer. Meteors (shooting stars) burn up here due to friction with air molecules. Temperature decreases with altitude.
4. Thermosphere / Ionosphere (80–400 km): The ionised gases in this layer reflect radio waves back to Earth, enabling long-distance communication. Temperature is very high but the air is extremely thin, so it does not feel hot. Auroras (Northern and Southern Lights) occur here.
5. Exosphere (400–800+ km): The outermost layer. Air is extremely thin here. Satellites orbit in this region. Gas molecules can escape into space from this layer.
The atmosphere is essential because it provides oxygen for breathing, protects us from UV radiation, regulates temperature, enables weather and the water cycle, and burns up meteors before they reach Earth.
VIII. Higher Order Thinking Skills (HOTS)
Q1. Why do fire extinguishers use CO₂ instead of water for certain types of fires?
CO₂ fire extinguishers are preferred over water for several reasons:
1. Electrical fires: Water conducts electricity. Using water on an electrical fire can electrocute the person holding the hose and spread the fire. CO₂ is non-conductive, making it safe for electrical fires.
2. Oil and petrol fires: Water is heavier than oil and petrol. If water is sprayed on an oil fire, it sinks below the oil and the oil continues to burn on top. CO₂ is heavier than air and forms a blanket over the fire.
3. No residue: CO₂ leaves no residue since it sublimes directly to gas. Water can damage electronics, documents, and other materials.
4. Dual action: CO₂ both displaces oxygen (smothering the fire) and produces a cooling effect on expansion.
5. Non-reactive: CO₂ does not react with burning materials, so it does not produce toxic by-products or fuel the fire further.
Q2. Why do mountaineers need oxygen cylinders even though air contains 21% oxygen?
At sea level, air is dense and each breath contains sufficient oxygen molecules. However, as altitude increases:
1. Lower air density: The number of air molecules per unit volume decreases. Even though oxygen is still 21% of air, there are far fewer molecules in each breath. 2. Lower air pressure: Atmospheric pressure drops significantly. The lungs have to work harder to draw in air, and less oxygen enters the bloodstream. 3. Reduced oxygen saturation: With fewer oxygen molecules per breath, haemoglobin in the blood cannot bind enough oxygen, leading to altitude sickness — dizziness, nausea, fatigue, and in severe cases, life-threatening conditions.
Mountaineers carry oxygen cylinders to supplement the reduced atmospheric oxygen, ensuring their bodies receive adequate oxygen for metabolism and brain function.
Q3. Why is nitrogen used in food packaging instead of just air?
Nitrogen is preferred over air in food packaging for these reasons:
1. Oxygen causes spoilage: Air contains 21% oxygen, which causes oxidation of fats and oils in food (rancidity), supports growth of aerobic bacteria and fungi, and makes food stale.
2. Nitrogen is inert: Nitrogen (78% of air) does not react with food. By flushing the package with pure nitrogen, oxygen is displaced, and no chemical reaction occurs with the food.
3. Prevents oxidation: Without oxygen, the fats, oils, and vitamins in food do not oxidise, preserving freshness, flavour, and nutritional value.
4. Prevents microbial growth: Most spoilage bacteria require oxygen. A nitrogen atmosphere inhibits their growth.
5. Physical cushioning: In chip packets, nitrogen also acts as a cushion, preventing the chips from breaking during transport.
If regular air were used, the oxygen inside would continue to cause spoilage, defeating the purpose of packaging.
Common Mistakes to Avoid
1. Saying "air is only oxygen" — Air is a mixture of many gases. Oxygen is just one component (21%). Nitrogen (78%) is the most abundant gas. Never describe air as a single gas or a compound.
2. Confusing photosynthesis and respiration gas exchanges — 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 uses only about 5% of the oxygen in each breath. It never uses up all the oxygen.
4. Saying "air composition is always the same everywhere" — Air composition varies slightly by place and season. Industrial areas have more CO₂, coastal areas have more water vapour, and windy places carry more dust. The percentages (78%, 21%, 0.04%) are averages, not constants.
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" or "the ozone layer is in the troposphere."
6. 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%, noble gases ~0.93%).
- [ ] I can explain why air is called a mixture, not a compound.
- [ ] I can name all five layers of the atmosphere in order and state what occurs in each.
- [ ] I can distinguish the troposphere (weather) from the stratosphere (ozone layer).
- [ ] I can explain the role of oxygen in both burning and rusting, with experiments.
- [ ] I can describe how nitrogen controls the rate of combustion and its uses.
- [ ] I can explain why CO₂ is used in fire extinguishers (displaces oxygen, non-reactive, non-conductive, cooling).
- [ ] I can compare inhaled and exhaled air in a table (O₂, CO₂, water vapour, temperature).
- [ ] I can differentiate photosynthesis and respiration in terms of gas exchange.
- [ ] I can explain why nitrogen is used in food packaging instead of air.
- [ ] I can define dry ice, state its temperature (−78°C), and explain its uses.
- [ ] I can explain the ozone layer and why it is important (absorbs UV radiation).
- [ ] I can explain why mountaineers need oxygen cylinders at high altitudes.
- [ ] I can explain why biscuits lose crispness during monsoon (humidity, moisture absorption).
- [ ] I can describe the rusting experiment showing both oxygen and moisture are needed.
- [ ] I can state who discovered oxygen (Joseph Priestley) and who named it (Antoine Lavoisier).
- [ ] I can give examples of how air composition varies by place and season.
- [ ] I can explain why air pressure decreases with altitude.