NCERT Solutions for Class 6 Science Chapter 15 — Air Around Us

Key Concepts

Intext Questions (Page-by-Page)

1. On mountaintops: Air becomes thinner (less dense) as we go higher. At very high altitudes, the amount of air decreases significantly.

2. Underwater: Some air is dissolved in water, which is how aquatic animals breathe. But the amount of air in water is much less than in the atmosphere.

3. In soil: Air is present in the spaces between soil particles. This is how earthworms and other soil organisms breathe.

4. In outer space: Beyond the Earth's atmosphere, there is almost no air (vacuum). The atmosphere extends to about 300 km above the Earth, beyond which air is negligible.

So while air is present in most places on and near the Earth's surface, there are places like outer space where there is no air.

• Place a burning candle on a plate and cover it with a glass tumbler.
• The candle continues to burn for a short time using the oxygen trapped inside the glass.
• Once the oxygen inside the glass is used up, the candle flame goes out.

This shows that air (specifically oxygen) is necessary for burning. The more oxygen available, the better things burn.

1. We can feel air when it moves — as wind or a breeze on our skin. 2. We can see the effects of air — a flying kite, a fluttering flag, leaves moving on trees, clouds moving in the sky. 3. We can see dust particles in air when a beam of sunlight enters a dark room — the tiny particles reflecting light are carried by air. 4. We can see water vapour in air when it condenses on cold surfaces.

So while air itself is invisible, its presence can be detected through its effects.

Animals (including humans):

• Take in oxygen from the air during respiration.
• Release carbon dioxide into the air as a waste product of respiration.

Plants:

• Take in carbon dioxide from the air during photosynthesis (to make food).
• Release oxygen into the air as a by-product of photosynthesis.

This creates a natural balance:

• Animals produce carbon dioxide, which plants need for photosynthesis.
• Plants produce oxygen, which animals need for respiration.

This interdependence maintains the balance of oxygen and carbon dioxide in the atmosphere. Without plants, the amount of carbon dioxide would increase and oxygen would decrease. Without animals, the amount of oxygen would increase and carbon dioxide would decrease. Both are essential for maintaining the balance of gases in air.

1. Thin air at high altitudes: As we go higher above sea level, the amount of air (and therefore oxygen) decreases. The atmosphere becomes less dense, and there is significantly less oxygen available to breathe.

2. Difficulty in breathing: At high altitudes (above 3,000-4,000 metres), the reduced oxygen makes it difficult to breathe. This condition is called altitude sickness and can cause dizziness, nausea, headache, and in severe cases, can be life-threatening.

3. Normal oxygen requirement: The human body needs a certain amount of oxygen to function properly. At high altitudes, the natural oxygen in the air is not sufficient for normal body functions.

4. Physical exertion: Climbing mountains requires a lot of energy, which means the body needs even more oxygen than usual. The thin air at high altitudes cannot provide enough oxygen for this increased demand.

Therefore, mountaineers carry oxygen cylinders to supplement the reduced oxygen available in the atmosphere and ensure they can breathe properly.

Exercise Questions (NCERT Textbook)

(b) The component of air that supports burning is ________.

(c) The component of air that does not support burning is ________.

(d) The process of burning of a substance in the presence of oxygen is called ________.

(b) Plants do not need oxygen for respiration.

(c) Air contains water vapour.

(d) Air is transparent.

Key points about the composition:

• Nitrogen is the most abundant gas in air (78%). It does not support burning but is essential for plant growth (plants use nitrogen to make proteins).
• Oxygen is the second most abundant gas (21%). It supports burning and is essential for respiration.
• Carbon dioxide is present in very small amounts (0.04%) but is vital for photosynthesis in plants.
• Water vapour content varies depending on the weather, location, and season.
• Dust particles and smoke are also present in air in varying amounts.

Experiment 1 — Cold glass method: 1. Take a clean, dry glass tumbler. 2. Fill it with ice-cold water. 3. Wipe the outside of the glass to make sure it is completely dry. 4. Leave the glass on a table for a few minutes. 5. Observe the outer surface of the glass — you will see tiny water droplets forming on it. 6. These water droplets come from the water vapour present in the air, which condenses on the cold surface of the glass.

Experiment 2 — Mirror method: 1. Breathe onto a clean, cold mirror. 2. You will see a foggy layer form on the mirror. 3. This fog is caused by water vapour in your breath condensing on the cold mirror surface.

Experiment 3 — Observation method: 1. Observe the formation of dew on grass and leaves early in the morning. 2. Dew forms because water vapour in the air condenses on cold surfaces during the night.

These experiments prove that air contains water vapour.

1. Respiratory diseases: Breathing polluted air causes diseases like asthma, bronchitis, lung cancer, and other respiratory problems. Children and elderly people are especially vulnerable.

2. Global warming: Increased carbon dioxide and other greenhouse gases in the atmosphere trap heat, causing the Earth's temperature to rise. This leads to melting of glaciers, rising sea levels, and extreme weather events.

3. Acid rain: Pollutants like sulphur dioxide and nitrogen oxides react with water vapour in the atmosphere to form acids. These acids fall as acid rain, which damages buildings, crops, forests, and aquatic life.

4. Damage to plants: Air pollutants damage plant leaves, reduce photosynthesis, and stunt plant growth. This affects food production and the natural balance of ecosystems.

5. Smog formation: Smoke and fog combine to form smog, which reduces visibility and causes breathing problems in cities.

6. Damage to the ozone layer: Certain chemicals (like CFCs) damage the ozone layer, which protects us from harmful ultraviolet radiation of the sun. A thinning ozone layer increases the risk of skin cancer.

7. Effect on animals and aquatic life: Pollutants in air dissolve in water bodies, harming fish and other aquatic organisms. Animals that breathe polluted air also suffer health problems.

1. Variable composition: The composition of air varies from place to place and time to time. The amount of water vapour, dust, and carbon dioxide can change. A compound always has a fixed composition.

2. No chemical bonding: The gases in air (nitrogen, oxygen, carbon dioxide, etc.) are not chemically bonded to each other. They simply exist together in the same space. In a compound, elements are chemically bonded.

3. Components retain their properties: Each gas in air retains its own individual properties. Nitrogen still behaves like nitrogen, and oxygen still behaves like oxygen. In a compound, the constituent elements lose their individual properties.

4. No energy change on mixing: When the gases of air are mixed together, no energy (heat or light) is released or absorbed. When a compound is formed, energy is usually released or absorbed.

5. Separable by physical means: The components of air can be separated by physical methods like fractional distillation of liquid air. Compounds cannot be separated into their elements by physical methods.

6. No fixed boiling/melting point: Air does not have a fixed boiling point or melting point because its components boil at different temperatures. A compound has a fixed boiling point and melting point.

Respiration (by both plants and animals):

• Living organisms take in oxygen from the air.
• Oxygen is used to break down food (glucose) and release energy.
• Carbon dioxide and water are released as waste products.
• This process consumes oxygen and produces carbon dioxide.

Photosynthesis (by green plants):

• Green plants take in carbon dioxide from the air.
• Using sunlight, water, and chlorophyll, plants convert carbon dioxide and water into glucose (food).
• Oxygen is released as a by-product.
• This process consumes carbon dioxide and produces oxygen.

The balance:

• Animals breathe in oxygen and breathe out carbon dioxide.
• Plants take in carbon dioxide and release oxygen during photosynthesis.
• The carbon dioxide produced by animals is used by plants for photosynthesis.
• The oxygen produced by plants is used by animals for respiration.
• This cycle keeps the levels of oxygen and carbon dioxide in the atmosphere relatively constant.

This is why forests and green plants are vital for maintaining the quality of air. Deforestation (cutting down trees) disturbs this balance and leads to an increase in carbon dioxide and a decrease in oxygen in the atmosphere.

1. Oxygen is consumed: The candle flame needs a continuous supply of oxygen from the air to keep burning. When the jar is placed over the candle, it traps a limited amount of air inside.

2. Limited supply: The oxygen inside the closed jar is finite. The burning candle uses up this oxygen through the process of combustion.

3. No new air enters: Since the jar is closed, no fresh air (and therefore no new oxygen) can enter to replace the consumed oxygen.

4. Oxygen depletion: Once all the oxygen inside the jar is used up, the candle can no longer sustain the combustion reaction, and the flame goes out.

5. Other gases do not support burning: The remaining air in the jar is mostly nitrogen and carbon dioxide, neither of which supports burning.

This experiment clearly demonstrates that oxygen in the air is necessary for burning.

1. Dilutes oxygen: Nitrogen (78%) dilutes oxygen (21%) in the air. If air contained only oxygen, even a small spark would cause everything to burn violently. Nitrogen acts as a diluent and prevents things from burning too rapidly.

2. Plant nutrition: Nitrogen is an essential nutrient for plants. Plants absorb nitrogen from the soil (in the form of nitrates and ammonium compounds) to make proteins, which are building blocks of life. Certain bacteria in the soil convert atmospheric nitrogen into forms that plants can use — this process is called nitrogen fixation.

3. Manufacture of fertilisers: Nitrogen from the air is used to manufacture chemical fertilisers like urea and ammonium nitrate, which are essential for agriculture.

4. Manufacture of ammonia: Nitrogen is combined with hydrogen to produce ammonia (NH₃), which is used in making fertilisers, explosives, and many industrial products.

5. Food preservation: Nitrogen is used in food packaging (like chips packets) to create an inert atmosphere that prevents oxidation and spoilage of food.

6. Biological importance: Nitrogen is a component of amino acids, proteins, DNA, and RNA — all essential for living organisms.

7. Inert atmosphere: Nitrogen is used in industries to create inert atmospheres where oxygen needs to be excluded (e.g., in welding, in electronics manufacturing).

Multiple Choice Questions (MCQs)

Common Mistakes

1. Thinking air is a compound: Many students mistakenly believe air is a compound because it has a fairly consistent composition. Air is a mixture — its components can vary in proportion, they are not chemically bonded, and they can be separated by physical means. A compound has a fixed composition and cannot be separated by physical methods.

2. Confusing respiration and photosynthesis in plants: Students often think plants only perform photosynthesis and not respiration. In reality, plants perform both processes. During the day, photosynthesis produces more oxygen than respiration consumes, so plants appear to release only oxygen. At night, when there is no sunlight, plants only respire and release carbon dioxide.

3. Assuming nitrogen is unimportant: Because nitrogen does not support burning, students sometimes think it is not important. Nitrogen is essential — it dilutes oxygen (preventing everything from burning), is needed for protein synthesis in plants and animals, and is used in making fertilisers and many industrial products.

4. Ignoring water vapour as a component of air: Students often forget that water vapour is a variable component of air. Its presence is important because it causes weather phenomena like rain, fog, and dew. The amount of water vapour varies from place to place and season to season.

5. Thinking carbon dioxide is the main gas that does not support burning: While it is true that carbon dioxide does not support burning, the main gas that does not support burning in air is nitrogen (78%). Carbon dioxide is present only in trace amounts (0.04%). Nitrogen is the gas that prevents everything from catching fire easily.

Exam Checklist

• [ ] I can state the composition of air with approximate percentages of each gas.
• [ ] I can explain why oxygen is important for burning and respiration, with experimental evidence.
• [ ] I can explain why air is a mixture and not a compound, giving at least three reasons.
• [ ] I can describe how plants and animals maintain the balance of oxygen and carbon dioxide in the atmosphere.
• [ ] I can demonstrate that air contains water vapour using simple experiments.
• [ ] I can list the harmful effects of air pollution on human health, plants, and the environment.
• [ ] I can explain the role of nitrogen in air and its importance for life and industry.
• [ ] I can explain why a candle goes out when covered with a glass tumbler and relate it to the necessity of oxygen for burning.