Overview
This chapter introduces 'Air Around Us' — the invisible mixture of gases that surrounds the Earth and forms the atmosphere. It explains why air is essential for life and many natural processes, describes its main components (nitrogen, oxygen, argon, carbon dioxide and variable water vapour) and shows simple properties: air has mass, occupies space, and exerts pressure. Key themes include how air helps in breathing and photosynthesis, supports combustion, enables movement of seeds and pollination, and drives weather and wind. The chapter also uses easy classroom activities and observations (balloon, syringe, inverted glass, burning candle demonstrations) to build experimental skills: observing, recording and drawing conclusions. Finally it covers air pollution — common pollutants, their harmful effects and simple preventive measures — so students appreciate the importance of protecting air quality. By the end, students will understand what air is made of, why it matters for living things and the environment, and how to demonstrate and explain its basic properties through experiments.
Learning Objectives
- Define air and atmosphere in simple terms.
- List the major components of air with their approximate percentage composition.
- Explain why air is a mixture of gases and not a single substance.
- Demonstrate, by simple classroom experiments, that air occupies space and exerts pressure.
- Explain the role of oxygen in combustion using the glowing-splint test as evidence.
- Explain how air (oxygen and carbon dioxide) is essential for respiration in animals and photosynthesis in plants.
- Identify the presence of oxygen and carbon dioxide in a sample of air using simple tests (glowing splint, limewater) and interpret the results.
- Describe how plants help maintain the composition of air through photosynthesis and respiration.
Topics in this chapter
9 topics · tap a topic title to jump straight to it.
What is Air
What is air? Air is a mixture of gases that surrounds the Earth. It is mostly invisible and odorless, but it occupies space, has mass, and can be felt as wind. Air is essential for life — animals and humans breathe oxygen from air and plants use carbon dioxide in photosynthesis.
Composition (by volume): roughly 78% nitrogen (N2), 21% oxygen (O2), ~0.93% argon (Ar), ~0.04% carbon dioxide (CO2), plus small amounts of other gases and variable water vapour. These percentages change slightly with altitude, location and pollution.
Key properties of air:
- Air occupies space (it has volume) — e.g., a filled balloon takes up room.
- Air has mass — a filled balloon weighs more than an empty one.
- Air exerts pressure on objects in all directions (atmospheric pressure).
- Air is a mixture, not a single substance; its components can be separated physically.
- Air can be compressed (squeezed into smaller volume) and expands when heated.
Why air matters: Air supports breathing and combustion (oxygen), moderates Earth’s temperature (greenhouse gases), transmits sound, enables flying and wind energy, and transports moisture and pollutants.
Simple demonstrations (classroom experiments): extinguishing a candle by covering it with a glass jar shows air is needed for combustion; pushing a deflated balloon into a bottle and blowing shows air occupies space and can be trapped; balancing a straw in water or using a syringe shows air pressure and compressibility.
- Breathing: lungs take in oxygen from air and expel carbon dioxide.
- Inflating a balloon: demonstrates that air occupies space and has mass.
- Wind moving leaves or a kite: movement of air (wind) exerting force.
- Tyres and air pumps: compressed air provides cushioning and support.
- Wind turbines: moving air (wind) is converted to electrical energy.
- Smell spreading in a room: diffusion of gas particles through air.
- Pressure = Force / Area (P = F / A) — describes how air exerts force on surfaces.
- Density = Mass / Volume (ρ = m / V) — air has measurable density (around 1.2 kg/m³ at sea level, 15°C).
- Ideal gas law (advanced) PV = nRT — links pressure (P), volume (V), amount (n), gas constant (R) and temperature (T).
- Boyle's law (qualitative for learners) P1V1 = P2V2 — for a fixed amount of gas at constant temperature, pressure and volume are inversely related.
- Approximate pressure decrease with altitude (advanced) P = P0 · e^(−h/H) — P0 is sea-level pressure, h is height, H ≈ 8.5 km (scale height).
Composition of Air
What is air? Air is a mixture of many gases, tiny water droplets (water vapour) and solid particles (dust, pollen, smoke). It is invisible, has mass, occupies space and exerts pressure.
Major components and their approximate proportions (by volume):
- Nitrogen (N2): about 78% — largely inert, reduces reactivity and helps maintain pressures.
- Oxygen (O2): about 21% — required for respiration and combustion.
- Argon (Ar): about 0.93% — an inert noble gas.
- Carbon dioxide (CO2): about 0.04% (≈410 ppm and rising) — used by plants in photosynthesis; amount varies with pollution and biological activity.
- Water vapour: variable (0–4% or more) — depends on humidity and temperature.
- Other gases and impurities (neon, helium, methane, ozone) and suspended particles: present in trace amounts.
Key points to understand:
- The composition of air is given most often as percent by volume because gases mix uniformly — e.g., 78% nitrogen means 78 cm³ of N₂ in 100 cm³ of dry air.
- Water vapour amount varies widely with weather and location — humid air contains more water vapour than dry air.
- Some components (like CO2 and pollutants) change over time and with human activities (burning fossil fuels, deforestation).
- Air is a homogeneous mixture of gases at the macroscopic scale; its components can be separated by physical methods (e.g., fractional distillation of liquefied air) but not by chemical bonds between different gases.
Why each component matters:
- Nitrogen: makes up most of air and dilutes oxygen so that combustion and biological processes are controlled.
- Oxygen: essential for animal respiration and for most fires to burn.
- Carbon dioxide: essential for plants (photosynthesis) and acts as a greenhouse gas influencing Earth’s temperature.
- Water vapour: affects weather, comfort, and processes like evaporation and condensation.
Simple classroom observations and experiments: A burning candle enclosed under a jar goes out because it consumes oxygen; a plant kept under a jar may produce gas (CO₂ and later oxygen under light) and can show that air composition changes with living processes. Collecting exhaled air and comparing it with inhaled air shows higher CO₂ and lower O₂ in exhaled air.
Note on variation: The values above are for dry, clean air near sea level. Local pollution, altitude and humidity change both absolute and relative amounts of some components.
- Breathing: Humans inhale oxygen from air and exhale air with less O2 and more CO2.
- Rusting and burning: Oxygen in air reacts with iron to form rust and supports combustion of fuels.
- Balloons: Filling a balloon with air shows air occupies space and exerts pressure; a helium balloon rises because helium is lighter than air.
- Plant photosynthesis: Plants take CO2 from air and produce oxygen, changing local air composition.
- Humidity and weather: Water vapour in air influences humidity, cloud formation and rain.
- Air pollution: Smoke and vehicle exhaust increase dust and CO2 levels, changing air quality and composition.
- Percentage by volume of a gas = (volume of that gas / total volume of mixture) × 100
- ppm (parts per million) conversion: ppm = percent × 10,000 (so 0.04% = 400 ppm)
- Partial pressure (basic idea): p_gas = x_gas × P_total, where x_gas is mole fraction (useful for advanced study)
- Density (useful concept): density = mass / volume. (Density of dry air at sea level ≈ 1.225 kg/m³ — value varies with temperature and humidity.)
Air Has Mass
What this means
Air is made of tiny particles (molecules). Each particle has mass, so a collection of many air particles also has mass. Although a small amount of air feels light, large volumes of air have noticeable mass and weight.
How we know air has mass — simple ideas and experiments
- Weighing a balloon: If you weigh an empty (deflated) balloon and then inflate it and weigh again, the inflated balloon is heavier. The extra mass comes from the air inside the balloon.
- Syringe experiment: Pull the plunger of a syringe to draw in air, seal the nozzle and weigh the syringe before and after. The one with air weighs slightly more.
- Balance and flask: Put a closed flask (or bottle) on a balance, then remove some air by pumping (partial vacuum) and weigh again. Removing air reduces the mass slightly.
Simple explanation with numbers
Use the relation mass = density × volume. The density of air at sea level and room temperature is about 1.2 kg/m³ (approx.). So:
- Mass of 1 cubic metre (1 m³) of air ≈ 1.2 kg.
- Mass of air in a small balloon (say 5 litres = 0.005 m³): m = 1.2 × 0.005 = 0.006 kg = 6 g.
- Mass of air in a classroom (example: 10 m × 8 m × 3 m = 240 m³): m ≈ 1.2 × 240 = 288 kg (about the mass of two or three grown adults).
Why this matters
Because air has mass, it exerts weight and pressure. This is important in weather (movement of air masses), in flying (lift and buoyancy for balloons), and in everyday things like tyres (air adds mass and exerts force on the tyre walls).
Key points to remember
- Air is matter — it occupies volume and has mass.
- Mass of air in a volume = density of air × that volume.
- Density of air changes with temperature and altitude (less dense at higher altitudes), so the mass for the same volume can change.
- Weigh a deflated balloon and then an inflated one; the inflated balloon is heavier because of the air inside.
- Pull the plunger of a syringe, seal the tip and weigh: the syringe with air weighs more than the empty one.
- Calculate mass of air in a room: volume × density (e.g., a 240 m³ room contains ≈ 288 kg of air at 1.2 kg/m³).
- A car tyre contains several kilograms of air — inflating a tyre increases the mass of the car slightly.
- Hot-air balloons rise because heated air inside is less dense (mass per volume smaller) than surrounding cold air.
- Breathing out: the air you exhale has mass, so exhaling and inhaling exchange small amounts of mass with the surroundings.
- mass (m) = density (ρ) × volume (V) — m = ρ × V
- density (ρ) = mass (m) / volume (V) — ρ = m / V
- weight (W) = mass (m) × gravitational acceleration (g) — W = m × g (use g ≈ 9.8 or ≈ 10 N/kg for rough calculations)
Air Exerts Pressure
What is air pressure?
Air is made of tiny moving particles (molecules). When these molecules collide with any surface, they apply tiny forces. The sum of these forces per unit area is called air pressure. Air pressure acts on objects in all directions — from above, below and sides.
Why does air exert pressure?
Because air molecules are constantly moving. Each collision with a surface transfers momentum. Many collisions over an area produce a measurable force. The greater the number or speed of collisions (for example when air is denser or warmer), the larger the pressure.
Key ideas and simple demonstrations
- Card and glass experiment: If you fill a glass with water, place a card on top, and invert the glass, the card stays in place and the water does not fall out. This happens because the air pressure outside (below the card) pushes up against the card and is greater than the pressure of the air (or partial vacuum) inside the inverted glass.
- Drinking through a straw: When you suck, you reduce the air pressure inside your mouth and the straw. External atmospheric pressure on the liquid surface pushes the liquid up the straw into your mouth.
- Squeezing a balloon or crushing a heated-can: Changing the amount or temperature of the air inside changes internal pressure. If the internal pressure becomes much lower than outside pressure, the outside pressure can crush the object.
- Weather and wind: Air moves from regions of higher pressure to lower pressure, producing wind. Changes in atmospheric pressure are what weather forecasts measure.
Important properties
- Air pressure acts in all directions equally at a point.
- Atmospheric pressure decreases with increasing altitude (higher up the air is thinner).
- Air pressure increases with an increase in the number of collisions per unit area (more molecules or higher temperature) and decreases if molecules are fewer or cooler.
Practical consequences
Atmospheric pressure holds up mercury in a barometer, helps you drink through a straw, enables suction cups to stick, causes your ears to pop during takeoff/landing, and affects breathing at high altitudes.
Quick numerical idea
Standard atmospheric pressure at sea level ≈ 101325 pascal (Pa). A pascal is one newton per square metre (1 Pa = 1 N/m2). Even on a small area this produces noticeable force (for example, on 1 cm2 area the atmospheric force is about 10 N).
- Card-on-glass: Fill a glass with water, cover with a card and invert it. The card stays because outside air pressure pushes up and supports the water.
- Using a straw: When you suck, you lower pressure in the straw so outside air pressure pushes the liquid up into your mouth.
- Suction cup: Pressing a suction cup against a surface expels some air. Outside atmospheric pressure pushes the cup onto the surface and keeps it stuck.
- Crushing a can: Heat a small amount of water in an empty can to make steam, then quickly cool the can (invert in cold water). The steam condenses, internal pressure drops and outside air pressure crushes the can.
- Barometer: Mercury in a barometer is supported by atmospheric pressure; the column height shows how strong the air pressure is.
- Why ears pop: Rapid altitude changes change external pressure; your ear pressure adjusts causing a pop until pressures equalize.
- Pressure = Force / Area (P = F/A)
- SI unit: Pascal (Pa) where 1 Pa = 1 N/m²
- Standard atmospheric pressure at sea level: 1 atm ≈ 101325 Pa ≈ 760 mmHg
- Force from pressure on an area: F = P × A (use same units: Pa and m²)
- Useful conversion: 1 cm² = 1×10⁻⁴ m² (so force on 1 cm² at 101325 Pa is ≈ 10.13 N)
Wind
What is wind? Wind is moving air. It is the horizontal movement of air from regions of higher air pressure to regions of lower air pressure. Wind is produced because the Sun heats different parts of the Earth unevenly. Warm air rises and creates a low-pressure area; cooler air moves in to replace it, causing wind.
How wind forms (simple explanation):
- Sunlight heats land and water differently. Land usually warms and cools faster than water.
- Warm air expands and rises, creating lower pressure at the surface.
- Cooler, denser air moves from high-pressure zones toward the low-pressure zone.
- This movement of air is what we feel as wind. The greater the pressure difference, the stronger the wind.
Types of winds (basic):
- Local winds: small-scale winds such as sea breezes and land breezes that occur near coastlines due to daily heating and cooling.
- Global winds: large-scale winds like the trade winds, westerlies and polar winds that blow over long distances and help move weather systems.
- Seasonal winds: e.g., monsoon winds that change direction with the season because of large-scale heating differences between land and sea.
Naming wind: Winds are named by the direction they come from. A north wind blows from the north toward the south.
Measuring wind: Wind speed is measured by an anemometer; direction is shown by a wind vane. Common units: metres per second (m/s), kilometres per hour (km/h), and knots (nautical miles per hour).
Effects and uses of wind: Wind can cool us, move clouds, cause storms, erode soil and rocks, help pollination and seed dispersal, and provide energy (windmills and turbines).
- Sea breeze: During the day, land heats faster than sea. Warm air above land rises and cool air from the sea moves inland as a sea breeze.
- Land breeze: At night, land cools faster than sea. Air above the sea is now warmer and rises, and cool air from land moves toward the sea.
- Flying a kite: Wind provides the force that lifts and keeps a kite in the air.
- Windmills and wind turbines: Moving air turns blades to pump water or generate electricity.
- Seed dispersal (anemochory): Seeds like those of dandelions are carried by the wind to new places.
- Blowing out a candle: A short gust of wind disrupts the steady air around the flame and extinguishes it.
- Speed = Distance / Time (e.g., wind speed in m/s = metres travelled per second)
- Convert m/s to km/h: km/h = m/s × 3.6
- Convert km/h to m/s: m/s = km/h ÷ 3.6
Water Vapour and Humidity
What is water vapour?
Water vapour is water in the form of a gas. It is produced when liquid water evaporates (turns into gas) from seas, rivers, lakes, wet clothes, soil and even from plants. Water vapour is invisible – you cannot see it, though you can see its effects (for example, clouds, fog, or steam condensing on a cold surface).
Evaporation and condensation
Evaporation is the change of water from liquid to gas. Evaporation happens faster when the air is warm, when there is wind, or when the surface area of water is large. Condensation is the reverse: water vapour turns back into tiny liquid droplets when the air cools (for example, when warm moist air meets a cold surface). Condensation forms dew, fog, clouds and water droplets on a cold glass.
What is humidity?
Humidity is the amount of water vapour present in the air. Humidity affects how comfortable we feel, how quickly clothes dry, and weather phenomena. Air that contains the maximum possible water vapour at a given temperature is called saturated. Warmer air can hold more water vapour than cooler air.
Types of humidity (simple)
- Absolute humidity: the actual amount (mass) of water vapour present in a unit volume of air (for example grams per cubic metre).
- Relative humidity: how full the air is of water vapour compared to the maximum it can hold at that temperature (expressed as a percentage). A relative humidity of 100% means the air is saturated and condensation (fog or dew) may form.
Why humidity matters (effects)
High humidity makes hot days feel hotter because sweat evaporates more slowly, so the body cools less efficiently. Low humidity makes the air feel dry and can dry out skin and plants. Humidity also influences weather: clouds, rain, fog and dew form when moist air cools and condenses.
How humidity is measured
A hygrometer measures humidity. Weather reports often give relative humidity. Simple classroom demonstrations (wet and dry-bulb thermometers) can show how to estimate relative humidity.
- Morning dew on grass: overnight cooling causes water vapour in air to condense into droplets on cooler surfaces.
- Clothes drying faster on a sunny, windy day: higher temperature and wind increase evaporation.
- Breathing out on a cold day: the warm moist breath condenses into visible mist when it meets cold air.
- Bathroom mirror fogging after a hot shower: warm moist air from the shower condenses on the cooler mirror surface.
- Feeling sticky on a humid summer day: high relative humidity slows evaporation of sweat, making you feel hotter.
- Absolute humidity = mass of water vapour (g) / volume of air (m³). Example unit: g/m³.
- Relative humidity (RH) = (actual amount of water vapour present ÷ maximum amount of water vapour air can hold at that temperature) × 100%.
- Relative humidity (using vapour pressures) = (actual vapour pressure ÷ saturation vapour pressure at same temperature) × 100%.
Importance of Air
What is air? Air is a mixture of gases that surrounds the Earth. It is mostly nitrogen and oxygen, with small amounts of other gases and water vapour. Although invisible, air has weight, takes up space, and exerts pressure.
Why air is important
- Supports life: Animals and humans breathe in oxygen from air for respiration. Plants take in carbon dioxide from air for photosynthesis and release oxygen.
- Needed for burning: Combustion (burning) requires oxygen from air. Without air, fire cannot continue.
- Helps in seed and pollen dispersal: Wind (moving air) carries seeds and pollen to new places so plants can reproduce.
- Makes weather and climate: Air movement creates winds and distributes heat and moisture, causing weather patterns like rain and storms.
- Protects Earth: The atmosphere filters harmful ultraviolet rays (ozone layer) and slows small meteors so they burn up before reaching the surface.
- Transmits sound: Sound waves travel through air, allowing us to hear.
- Useful in everyday life and technology: Air inflates tyres and balls, drives windmills and turbines, helps cool by evaporation, and provides lift for flying objects.
- Regulates temperature: Air and its water vapour help keep Earth's temperature suitable for life (greenhouse effect in balanced amounts).
Simple observations and experiments
- A candle placed under a jar goes out after the oxygen inside is used up — showing oxygen in air is needed for burning.
- Inflating a balloon shows that air occupies space. Squeezing a closed bottle with no air flow demonstrates air exerts pressure.
Effects of polluted air
When air contains harmful gases, smoke, or dust (pollutants), it causes health problems, damages plants and buildings, and changes weather patterns. Clean air is essential for healthy life.
- Breathing: Humans and animals inhale oxygen from air and exhale carbon dioxide.
- Photosynthesis: Plants use carbon dioxide from air and sunlight to make food, releasing oxygen.
- Burning a candle: A candle flame goes out in a closed jar because the oxygen in the trapped air is used up.
- Seed dispersal: Light seeds (like those of dandelion) are carried away by wind to grow elsewhere.
- Wind energy: Wind turbines convert moving air into electrical energy used in homes and schools.
- Inflating a tyre or balloon: Air occupies space and provides pressure to keep tyres and balloons firm.
- Density of air (or any substance): density = mass / volume (ρ = m / V)
- Pressure exerted by air: pressure = force / area (P = F / A)
- Approximate composition of dry air by volume: Nitrogen ≈ 78%, Oxygen ≈ 21%, Argon ≈ 0.93%, Carbon dioxide ≈ 0.04% (rest = trace gases)
- General note (Class 6 level): Air has weight — a column of air above an area exerts atmospheric pressure on that area.
Air Pollution
What is air pollution? Air pollution is the presence of harmful or excessive quantities of substances (particles or gases) in the air that can cause health problems, damage the environment, and reduce visibility. Clean air becomes polluted when natural balance is disturbed by added substances.
Sources of air pollution
- Natural sources: dust storms, volcanic eruptions, forest fires, pollen.
- Human (anthropogenic) sources: vehicle exhaust, industrial chimneys, burning of coal, biomass and crop residues, construction dust, burning of garbage, household cooking with solid fuels, use of solvents and paints.
Major types of pollutants
- Particulate matter (PM): dust, smoke, soot — tiny solid particles suspended in air (PM10, PM2.5).
- Gaseous pollutants: carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), ozone (O3) at ground level, volatile organic compounds (VOCs), methane (CH4), carbon dioxide (CO2).
- Biological pollutants: pollen, mold spores, bacteria.
Effects of air pollution
- On health: irritation of eyes, nose and throat; coughing; asthma attacks; bronchitis; reduced lung function; in severe cases heart disease and premature death.
- On environment: acid rain (from SO2 and NOx) damages plants and buildings; eutrophication of water bodies; ozone near ground level harms crops; reduced visibility (smog).
- Climate effects: greenhouse gases like CO2 and CH4 trap heat and contribute to global warming.
How we can reduce air pollution
- Use public transport, cycle or walk; reduce vehicle use; maintain vehicles to lower exhaust emissions.
- Switch to cleaner fuels and renewable energy (solar, wind); use cleaner industrial technologies and filters (scrubbers, electrostatic precipitators).
- Avoid burning waste and crop residue; adopt waste segregation and recycling.
- Plant trees and create green belts; use dust-control measures at construction sites.
- Use clean cooking fuels and improved stoves at home.
Quick summary: Air pollution comes from both natural and human activities. It contains particles and gases that harm health, damage the environment and affect climate. Simple actions by individuals and policies at community and national level can greatly reduce pollution.
- Vehicle exhaust on busy city roads causing smog and breathing problems.
- Smoke from a coal-fired power plant releasing sulfur dioxide and soot into the air.
- Open burning of crop stubble after harvest producing dense smoke and particulate pollution in rural areas.
- Household cooking with wood or animal dung indoors leading to indoor air pollution and respiratory illness.
- Construction sites generating large amounts of dust (particulate matter) that spread to nearby areas.
- Percentage concentration = (Volume of pollutant / Total volume of air) × 100
- ppm (parts per million) = (Volume of pollutant / Total volume of air) × 10^6
- ppb (parts per billion) = (Volume of pollutant / Total volume of air) × 10^9
- Basic AQI interpolation (simplified form): AQI = (Ihi - Ilo)/(BPhi - BPlow) × (C - BPlow) + Ilo (where C = pollutant concentration, BP = breakpoint concentrations, I = AQI index values)
- Relative change (%) = ((Final concentration - Initial concentration) / Initial concentration) × 100
Activities and Experiments
Overview
The chapter section 'Activities and Experiments' for Air Around Us shows, by simple hands‑on tests, the basic properties of air: it occupies space, has weight, can be compressed, exerts pressure, contains moisture (water vapour) and oxygen, and carries smells and dust. Below are clear classroom‑safe experiments with materials, steps, observations and short conclusions.
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Air occupies space
- Materials: a beaker of water, a small glass or jar, bowl (or basin).
- Procedure: Fill the bowl with water. Take the small glass, turn it mouth down and push it straight into the water (or invert the glass and push under water). Observe the trapped air bubbles and that water does not fill the entire inside of the inverted glass.
- Observation: Air is trapped in the glass and displaces some water; bubbles rise when the angle changes.
- Conclusion: Air occupies space and prevents water from entering the whole volume.
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Air can be compressed (syringe experiment)
- Materials: plastic syringe without needle (20–60 ml).
- Procedure: Pull the plunger out and then push it in slowly while blocking the nozzle with your finger (or attach a cap). Note change of plunger position and feel resistance.
- Observation: The same amount of air takes less volume when you push the plunger in; pressure increases (you feel resistance).
- Conclusion: Air occupies space and can be compressed (volume decreases when pressure increases).
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Air has weight
- Materials: two identical balloons, simple weighing scale or a balance.
- Procedure: Weigh two deflated balloons together or on two pans so they balance. Inflate one balloon and compare weights.
- Observation: The inflated balloon is slightly heavier than the deflated one.
- Conclusion: The air inside the balloon has mass, therefore weight.
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Air exerts pressure (card and glass experiment)
- Materials: a glass filled with water, stiff card or plastic index card, a tray.
- Procedure: Fill the glass to the brim, place the card tightly over the mouth, invert the glass quickly while holding the card, then remove your hand supporting the card.
- Observation: The card stays stuck to the rim and water does not fall out immediately.
- Conclusion: Atmospheric air pushes up on the card (air pressure) and prevents water from falling out; air outside exerts pressure on surfaces.
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Air contains water vapour (condensation test)
- Materials: hot water, metal spoon or cold mirror, or a cold plate.
- Procedure: Hold the cold spoon or mirror over the steam rising from hot water or breathe onto the cold surface.
- Observation: Tiny water droplets form (fog/condensation) on the cold surface.
- Conclusion: Air contains water vapour which condenses when cooled.
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Air contains oxygen (burning test)
- Materials: a small candle, a matchstick, transparent jar.
- Procedure: Light the candle and cover it with the jar. Observe how long it takes to go off. (Do not leave unattended.)
- Observation: The flame goes off after some time when the oxygen inside the jar is used up.
- Conclusion: Air contains oxygen which supports burning; removing oxygen extinguishes the flame.
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Air spreads smell and dust
- Materials: incense stick or a small strong-scented item.
- Procedure: Light the incense in one corner of the room and notice that people a distance away can smell it.
- Observation & Conclusion: Air carries particles of smell and dust, showing it acts as a medium for transport.
Safety notes: Adults should supervise burning experiments and hot water. Do not seal containers completely when compressing air without release: trapped high pressure can be hazardous.
How these experiments link to ideas
- Simple observations lead to scientific conclusions: air has mass, occupies space, exerts pressure, and contains components (oxygen, water vapour).
- These properties explain many everyday phenomena: why balloons inflate, why we feel wind, why ears pop on ascent/descent, and why weather and breathing depend on air.
- Inflating a bicycle tyre: air under pressure makes the tyre firm; this uses the fact that air can be compressed and exerts pressure on tyre walls.
- Breathing: lungs take in air (oxygen) and expel carbon dioxide; air occupies space and flows from high to low pressure.
- Kite flying: wind (moving air) exerts force (pressure) on the kite surface to lift it.
- Fog on a cold morning: water vapour in warm air condenses when temperature drops, forming visible droplets.
- Using a vacuum pump (syringe model): large suction devices remove air to reduce pressure for experiments or packaging.
- Pressure = Force / Area (P = F / A) — explains how air exerts force over surfaces.
- 1 atmosphere (atm) ≈ 101325 pascal (Pa) ≈ 101.3 kilopascal (kPa) — standard atmospheric pressure at sea level.
- Boyle's law (qualitative for students): At constant temperature, pressure × volume = constant (P1 × V1 = P2 × V2) — shows inverse relation between pressure and volume when compressing air.
Key Concepts
- Air
- A mixture of gases that surrounds the Earth and is essential for life.
- Atmosphere
- The layer of air that surrounds the Earth, made of several gases and layers.
- Nitrogen
- The most abundant gas in air (about 78%) that is mostly inert and helps dilute oxygen.
- Oxygen
- A gas (about 21% of air) needed by most living organisms for respiration and for burning.
- Carbon dioxide
- A small but important gas in air produced by respiration and combustion and used by plants for photosynthesis.
- Water vapour
- The gaseous form of water present in air; its amount determines humidity.
- Argon (and other noble gases)
- Inert gases present in small amounts in air that do not react easily with other substances.
- Dust and smoke (Particulate matter)
- Tiny solid or liquid particles suspended in air that can come from soil, burning, or pollution.
- Air pressure
- The force exerted by air molecules pushing on a surface; it acts in all directions.
- Wind
- The movement of air from a region of higher pressure to a region of lower pressure.
- Breeze
- A gentle, light wind.
- Humidity
- The amount of water vapour present in the air.
- Air pollution
- The presence of harmful substances in air, such as gases, smoke, or dust, that can damage health and the environment.
- Smog
- A type of air pollution formed by the mixture of smoke and fog, often seen in cities.
- Respiration
- A biological process where living organisms take in oxygen and release carbon dioxide to produce energy.
- Photosynthesis
- The process by which green plants use sunlight, carbon dioxide, and water to make food and release oxygen.
- Combustion
- A chemical process of burning in which a substance reacts with oxygen and produces heat and often smoke.
- Diffusion
- The natural spreading of particles (like gases) from an area of higher concentration to an area of lower concentration.
- Ventilation
- The process of replacing stale or polluted indoor air with fresh outdoor air.
- Ozone
- A form of oxygen (O3) found in the upper atmosphere that protects us from ultraviolet rays; at ground level it can be a pollutant.
End-of-Chapter Trial Paper & Test Questions
Topic-wise questions to test your understanding of every concept in this chapter.
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What is the approximate percentage of nitrogen in dry air? / शुष्क वायु में नाइट्रोजन का लगभग कितना प्रतिशत है? (a) 21% (b) 0.04% (c) 78% (d) 0.93%
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(c) 78% — Nitrogen is the most abundant gas in dry air at approximately 78% by volume. Oxygen comes second at about 21%. / नाइट्रोजन शुष्क वायु में आयतन के अनुसार लगभग 78% के साथ सबसे प्रचुर गैस है। ऑक्सीजन दूसरे स्थान पर लगभग 21% है।
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A candle placed under a glass jar goes out after some time. The best explanation for this is: / एक काँच के जार के नीचे रखी मोमबत्ती कुछ समय बाद बुझ जाती है। इसका सबसे अच्छा स्पष्टीकरण है: (a) The wax runs out / मोम खत्म हो जाता है (b) The jar cools the flame / जार लौ को ठंडा कर देता है (c) The oxygen inside the jar is used up by burning / जार के अंदर ऑक्सीजन जलने से समाप्त हो जाती है (d) Nitrogen extinguishes the flame / नाइट्रोजन लौ बुझा देती है
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(c) The oxygen inside the jar is used up by burning / जार के अंदर ऑक्सीजन जलने से समाप्त हो जाती है — Combustion needs oxygen. When the limited oxygen inside the sealed jar is consumed, the candle goes out. This shows air contains oxygen. / दहन के लिए ऑक्सीजन आवश्यक है। जब बंद जार के अंदर सीमित ऑक्सीजन समाप्त हो जाती है, मोमबत्ती बुझ जाती है।
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Which of the following demonstrates that air has mass? / निम्नलिखित में से कौन सा दर्शाता है कि वायु में द्रव्यमान होता है? (a) Wind moves leaves / हवा पत्तियाँ हिलाती है (b) An inflated balloon weighs more than a deflated one / फुला हुआ गुब्बारा पिचके गुब्बारे से अधिक भारी होता है (c) We can hear sound in air / हम वायु में ध्वनि सुन सकते हैं (d) Air can be seen in fog / कोहरे में वायु दिखाई देती है
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(b) An inflated balloon weighs more than a deflated one / फुला हुआ गुब्बारा पिचके गुब्बारे से अधिक भारी होता है — The extra mass of the inflated balloon comes from the air trapped inside it, demonstrating that air has mass. / फुले हुए गुब्बारे का अतिरिक्त द्रव्यमान उसके अंदर भरी हवा से आता है, जो दर्शाता है कि वायु में द्रव्यमान होता है।
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Fill in the blank: The movement of air from a region of high pressure to a region of low pressure is called ______. / रिक्त स्थान भरें: अधिक दाब वाले क्षेत्र से कम दाब वाले क्षेत्र की ओर वायु की गति ______ कहलाती है।
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Wind / पवन — Wind is caused by uneven heating of different parts of Earth's surface by the Sun. Warm air rises (low pressure) and cooler air moves in to replace it. / पवन पृथ्वी की सतह के विभिन्न भागों के असमान ताप के कारण होती है। गर्म वायु ऊपर उठती है और ठंडी वायु उसकी जगह लेती है।
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Fill in the blank: The gas in air that plants use during photosynthesis to make food is ______. / रिक्त स्थान भरें: वायु में वह गैस जिसे पौधे भोजन बनाने के लिए प्रकाश संश्लेषण में उपयोग करते हैं, ______ है।
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Carbon dioxide / कार्बन डाइऑक्साइड — Plants absorb carbon dioxide from air and combine it with water using sunlight energy to produce glucose and oxygen (photosynthesis). / पौधे वायु से कार्बन डाइऑक्साइड अवशोषित करते हैं और सूर्य की ऊर्जा का उपयोग करके जल के साथ मिलाकर ग्लूकोज और ऑक्सीजन बनाते हैं।
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True or False: Air is a pure substance with a fixed chemical formula. / सत्य या असत्य: वायु एक शुद्ध पदार्थ है जिसका एक निश्चित रासायनिक सूत्र होता है।
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False / असत्य — Air is a mixture of several gases (mainly nitrogen, oxygen, argon, carbon dioxide and water vapour). Its composition can vary with location, altitude and pollution, so it does not have a fixed chemical formula. / वायु कई गैसों का मिश्रण है (मुख्यतः नाइट्रोजन, ऑक्सीजन, आर्गन, कार्बन डाइऑक्साइड और जलवाष्प)। इसकी संरचना स्थान, ऊँचाई और प्रदूषण के साथ बदल सकती है।
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List three major sources of air pollution and explain one harmful effect of air pollution on human health. / वायु प्रदूषण के तीन प्रमुख स्रोत बताइए और मानव स्वास्थ्य पर वायु प्रदूषण का एक हानिकारक प्रभाव समझाइए।
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Sources: (1) Vehicle exhaust fumes. (2) Industrial chimneys releasing smoke and gases. (3) Open burning of garbage or crop residues. Harmful effect: Particulate matter (PM2.5) and gases like NO₂ and SO₂ irritate the respiratory tract, causing coughing, asthma attacks and reduced lung function over time. / स्रोत: (1) वाहनों का धुआँ। (2) उद्योगों की चिमनियाँ। (3) कूड़ा या फसल अवशेष जलाना। हानिकारक प्रभाव: पार्टिकुलेट मैटर और गैसें श्वसन तंत्र को प्रभावित करती हैं।
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Explain how you can demonstrate that air exerts pressure using a glass filled with water and a card. / काँच के पानी भरे गिलास और कार्ड का उपयोग करके दिखाइए कि वायु दाब डालती है।
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Fill a glass to the brim with water, place a stiff card on top, and quickly invert the glass. The card stays in place and water does not fall out. This happens because the atmospheric air pressure below the card pushes up on it, which is greater than the pressure of the water pushing down. / गिलास को पूरा पानी से भरें, ऊपर एक कड़ा कार्ड रखें और तुरंत उलट दें। कार्ड लगा रहता है और पानी नहीं गिरता क्योंकि नीचे से वायुमंडलीय दाब ऊपर की ओर धकेलता है।
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