NCERT Solutions for Class 9th Social Science Chapter 3 Chapter opening — The Big Questions
Book page 39 Updated on2026-09-08
Q1.
What is the composition of the atmosphere?
Answer
The atmosphere is a mixture of gases, not a single gas. Two gases do almost all of it — nitrogen (78%) and oxygen (21%) — and everything else shares the last 1%. On top of this mixture the air also carries water vapour and tiny dust particles.
Fig. A — The composition of the atmosphere, drawn from the figures printed in Fig. 3.2. Nitrogen and oxygen together make 99% of dry air; everything else shares the last 1%.
Add up what Fig. 3.2 prints:
78 + 21 = 99% — nitrogen and oxygen alone
0.93 (argon) + 0.04 (carbon dioxide) + 0.03 (others) = 1.00%
99 + 1 = 100% — the pie is complete
Water vapour is counted separately, and it is the one part of the mixture that keeps changing: the chapter says it generally ranges from 0.1 per cent to 0.4 per cent. That is a fourfold swing, and it matters far more than its small size suggests, because water vapour is what makes clouds and precipitation possible.
The other gases named in the chapter — beyond argon and carbon dioxide — are helium, neon, krypton, xenon, ozone and hydrogen. They are present, but in very small quantities.
Why the proportions are not the same everywhere: the chapter says plainly that the composition of the atmosphere also varies with altitude. Gravity holds the heavier gases close to the surface, so the air thins out as you rise, and by the exosphere only the lightest gases — helium and hydrogen — are left, and even those float away into space. So the 78:21 figures describe the air near the ground, which is where we live and where the weather happens.
Did you know? Carbon dioxide is only 0.04% of the air — four parts in ten thousand. Yet this tiny share is the gas at the centre of the climate change discussion later in the chapter, because it traps heat. Small in quantity does not mean small in effect.
Q2.
How do the different layers of the atmosphere affect the planet Earth?
Answer
Each layer does a different job for the Earth, and the reason each layer can do that job is the way its temperature changes with altitude. That is exactly how the layers are defined — the chapter says they are marked out on the basis of changes in temperature and density with increasing altitude.
Fig. C — The layers of the atmosphere and how temperature behaves in each. The red line follows Fig. 3.3 of the book: it falls in the troposphere, rises in the stratosphere, falls in the mesosphere and rises steeply in the thermosphere.
Layer
Extent
What it does for the Earth
Troposphere
about 12 km
Holds the air we breathe, most of the water vapour and the clouds. Nearly all weather — rainfall, fog, hail — happens here. Without it there would be no weather at all.
Stratosphere
up to 50 km
Carries the ozone layer, which filters the Sun's harmful radiation, including ultraviolet. Being free of clouds and weather disturbances, it is also where aeroplanes fly.
Mesosphere
up to 80 km
Acts as the Earth's shield against space debris — most meteorites burn up here before they can reach the ground.
Thermosphere
80 – 700 km
Absorbs the Sun's X-rays and short-wave ultraviolet. Its ionosphere reflects radio waves back to the Earth, which is what makes long-distance radio transmission possible. The auroras occur here.
Exosphere
outermost
Very thin air and weak gravity, so helium and hydrogen escape into space from here. It is the boundary between the Earth's air and outer space.
Why the temperature rises in some layers and falls in others — the whole logic in one paragraph: a layer warms where something in it absorbs the Sun's energy, and cools where nothing does. In the troposphere the heat comes from below — from the ground warmed by the Sun — so the further up you go, the further you are from the heat source, and the colder it gets. In the stratosphere the ozone absorbs ultraviolet, so the top of that layer is warmer than the bottom. The mesosphere has no ozone and is far above the ground, so temperature falls again. In the thermosphere the gas molecules absorb X-rays and short-wave ultraviolet — the most energetic radiation of all — so temperature climbs very rapidly. That is why the chapter's DON'T MISS OUT box can say: temperature decreases with altitude only in the troposphere and mesosphere.
Tip for remembering: the layers alternate — fall, rise, fall, rise — and each boundary between them is a '-pause': tropopause, stratopause, mesopause. A pause is where the temperature stops going one way and starts going the other.
Q3.
What is the mechanism of monsoon?
Answer
The mechanism is a seasonal reversal of the wind, and it is driven by one simple fact: land heats up and cools down faster than the sea. The chapter's own definition in the margin says it exactly — monsoon refers to the seasonal reversal in the wind direction during a year.
Fig. E — The mechanism of the monsoon. The same rule — wind blows from high pressure to low pressure — produces opposite winds in summer and winter, because land heats and cools faster than the sea.
Step by step, in summer (the south-west monsoon, June to September):
During summer the landmass of India heats up faster than the surrounding oceans.
Hot air over the land expands and rises, leaving a low-pressure area over the Indian subcontinent.
The Indian Ocean stays relatively cooler, so the air over it is heavier and sinks — a high-pressure area.
Winds move from high pressure to low pressure. So moist winds blow from the ocean towards the land, across the Indian Ocean, the Arabian Sea and the Bay of Bengal.
These winds are loaded with water vapour picked up over the sea. When they rise over the land and the hills, that vapour condenses and falls as rain. This monsoon accounts for most of the rainfall in the country throughout the year.
In winter (the north-east monsoon, October to February) everything reverses: the Indian landmass now cools faster than the surrounding oceans, so high pressure sits over the land and low pressure over the seas. The wind therefore blows from land to sea — cold and dry, bringing no rain to most of India. The exception is important: when these winds cross the Bay of Bengal they pick up moisture, and they rain on the eastern coast — especially Tamil Nadu, Andhra Pradesh and parts of Karnataka.
Why land and sea heat so differently: this is the single fact the whole monsoon rests on. Water needs far more heat to warm up than soil and rock do, and the sea also mixes its heat down through a great depth, while the land can only heat its thin top layer. So in summer the land races ahead of the sea in temperature, and in winter it falls behind. The pressure difference that follows is what turns the wind around — twice every year.
Where the name comes from: the Arab traders sailing to India named this reversal monsoon, from the Arabic word mausim, which literally means season. Sailors were the first to notice it, because in the age of sailing ships they were completely at the mercy of the winds — and a wind that reverses on schedule is a wind you can plan a voyage around.
Q4.
How can we reduce our carbon footprint?
Answer
By cutting the greenhouse gases our daily choices release. A carbon footprint is defined in the chapter as the total amount of greenhouse gases released into the atmosphere as a result of human activities, such as energy use, transportation, or the production of goods and services — so those three are exactly where the reductions have to come from.
The chapter's four collective actions:
Reducing carbon footprints — the everyday choices below.
Using renewable energy — solar and wind instead of burning fossil fuels.
Protecting forests — trees absorb carbon dioxide, and deforestation is named as one of the main causes of climate change.
Adopting sustainable lifestyles — using less, and using it longer.
What that means for a student, taken from the four habits the LET'S EXPLORE box asks you to score:
Area
The high-impact habit
The low-impact habit to move to
Transport
Travelling by private car even for short distances; taking flights more than twice a year
Walk or cycle for short trips; use public transport or carpool for longer ones
Electricity
Leaving lights and fans on frequently
Switch off every appliance when it is not in use
Water
Long showers, running taps, several buckets for a bath
Use water judiciously — one bucket for a bath
Waste and plastics
Throwing away plastic and not recycling
Reuse, recycle and avoid single-use plastics
Why saving electricity and water counts as saving carbon: most of the electricity we use is still generated by burning fossil fuels, so a fan left running is coal burnt somewhere and carbon dioxide added to the atmosphere. Water is the same story once removed — it has to be pumped, treated and delivered, and all of that runs on energy. This is why the chapter can put transport, electricity, water and plastic in one scorecard: they all end at the same place, the greenhouse gases in the air.
The chapter's own closing line is worth keeping: 'Every small step counts, and every human being plays a vital role in shaping a healthier and greener future.' One student's saved bucket of water is small; a class of forty doing it every day for a year is not.