NCERT Solutions for Class 9th Science Chapter 13 Introduction — Activity 13.1: Let us explore

Book page 253 Updated on2026-09-08

Q1.

Observe the features of the Earth as shown in Fig. 13.1. Identify and circle one example representing each of the geosphere, hydrosphere, cryosphere, atmosphere, and biosphere.

Answer

Fig. 13.1 shows a high mountain valley with a lake, a snow-covered range behind it, and a shepherd grazing his flock on the grass at the water's edge. Every sphere is present in that one picture:

Sphere	What to circle in Fig. 13.1	Why it belongs there
Geosphere	The bare rock and boulders on the valley floor and the mountain slopes	Solid rock, soil and landforms
Hydrosphere	The lake	Liquid water at the surface
Cryosphere	The snow and ice capping the mountains	Water in the solid state
Atmosphere	The sky and clouds above the valley	The layer of air held by the Earth's gravity
Biosphere	The sheep, the shepherd and the grass	Living organisms and their habitat

Check it yourself: the picture is chosen so that all five spheres touch each other. The snow rests on rock, the melt water fills the lake, the lake water evaporates into the air, and the grass — which needs soil, water and air together — feeds the sheep. That overlap is the whole point of the activity.

Q2.

How does snow (cryosphere) eventually become part of the lake (hydrosphere)?

Answer

By melting. When the air and the ground warm up in spring and summer, snow and glacier ice absorb heat, change from the solid state to the liquid state, and the melt water runs down the slope as streams into the lake.

Snow / ice (solid, cryosphere)
+ heat from solar radiation and warmer air
→ melting at 0 °C
→ melt water runs down the slope
→ streams → lake (liquid, hydrosphere)

Why it happens: snow is the same substance as lake water — only its state is different. Melting needs energy, and that energy comes from the Sun. Once the temperature at the snow surface reaches 0 °C, further heat goes into breaking the bonds of the ice crystal (latent heat of fusion) rather than raising the temperature, and the solid turns to liquid. Gravity then does the rest: the water flows downhill until it reaches the lowest point in the valley, which is the lake.

Tip: a small part of the snow does not go through the liquid stage at all — it sublimes, changing straight from ice to water vapour in the dry mountain air. That water joins the atmosphere instead, and returns as precipitation elsewhere.

Q3.

If there is less snowfall during winters for a few years, how would this affect the lake's level and the grass available for the sheep?

Answer

The lake level would fall in summer, and with less water the grass would grow thinner — so there would be less fodder for the sheep.

Less snowfall in winter → smaller store of snow and ice
Smaller store → less melt water in summer
Less melt water → lake level falls
Lower lake and drier soil → poorer growth of grass
Less grass → less food for the sheep

Why it happens: the snowpack is a natural reservoir. It collects the winter precipitation as a solid and releases it slowly as liquid through the summer, exactly when there is no rain and the grass needs water most. Cut the winter deposit and the summer withdrawal must fall too. The chapter puts it in the same words: "less snowfall in winters may lead to less water in the lake in summers, resulting in less water to support the growth of grass."

Follow it one step further: fewer sheep can be grazed, so the shepherd's income falls. A change that began in the cryosphere has ended in a human household — which is why the Earth's spheres must be studied together.

Q4.

Discuss with your classmates and write down how all the spheres are interconnected, and how a disturbance in one can lead to changes in others.

Answer

The five spheres are joined by two things that never stop moving between them — energy (mainly from the Sun) and matter (water, carbon, nitrogen, oxygen and minerals). Because they share the same energy and the same matter, a change forced on one sphere must appear in the others.

The five spheres are not stacked layers — they overlap and exchange energy and matter along every line. That is why a disturbance travels.

Disturbance	Sphere it starts in	Where it shows up next
Less snowfall in winter	Cryosphere	Lake level falls (hydrosphere) → grass grows poorly → sheep go hungry (biosphere)
Arabian Sea warms	Hydrosphere	More evaporation → erratic monsoon (atmosphere) → floods and droughts → crops fail (biosphere)
Extra CO₂ from burning fuels	Atmosphere	Warming melts glaciers (cryosphere) → sea level rises (hydrosphere) → coastal habitats lost (biosphere)
Forests cleared	Biosphere	Less transpiration (atmosphere), more run off and erosion (geosphere), silt-choked rivers (hydrosphere)

The underlying reason: the Earth is a closed system for matter — no new water, carbon or nitrogen arrives from outside — but an open system for energy, which flows in from the Sun and leaves as infrared radiation. Because the same fixed stock of matter is shared by all five spheres, taking it out of one sphere necessarily changes the amount in another. The spheres are held in what the chapter calls "a delicate balance".

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