Class 9 Science Exploration Chapter 13 Solutions (NCERT 2026–27) – Earth as a System: Energy, Matter, and Life

These Class 9 Science Exploration Chapter 13 solutions cover Earth as a System: Energy, Matter, and Life from the new NCF-2023 textbook (2026–27).

Class: 9 Subject: Science Book: Exploration Chapter: 13 Exercise: Revise, Reflect, Refine (15 Qs) Session: 2026–27
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Class 9 Science Exploration Chapter 13 Solutions – Overview

Chapter 13 of Exploration, Earth as a System: Energy, Matter, and Life, shows how the Earth works as a single connected system. It covers the four spheres (lithosphere, hydrosphere, atmosphere and biosphere) and how they interact, the flow of energy from the Sun, the greenhouse effect and albedo, the biogeochemical cycles (carbon, oxygen, nitrogen and water), winds and the atmosphere, and the effects of human activities such as deforestation and climate change. These Class 9 Science Exploration Chapter 13 solutions answer every textbook question step by step.

Key Concepts & Definitions

The four spheres: lithosphere (land), hydrosphere (water), atmosphere (air) and biosphere (living things) — they constantly exchange energy and matter.

Biogeochemical cycle: the recycling of nutrients (carbon, oxygen, nitrogen, water) between living and non-living parts of the ecosystem.

Greenhouse effect: greenhouse gases (CO2, water vapour, methane) trap the heat re-radiated by the Earth, keeping it warm.

Albedo: the fraction of sunlight a surface reflects — high for ice/snow, low for dark surfaces.

Troposphere: the lowest layer of the atmosphere, where weather occurs.

Climate change: long-term changes in temperature and weather patterns, largely due to rising greenhouse gases.

“Think It Over” — Answers

If a large forest in an area is cut down, what might happen in that area?

ANSWERCutting down a large forest means less photosynthesis (so more CO2 and less O2), a disturbed water cycle (less transpiration, so less rainfall), soil erosion, loss of habitat and biodiversity, and a rise in local temperature — upsetting the balance of the spheres.

What might happen if glaciers keep melting faster?

ANSWERFaster melting raises sea levels (flooding coasts), first floods then reduces glacier-fed rivers, and lowers the Earth’s albedo (less ice to reflect sunlight → even more warming) — threatening water supplies and habitats.

Class 9 Science Exploration Chapter 13 Solutions — Revise, Reflect, Refine

1. Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem. (i) To provide food directly to all organisms. (ii) To recycle essential nutrients between biotic and abiotic components. (iii) To create new elements for use by living things. (iv) To remove pollutants and toxins from the organism.

ANSWERCorrect option: (ii) — biogeochemical cycles recycle essential nutrients (carbon, oxygen, nitrogen, water) between the living (biotic) and non-living (abiotic) parts of an ecosystem.

2. Which of the following is primarily responsible for the warming of the Earth? (i) Solar radiation is immediately absorbed by carbon dioxide, which then releases it as heat. (ii) The atmosphere’s tiny particles absorb incoming solar radiation, which directly heats the Earth. (iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases. (iv) The Earth’s environment is heated only by the solar radiation reflected by the clouds.

ANSWERCorrect option: (iii). The Earth’s surface absorbs sunlight and re-radiates it as heat (infrared); greenhouse gases trap this heat — the greenhouse effect.

3. Explain how climate change affects the water cycle. Illustrate with examples.

ANSWER A warmer climate speeds up evaporation, putting more water vapour into the air, which changes rainfall patterns — some regions get heavier, more intense rain and floods, while others face longer droughts. It also melts glaciers and ice faster and raises sea levels. Examples: heavier monsoon downpours and flash floods, prolonged droughts in some areas, and the rapid melting of Himalayan glaciers.

4. Describe how albedo affects the Earth’s surface temperature and its climate.

ANSWER Albedo is the fraction of sunlight a surface reflects. High-albedo surfaces (snow, ice, light surfaces) reflect most sunlight, so they absorb less and stay cooler; low-albedo surfaces (dark ocean, forests, soil) absorb more and become warmer. So albedo controls how much solar energy is absorbed. If ice melts, albedo falls, more heat is absorbed and warming increases — a feedback that affects regional and global climate.

5. How are mountain and valley breezes formed? If one mountain is grass-covered and another is barren rock, would the temperatures of the two mountain breezes differ? How?

ANSWER By day, the mountain slopes heat up faster than the valley; the warm air rises and air flows up the slope from the valley — a valley breeze (upslope). By night, the slopes cool faster, the cool dense air sinks and flows down into the valley — a mountain breeze (downslope). Yes, the temperatures would differ. Barren rock heats up (and cools) faster than grass, so the breeze from the barren mountain is warmer in the day (and cooler at night), while the grassy mountain, holding moisture, gives a more moderate breeze.

6. Which atmospheric layer is mainly responsible for weather phenomena such as winds, storms and rainfall, and what is the primary reason for its occurrence?

ANSWER The troposphere — the lowest layer of the atmosphere — is responsible for weather. It contains most of the air and almost all the water vapour, and it is heated unevenly from the Earth’s surface. This uneven heating sets up convection currents and pressure differences that cause winds, clouds, storms and rainfall.

7. Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?

ANSWER Nitrogen fixation: atmospheric N2 is converted into usable compounds (ammonia/nitrates) by nitrogen-fixing bacteria (e.g., Rhizobium), lightning and industry. Nitrification: bacteria change ammonia into nitrites and then nitrates. Assimilation: plants absorb nitrates to make proteins, and animals get nitrogen by eating plants. Ammonification: decomposers turn dead matter and waste back into ammonia. Denitrification: bacteria return nitrogen to the air as N2. If nitrogen were not cycled, plants could not get nitrogen to make proteins and DNA, so they could not grow, food chains would collapse, and life could not be sustained.

8. What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?

ANSWER With fewer trees there is less photosynthesis, so less oxygen is released and less CO2 is absorbed — atmospheric CO2 rises, strengthening the greenhouse effect and global warming. Other consequences: loss of habitat and biodiversity, soil erosion, a disturbed water cycle (less rainfall), floods, and desertification.

9. Explain with a suitable diagram the path that carbon takes to go back to the atmosphere, starting from plants using CO2.

ANSWER Plants take in CO2 from the atmosphere by photosynthesis and store carbon as food (biomass). Animals eat the plants, so carbon passes into animals. Carbon returns to the atmosphere as CO2 through respiration (by plants and animals), decomposition of dead organisms and waste by decomposers, and combustion (burning of wood and fossil fuels). Diagram (in words): Atmospheric CO2 → plants (photosynthesis) → animals (feeding) → back to atmospheric CO2 through respiration, decomposition and burning; fossil fuels formed from buried organisms also release CO2 when burnt.

10. Why is an excess of CO2 in the atmosphere considered undesirable even though plants need it?

ANSWER Plants use CO2 for photosynthesis, but they can absorb only a limited amount. Excess CO2 acts as a greenhouse gas and traps more heat, causing global warming and climate change — rising temperatures, melting ice, sea-level rise, extreme weather and ocean acidification. So an excess is harmful.

11. How is heat lost from the surface of the Earth? What is its significance?

ANSWER The warmed surface loses heat mainly by re-radiating it as infrared (long-wave) radiation, and also by conduction and convection to the air and by evaporation (latent heat). This heat loss balances the energy received from the Sun, keeping the Earth’s temperature steady. Greenhouse gases trap part of this outgoing radiation, keeping the planet warm enough for life.

12. If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?

ANSWER On a sphere, sunlight strikes the equator almost directly (concentrated, hot) and the poles at a slant (spread out, cold), creating temperature zones and varied climates. A flat disc facing the Sun would receive sunlight at nearly the same angle everywhere, so the heating would be much more uniform — there would be no strong equator-to-pole temperature gradient and none of the climate zones we have today.

13. Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?

ANSWER Cryosphere: glaciers, ice caps and snow melt and shrink, reducing the ice cover. Hydrosphere: sea levels rise (from melted ice and the thermal expansion of water), oceans warm and currents and rainfall patterns change. Biosphere: habitats shrink or shift, species migrate or face extinction, and ecosystems, breeding cycles and food supplies are disrupted (for example, coral bleaching).

14. Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive.

ANSWER The atmosphere acts like a blanket: greenhouse gases (CO2, water vapour, methane) absorb and re-radiate the heat given off by the Earth’s surface, keeping it warm. Without this greenhouse effect the Earth would be far too cold (about −18 °C). The atmosphere also moderates the difference between day and night temperatures and blocks harmful radiation, keeping the temperature in a range suitable for life.

15. Describe the interrelationship between the different spheres of the Earth. Illustrate with an example how these spheres function in a delicate balance.

ANSWER The lithosphere (land), hydrosphere (water), atmosphere (air) and biosphere (life) constantly interact and exchange matter and energy. Example: water from the hydrosphere evaporates into the atmosphere and falls as rain on the land (lithosphere); plants (biosphere) absorb this water and soil nutrients, and release water vapour and oxygen back into the atmosphere; animals breathe this air and drink the water. Because the spheres are linked, a change in one affects the others — for example, deforestation (biosphere/lithosphere) raises atmospheric CO2 and disturbs the water cycle — so they work in a delicate balance.

Common Mistakes to Avoid

Watch out for these

  • Thinking the atmosphere is heated directly by sunlight — it is mainly warmed by the heat re-radiated from the Earth’s surface.
  • Saying biogeochemical cycles “create” elements — they only recycle existing nutrients.
  • Confusing high albedo (reflects, cooler) with low albedo (absorbs, warmer).
  • Forgetting that the greenhouse effect is natural and necessary — only its excess causes harmful warming.
  • Mixing up the atmospheric layers — weather happens in the troposphere.
  • Treating the spheres as separate — they are closely interlinked, so a change in one affects the rest.

Extra Practice Questions

Very Short Answer Type Questions

Q1. Name the sphere of the Earth that contains all living things.

ANSWERThe biosphere.

Q2. Name two greenhouse gases.

ANSWERCarbon dioxide and methane (also water vapour).

Q3. What is the main source of energy for the Earth’s systems?

ANSWERThe Sun.

Short Answer Type Questions

Q1. State two important roles of the oxygen cycle.

ANSWERIt keeps oxygen available for respiration in living organisms, and it balances the oxygen released by photosynthesis with the oxygen used in respiration and combustion.

Q2. Why is the carbon cycle important for life?

ANSWERCarbon is the basis of all organic molecules; the cycle keeps recycling carbon between the atmosphere, living organisms and the Earth, ensuring a continuous supply for making food and body materials.

Long Answer Type Question

Q1. Explain the greenhouse effect and why it is both essential and a cause for concern.

ANSWER Sunlight reaches and warms the Earth’s surface, which then re-radiates the energy as infrared (heat) radiation. Greenhouse gases in the atmosphere absorb and re-radiate this heat, keeping the Earth warm enough for life — without it the Earth would be far too cold, so the effect is essential. However, human activities (burning fossil fuels, deforestation) increase greenhouse gases, trapping extra heat and causing global warming and climate change — which is why the excess is a serious concern.

MCQs & Assertion–Reason

1. The sphere of the Earth that contains all water is the:

(a) lithosphere    (b) hydrosphere    (c) atmosphere    (d) biosphere

2. Weather phenomena mainly occur in the:

(a) stratosphere    (b) troposphere    (c) mesosphere    (d) thermosphere

3. Biogeochemical cycles mainly:

(a) create new elements    (b) recycle nutrients    (c) destroy nutrients    (d) provide light

4. Which surface has the highest albedo?

(a) dark ocean    (b) forest    (c) fresh snow    (d) bare soil

5. The greenhouse effect is caused by gases that trap:

(a) visible light    (b) infrared (heat) radiation    (c) ultraviolet rays    (d) X-rays

6. Nitrogen-fixing bacteria convert:

(a) nitrates to N2    (b) N2 to usable compounds    (c) ammonia to oxygen    (d) CO2 to oxygen

7. Deforestation tends to:

(a) lower atmospheric CO2    (b) raise atmospheric CO2    (c) raise oxygen levels    (d) have no effect

8. Carbon returns to the atmosphere mainly through:

(a) photosynthesis    (b) respiration, decomposition and combustion    (c) condensation    (d) reflection

9. A rise in global temperature would cause the cryosphere to:

(a) expand    (b) melt and shrink    (c) stay the same    (d) become darker only

10. The ultimate source of energy for the Earth’s biogeochemical cycles is the:

(a) wind    (b) Sun    (c) ocean    (d) soil

Answer key: 1-(b), 2-(b), 3-(b), 4-(c), 5-(b), 6-(b), 7-(b), 8-(b), 9-(b), 10-(b).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: The greenhouse effect keeps the Earth warm enough for life.

Reason: Greenhouse gases trap the heat re-radiated by the Earth’s surface.

A-R 2. Assertion: Melting ice can increase global warming.

Reason: Melting ice lowers the albedo, so more solar energy is absorbed.

A-R 3. Assertion: Weather occurs in the troposphere.

Reason: The troposphere holds most of the air and water vapour and is heated from the Earth’s surface.

A-R 4. Assertion: Biogeochemical cycles create new nutrients for organisms.

Reason: Nutrients move only from non-living to living things and are not returned.

A-R 5. Assertion: Deforestation raises atmospheric carbon dioxide.

Reason: Fewer trees carry out less photosynthesis, so less CO2 is absorbed.

Answer key: 1-(A), 2-(A), 3-(A), 4-(D), 5-(A).

Quick Revision Summary

  • The Earth’s four spheres — lithosphere, hydrosphere, atmosphere, biosphere — interact and exchange energy and matter.
  • Energy flows from the Sun; the greenhouse effect keeps the Earth warm.
  • Albedo is the fraction of sunlight reflected; high for ice, low for dark surfaces.
  • Biogeochemical cycles (carbon, oxygen, nitrogen, water) recycle nutrients between living and non-living parts.
  • Weather occurs in the troposphere due to uneven heating.
  • Human activities (deforestation, burning fuels) raise greenhouse gases and drive climate change.

Real-life Applications

Understanding the Earth as a system guides real decisions: predicting weather and monsoons, managing water resources and farming, designing cities to reduce heat and pollution, protecting forests and wetlands, and framing climate policy to cut greenhouse-gas emissions and slow global warming.

How to score full marks in this chapter

Link the four spheres and explain interactions with clear examples. Remember that the atmosphere is warmed by heat re-radiated from the surface, that biogeochemical cycles recycle (not create) nutrients, and that the greenhouse effect is natural but its excess causes climate change.

Frequently Asked Questions

What is Class 9 Science Exploration Chapter 13 about?

Earth as a system — the four spheres and their interactions, the flow of energy, the greenhouse effect and albedo, the carbon, oxygen, nitrogen and water cycles, winds, and the effects of deforestation and climate change.

What are biogeochemical cycles?

They are natural pathways that recycle nutrients such as carbon, oxygen, nitrogen and water between the living and non-living parts of an ecosystem.

What is the greenhouse effect?

The trapping of the Earth’s re-radiated heat by greenhouse gases such as CO2 and water vapour, which keeps the planet warm enough for life.

Are these Class 9 Science Exploration Chapter 13 solutions free?

Yes. All solutions are free and follow the official NCERT Exploration textbook for 2026–27.

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