Where Mindful Learning Begins
Where Mindful Learning Begins
NCERT Class 9 Science Chapter 13 question answers for Earth as a System: Energy, Matter, and Life (CBSE 2026-27), from uneven heating and ocean currents to the carbon and nitrogen cycles, with every Think It Over, Activity, Pause & Ponder, Revise Reflect Refine and Journey Beyond question solved. Use it with your Class 9 Science Ch 13 notes.
As the closing chapter of Class 9 Science, Earth as a System ties together energy, cycles and ecosystems, and these Class 9 Science Chapter 13 NCERT solutions cover uneven heating and albedo, land, sea and mountain-valley breezes, ocean currents, the water, carbon, nitrogen and oxygen cycles, the greenhouse effect and temperature-with-height numericals, with the reasoning behind every answer.
Last updated 20 October 2026 · Reviewed by Suchita Arora, 13+ years teaching CBSE Maths & Science
Earth as a System: Energy, Matter, and Life is the closing chapter of Class 9 Science. It brings ideas from across the book into one picture of the Earth as five interacting spheres: geosphere, hydrosphere, cryosphere, atmosphere and biosphere. It builds from uneven heating and insolation, through local and planetary winds and ocean currents, to the water, carbon, nitrogen and oxygen cycles that keep matter moving between living and non-living systems, and finally to how human activity is disturbing this balance. Every question is solved here, section by section, as complete Class 9 Science Chapter 13 NCERT solutions with answers.
Geosphere, hydrosphere, cryosphere, atmosphere and biosphere — and how a disturbance in one ripples through all the others.
Insolation, albedo, atmospheric layers, local breezes, planetary wind belts, and ocean gyres like the Gulf Stream.
Water, carbon, nitrogen and oxygen cycles, and how fossil fuels, deforestation and fertiliser overuse disturb this balance.
Step 1: warmer sea water makes more water evaporate from the sea.
Step 2: the air above the sea gets more moisture, and the southwest monsoon winds carry it over India.
Step 3: the extra moisture and the changed heating of sea and land make the monsoon fluctuate.
Result: rainfall becomes variable. Some regions get floods and others face drought.
This is a link between the hydrosphere and the atmosphere.
Step 1: without trees there is less transpiration, so less water vapour goes to the air and local rainfall can fall.
Step 2: without leaves and roots, the rain falls directly on the soil. Less water seeps in (infiltration), and more water runs off.
Step 3: in the rainy season the river gets a sudden rush of water and floods. It also carries a lot of eroded soil.
Step 4: less water is stored in the ground, so in the dry months the river flow becomes weak or even dries up.
Step 1: melting glaciers and polar ice add water to rivers and oceans.
Step 2: the sea level rises.
Step 3: low-lying coastal cities (like Mumbai and Chennai) can get flooded, and the coast can erode.
Step 4: sea water can enter the groundwater and farm land, and people and wildlife lose their habitats.
Step 1: the ocean absorbs more CO₂ from the air.
Step 2: the sea water becomes more acidic.
Step 3: tiny plankton and shell-forming organisms find it difficult to build their shells and to grow. Coral reefs are also harmed.
Step 4: plankton are at the base of the marine food chain, so fish and other sea life also suffer.
Step 1: one example of each sphere in Fig. 13.1:
| Sphere | Example in the figure |
|---|---|
| Geosphere | Rocks, soil and the mountain |
| Hydrosphere | The lake |
| Cryosphere | Snow on the mountain tops |
| Atmosphere | Air and clouds in the sky |
| Biosphere | Grass, sheep and the shepherd |
Step 2: the Sun’s heat melts the snow. The melt water flows down the slope as streams and joins the lake.
Step 3: less snowfall gives less melt water, so the level of the lake falls in summer.
Less water in the lake and the soil means less grass grows. So the sheep get less food.
Step 4: the spheres are interconnected:
Snow (cryosphere) → melt water fills the lake (hydrosphere).
Lake water → wets the soil (geosphere) → grass grows (biosphere).
Sun’s heat, wind and clouds (atmosphere) → decide how fast snow melts and when it rains.
A change in one sphere changes the others.
| S. No. | Material | Albedo |
|---|---|---|
| 1. | Snow | 0.80 – 0.90 |
| 2. | Ice | 0.50 – 0.70 |
| 3. | Crushed rock | 0.25 – 0.30 |
| 4. | Light coloured soil | 0.25 – 0.45 (typical) |
| 5. | Black soil | 0.05 – 0.15 (typical) |
| 6. | Ocean water | 0.05 – 0.10 (typical) |
Pattern: light coloured surfaces (snow, ice) have a high albedo. They reflect most of the sunlight and stay cool.
Dark surfaces (black soil, ocean water) have a low albedo. They absorb more sunlight and get warmer.
This is why polar regions covered with snow and ice are very cold.
Step 1: open the simulation and keep the greenhouse gas concentration at a low value. Note the surface temperature.
Step 2: increase the concentration (for example from ‘none’ to ‘lots’). Note the temperature again.
Observation: more infrared radiation re-radiated by the surface is absorbed and sent back by the gas molecules. Less of it escapes to space.
Result: the surface temperature rises as the concentration increases.
Step 1: the mountain breeze is cool, dense air that flows down into the valley after sunset.
Step 2: it lowers the night temperature of the valley and reduces heat stress on crops.
Step 3: cool air holds the moisture, so dew can form. This adds moisture to the soil and the plants.
Step 4: there is less loss of soil water by evaporation. So the soil stays moist and healthy, and crops grow better.
Step 1: as the warm water moves towards the poles, it loses heat to the air and cools.
Step 2: the cooled water becomes denser and flows back towards the equator at deeper levels.
Impact: the warm current carries heat from the equator to the poles, so the temperature difference between them is reduced.
The areas it flows past become warmer. For example, the North Atlantic Drift keeps the ports of north-western Europe ice-free in winter.
It also carries nutrients, which support a large number of marine organisms.
Step 1: warmer ocean water can hold less dissolved CO₂, so the ocean absorbs less CO₂ from the air.
Step 2: more CO₂ stays in the atmosphere and warms the Earth even more.
Step 3: the balance of carbonate and bicarbonate ions in the water changes. Plankton and shell-forming animals cannot make their shells and food properly.
Step 4: warm water also holds less oxygen, and corals get stressed. Fish and other sea animals lose food and habitat.
Biogeochemical cycles return essential materials to the air, water and soil so that living things can use them again.
If they stop, the materials are used up and are not replaced. Examples:
1. Carbon cycle: plants would not get CO₂ for photosynthesis. No food or oxygen would be made.
2. Nitrogen cycle: soil nitrates would be used up. Plants could not make proteins, and animals would not get protein.
3. Water cycle: there would be no rain and no fresh water for living things.
4. Oxygen cycle: oxygen used in respiration would not be restored, so animals could not breathe.
How human activities increase greenhouse gases:
1. Burning fossil fuels (coal, petrol, diesel) in vehicles, homes and power plants releases CO₂.
2. Deforestation reduces the CO₂ absorbed by plants, and burning or rotting of trees releases CO₂.
3. Cattle farming, paddy fields and garbage in landfills release methane (CH₄).
4. Industries and the overuse of fertilisers release other greenhouse gases.
The graph shows the result: CO₂ rose from 315 ppm to 420 ppm.
What I can do as an individual:
1. Save electricity (switch off lights and fans, use LED bulbs).
2. Walk, cycle, or use public transport or car pools.
3. Plant trees and look after them.
4. Reduce, reuse and recycle, and avoid wasting food and water.
5. Prefer solar and other renewable energy.
Given: insolation = 1 kW m⁻² = 1000 J s⁻¹ m⁻², area = 1 m², time = 1 h = 3600 s
To find: energy received, E
E = intensity × area × time
This is equal to 1 unit of electricity (1 kWh) used in a household.
Data and assumptions (approximate, check the latest figures):
India uses about 1.8 × 10¹² kWh of electricity in a year.
Average solar energy on 1 m² in a day (day and night, all seasons) = about 5 kWh.
Efficiency of a solar panel = 20%.
Step 1: energy received by 1 m² in a year:
Step 2: electricity produced by 1 m² of panel in a year:
Step 3: area of panels required:
Step 4: compare with the Thar desert, whose area is about 2 × 10⁵ km².
Even if we double the area for spacing and losses, we need only about 5% of the Thar desert.
Step 1: plants would not make glucose. Food chains would collapse, because all organisms depend on plants for food.
Step 2: no oxygen would be added to the air. Respiration and burning would keep using up the oxygen.
Step 3: CO₂ would not be removed from the air, and its level would rise. This would strengthen the greenhouse effect and warm the Earth.
Step 4: the plants and animals would die, and the carbon, nitrogen and oxygen cycles would be disturbed.
(i) wrong: the cycles do not give food. Only producers (plants) make food.
(ii) correct: the cycles move carbon, nitrogen, oxygen and water between the living and non-living parts again and again.
(iii) wrong: elements cannot be created. They are only recycled.
(iv) wrong: cycles do not remove toxins from an organism.
(i) wrong: CO₂ does not absorb most of the incoming visible light. It absorbs the heat (infrared) re-radiated by the Earth.
(ii) wrong: the particles absorb only a part of the sunlight. They do not directly heat the Earth.
(iii) correct: the surface absorbs sunlight and re-radiates it as infrared. Greenhouse gases trap part of this heat.
(iv) wrong: clouds reflect sunlight back to space, so they do not heat the Earth.
Step 1: a warmer atmosphere holds more moisture. So some regions get heavier rain (for example, intensified monsoons), and other regions get drought.
Step 2: melting glaciers add more water to the rivers. In the long run, the sea level rises and threatens coastal cities like Mumbai and Chennai.
Step 3: sudden, intense rain gives more run-off and soil erosion, and less infiltration.
Step 4: less infiltration means less recharge of groundwater. This makes farming difficult in the dry months.
So the water cycle links the cryosphere (glaciers), hydrosphere (rivers, oceans), atmosphere (moisture), geosphere (soil erosion) and biosphere (crops, fisheries).
Albedo is the fraction of solar radiation reflected by a surface.
High albedo (snow 0.80 – 0.90, ice 0.50 – 0.70): most light is reflected, and little is absorbed. The surface stays cool.
Low albedo (black soil, ocean water): most light is absorbed, and the surface warms.
Effect on climate: polar regions covered with snow and ice are very cold. Dark roads and dark soil heat up quickly.
If ice melts, the dark water or land below absorbs more sunlight. This warms the surface further and melts more ice.
So the albedo of the surfaces of the Earth helps to decide the temperature of a region and the climate.
Valley breeze (day):
1. The slopes facing the Sun heat up faster than the valley floor.
2. The air above the slopes becomes warm and rises, and a low pressure region is formed.
3. Cooler air from the valley moves up the slope to take its place. This is the valley breeze.
Mountain breeze (night):
1. After sunset the slopes lose heat faster and become cooler than the valley floor.
2. The air over the slopes becomes cool and dense, and flows down into the valley. This is the mountain breeze.
Grass-covered and barren mountains: yes, the temperatures of the two mountain breezes will be different.
A barren rocky mountain heats up strongly in the day, but it also loses its heat quickly at night. It has no plants, no moisture and no cover to slow the loss of heat.
So the air over it becomes colder, and its mountain breeze is colder.
A grass-covered mountain stays cooler in the day (shade and transpiration), and it loses heat slowly at night. Its mountain breeze is less cold.
Layer: the troposphere (0 – 12 km above the surface).
Reason:
1. The troposphere is heated from the Earth’s surface, which absorbs sunlight and gives out heat.
2. The air near the ground is warm and the air above it is colder, as the temperature falls with height (about 6.5 °C per km).
3. Warm air rises and cold air sinks, so the air keeps mixing vertically. This drives winds and storms.
4. The troposphere has most of the water vapour, so evaporation, condensation and rain take place here.
In the stratosphere, the temperature rises with height. It is calm, so weather stays in the troposphere.
1. Nitrogen fixation: bacteria like Rhizobium (in root nodules of legumes) and Azotobacter (in soil) change atmospheric N₂ into ammonia (NH₃). Lightning also fixes nitrogen.
2. Nitrification: Nitrosomonas change ammonia into nitrite (NO₂⁻), and Nitrobacter change nitrite into nitrate (NO₃⁻).
3. Assimilation: plants absorb nitrates from the soil. Animals get nitrogen by eating plants or other animals.
4. Ammonification: when plants and animals die or give out waste, decomposers (bacteria and fungi) change the organic matter into ammonia.
5. Denitrification: denitrifying bacteria like Pseudomonas change some nitrates back into N₂ gas, which goes to the air.
If nitrogen were not cycled:
The soil nitrates would be used up, and plants could not make proteins and nucleic acids.
Plants would die, animals would have no food, and life would end.
Impact on the oxygen cycle: fewer trees mean less photosynthesis, so less oxygen is added to the air.
Impact on the carbon cycle:
1. Less CO₂ is absorbed by plants.
2. Burning or decay of the cut trees releases CO₂.
3. So the CO₂ in the air increases, and the greenhouse effect gets stronger.
Other consequences:
1. Less transpiration, so local rainfall can decrease.
2. The albedo of the surface changes.
3. Soil erosion increases, as there are no roots to hold the soil.
4. Habitats are destroyed, so biodiversity declines.
Step 1: plants take CO₂ from the air and make glucose by photosynthesis, using sunlight.
Step 2: animals eat the plants (or other animals) and get carbon.
Step 3 (respiration): plants and animals use glucose for energy and release CO₂ into the air.
Step 4 (decomposition): when they die, decomposers break down the bodies and release CO₂ into the air.
Step 5 (burning of fuels): over millions of years, buried dead organisms form coal, oil and gas. When these are burnt, CO₂ is released quickly into the air.
Step 6 (ocean exchange): the atmosphere and the ocean continuously exchange CO₂.
A small amount of CO₂ is necessary. Plants need it for photosynthesis and it keeps the Earth warm enough for life.
Excess CO₂ is bad, because:
1. It traps more heat and strengthens the greenhouse effect, causing global warming.
2. Glaciers and Arctic ice melt, and the sea level rises.
3. It causes more extreme weather and affects monsoons and farming.
4. More CO₂ dissolves in the ocean, which becomes more acidic and harms plankton and corals.
The CO₂ level rose from 315 ppm (1960) to 420 ppm (2025), which is an increase of about 33%.
How heat is lost:
1. The Earth’s surface absorbs sunlight and re-radiates the heat as infrared radiation.
2. Part of this heat is absorbed by greenhouse gases (CO₂, CH₄, water vapour) and sent back. The rest escapes to space.
3. The warm air rising from the surface and the evaporation of water also carry heat upwards.
Significance:
1. The heat lost balances the heat received from the Sun, so the average temperature stays steady.
2. Because greenhouse gases trap a part of it, the Earth stays warm enough for life.
3. If too much heat were trapped, the Earth would warm up. If all of it escaped, the Earth would be too cold.
On the sphere: the Sun’s rays strike different latitudes at different angles. At the equator the radiation is concentrated over a small area. At the poles it is spread over a large area.
So the equator is hot and the poles are cold.
On a flat disc facing the Sun:
1. The rays would fall at the same angle everywhere. All places would receive nearly equal radiation.
2. The temperature would be nearly the same everywhere, with no hot equator and cold poles.
3. With no uneven heating, there would be no pressure belts, no planetary winds and no ocean currents.
4. There would be no climate zones like the tropical and polar regions.
| Sphere | Effect of a rise in temperature |
|---|---|
| Cryosphere | Glaciers, snow and polar ice melt faster. Less ice means a lower albedo, so more warming |
| Hydrosphere | Sea level rises and low-lying areas flood. More evaporation changes the rain. Rivers get floods and droughts |
| Biosphere | Habitats are lost (coasts, glaciers, corals). Crops and fisheries are affected, and species may disappear |
1. It absorbs part of the incoming radiation. The ozone layer blocks harmful UV rays. Clouds and gases also absorb or reflect some sunlight.
2. It traps outgoing heat. The surface re-radiates infrared. Greenhouse gases (CO₂, CH₄, water vapour) absorb it and keep the Earth warm.
3. It spreads heat around. Winds and the water cycle move heat from the warm to the cold regions.
Without the atmosphere the average temperature would be about −18 °C, and the Earth would be too cold for life. With it, the average temperature is about 15 °C.
Excess greenhouse gases would make it too hot, so the balance matters.
The Earth has five interacting spheres: geosphere (rocks, soil), hydrosphere (water), cryosphere (ice and snow), atmosphere (air) and biosphere (living organisms).
Energy and matter move between them, so a change in one sphere changes the others.
Example 1 (monsoon): warm Arabian Sea water (hydrosphere) → more evaporation → moisture in the air (atmosphere) → rain on land (geosphere) → crops grow (biosphere).
Example 2 (rise in temperature): warming (atmosphere) → melting glaciers (cryosphere) → sea level rises (hydrosphere) → flooded coasts and habitat loss (biosphere).
Example 3 (deforestation): cutting trees (biosphere) → less rain (atmosphere) → soil erosion (geosphere) → muddy rivers and floods (hydrosphere).
A small disturbance in one sphere can grow into a big change in the others.
The Sun heats the equator more, so on both planets warm air rises at the equator and cold air sinks at the poles. This gives global pressure belts and planetary winds (deflected by rotation).
| Earth (land and sea) | Ocean planet | Land planet | |
|---|---|---|---|
| Heating | Uneven: land and water heat differently | Even and slow, since water heats and cools slowly | Fast and large changes in day and night |
| Local winds | Land and sea breezes, mountain and valley breezes | No land and sea breezes | Strong local winds on the hot ground |
| Wind pattern | Planetary belts disturbed by continents and monsoons | Regular, smooth planetary belts | Strong, gusty, dusty winds |
| Ocean currents | Gyres, blocked by continents | Currents flow freely around the planet | None |
The winds on the ocean planet would be the steadiest. The winds on the land planet would be the most extreme, with large temperature swings and little moisture.
Meal chosen: roti and dal (sample answer).
| Item | (i) Carbon from the air | (ii) Nitrogen into the plant |
|---|---|---|
| Roti (wheat) | Wheat plants took CO₂ from the air and made glucose by photosynthesis. It was stored as starch in the grain | The soil had nitrates from fertiliser (made by the Haber–Bosch process) and from soil bacteria. The roots absorbed them to make the protein |
| Dal (a pulse) | The plant used CO₂ in the same way to make carbohydrates and proteins | Rhizobium bacteria in the root nodules fixed N₂ of the air into ammonia. The plant used it to make the protein |
Human activities that add CO₂ or nitrogen to the environment:
1. Tractors and pumps burn diesel, which gives CO₂.
2. Making fertiliser needs a lot of energy (CO₂), and overuse of fertiliser washes nitrates into the rivers.
3. Transport of grain and dal by trucks releases CO₂.
4. Cooking on an LPG or wood stove releases CO₂.
5. Burning of crop residue releases CO₂.
Steps:
1. Download the June – September rainfall of your city for 5 years in the 1980s and 5 years in the 2020s.
2. Find the average of each decade, and count the days with rain of more than 50 mm.
3. Compare the two decades in a table.
Table format (the numbers are only illustrative, replace them with your own data):
| Decade | Average monsoon rainfall | Days with heavy rain (> 50 mm) |
|---|---|---|
| 1980s | about 600 mm | about 3 days |
| 2020s | about 620 mm | about 6 days |
Trend: in many places the total rainfall changes little, but the number of heavy-rain days increases. This means the rain is more intense.
Connection:
1. A warmer Arabian Sea gives more evaporation. Warm air holds more moisture and gives heavier rain in short bursts.
2. Forests turn to farms and cities, so there is less transpiration, less infiltration and more run-off. This makes floods more likely.
New tools: satellites, ocean buoys, drones, sensors, ice cores, and supercomputer and AI models.
What they can reveal:
1. How fast glaciers, polar ice and forests are changing, and how the sea level is rising.
2. Where CO₂ and methane are produced and absorbed, and how much the oceans absorb.
3. How the sea temperature, winds and monsoon are connected.
4. Hidden links, such as how ice melt changes ocean currents and weather.
How this helps:
1. Better forecasts of the monsoon, cyclones and floods, and early warnings.
2. More accurate models of future warming.
3. Better decisions on energy, farming and protecting forests and coasts.
The Earth works as one connected system — the geosphere, hydrosphere, cryosphere, atmosphere and biosphere constantly exchange energy and matter through uneven heating, winds, ocean currents, and the water, carbon, nitrogen and oxygen cycles, and a disturbance anywhere (like excess CO₂ from human activity) ripples through the whole system rather than staying contained in one place.
Seven extra questions in the style of the textbook, for independent practice. Attempt each one on paper first, then tap to check.
Reason:
1. The trees are gone, so rain falls directly on the soil. There are no leaves to slow it down and no roots to hold the soil.
2. Less water seeps into the ground (infiltration), and most of it runs off quickly into the stream. So the stream floods in the rainy season.
3. Less water is stored in the ground (groundwater recharge). So in summer there is not enough water to feed the stream and it nearly dries up.
4. Eroded soil also silts up the stream.
Ways to reduce the problem:
1. Plant trees (afforestation) along the road and the banks of the stream.
2. Build check dams or contour bunds, and dig recharge pits so that water seeps into the ground.
Given: T at sea level = 30 °C, rate = 6.5 °C per km, height = 8.8 km
Fall in temperature = rate × height
Temperature at the top = 30 − 57.2
Given: height = 12 km
Fall in temperature
Temperature at 12 km = 30 − 78
Step 1: black roofs have a low albedo, so they absorb most of the sunlight and become very hot.
Step 2: white roofs have a high albedo. They reflect most of the sunlight, so they absorb less heat and stay cooler.
Step 3: cooler roofs give out less heat (re-radiation) to the air. This reduces the urban heat island effect, and the city becomes cooler.
Step 4: the buildings stay cooler inside, so less air conditioning is used. This saves electricity, and so less CO₂ is released by power plants.
Disadvantages:
1. In winter the white roofs also reflect the useful sunlight, so heating needs may increase in cold regions.
2. The effect on the global temperature is small, and the white paint needs cleaning, because it gets dirty and its albedo falls.
Given: area = 20 m², insolation = 1 kW m⁻², time = 5 h
Energy = insolation × area × time
20% of the energy is converted.
1 kWh = 3.6 × 10⁶ J
Power of the fan = 100 W = 0.1 kW
Increase = 420 − 315 = 105 ppm
Time = 2025 − 1960 = 65 years
More rise needed = 450 − 420 = 30 ppm
Year = 2025 + 19 = 2044. The real rise is getting faster, so 450 ppm may be reached earlier.
The yearly dips: in the growing season, plants in the Northern Hemisphere take in more CO₂ by photosynthesis. So the CO₂ falls a little each year.
The rising trend: the CO₂ added by burning fossil fuels and deforestation is more than what plants and oceans can absorb. So, year after year, the CO₂ increases overall.
Hydrosphere: the lake level falls. The groundwater around it also falls, and streams that feed the lake or flow from it dry up.
Biosphere: fish and water plants die. Birds and animals that depend on the lake leave or die. Farmers have less water for irrigation, so crops fail.
Geosphere: the exposed lake bed dries and cracks. The soil is blown away as dust, and there is more erosion.
Atmosphere: there is less evaporation, so the air is drier and the chance of local rain falls. Dust in the air increases, and the land around becomes hotter than before, as water is not there to cool it.
Chain: the drought can feed on itself. Less water → less vegetation → less transpiration → less rain → more drought.
Step 1: solar radiation (UV, visible and infrared) reaches the Earth. The ozone layer absorbs the harmful UV.
Step 2: the visible light and infrared reach the surface. The land and water absorb it and warm up.
Step 3: the warm surface re-radiates the energy as infrared (heat) radiation.
Step 4: greenhouse gases (CO₂, CH₄, water vapour) absorb a part of this outgoing heat and send some of it back to the surface.
Step 5: the trapped heat keeps the lower atmosphere warm, with an average of about 15 °C. Without it, it would be about −18 °C.
Step 6: but with more greenhouse gases from human activities, more heat is trapped, and this causes global warming.
That's all 13 chapters of Class 9 Science (Exploration) covered. Revisit any earlier chapter, browse the full Class 9 Science hub, or revise with the Class 9 Science Notes, or test yourself with the free Class 9 Science Sample Paper and Marking Scheme.
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