Earth as a System: Energy, Matter, and Life Class 9 Notes and Questions

What Earth as a System: Energy, Matter, and Life is about

Weightage: This chapter is part of Unit IV, Earth as a System, which carries 5 of the 80 theory marks in the school annual exam.Source: CBSE Science (086) syllabus, Class IX 2026-27, course structure, Unit IV Earth as a System

Key concepts

The Earth's interacting spheres

NCERT section 13.1 Uneven Heating of the Earth

The Earth works as one system of five connected 'spheres':

Sphere	What it includes	Indian examples
Geosphere	rocks, soil, landforms, Earth's interior	Deccan plateau, Thar desert
Hydrosphere	liquid water: oceans, rivers, lakes, groundwater	Ganga–Brahmaputra system
Cryosphere	ice and snow	Himalayan glaciers, snow in Ladakh, polar ice
Atmosphere	the layer of air	cleaner air in hills and forests
Biosphere	all living things and their habitats	mangroves, forests, farms, plankton, coral reefs

Energy comes mainly from the Sun, with some from the Earth's hot interior and chemical reactions. Solar heating, moving air and water, and nutrient cycling link the spheres in a delicate balance.

A change in one sphere spreads to others (Activity 13.1): less winter snow (cryosphere) → less water in a lake in summer (hydrosphere) → less grass for sheep (biosphere). On a large scale: a warmer Arabian Sea → more evaporation → an erratic south-west monsoon → floods in some places and droughts in others; warmer air → faster melting of glaciers → floods, rising sea levels and loss of coastal habitats.

Worked example

A large forest on a hillside is cleared. Trace the effects through at least three spheres.

Try it first, then show the solutionSolution

1. Biosphere: habitats and species are lost; less photosynthesis and transpiration.
2. Atmosphere: less water vapour from leaves, so local rainfall can fall; more CO${}_2$ stays in the air.
3. Geosphere: without roots, rain washes soil away (erosion).
4. Hydrosphere: more runoff and less infiltration, so the river floods in the rains, carries more silt, and runs low in dry months.

Answer: Loss of life and habitat, drier local air with more CO${}_2$, soil erosion, and a river that floods then runs low.

Solar radiation and the electromagnetic spectrum

NCERT section 13.1 Uneven Heating of the Earth

Sunlight reaches the Earth as electromagnetic (EM) waves, which, unlike sound, need no medium and travel through vacuum at the speed of light, $3\times 10^8$ m s${}^{-1}$.

The electromagnetic spectrum, from high frequency (short wavelength, high energy) to low frequency (long wavelength):
gamma rays → X-rays → ultraviolet (UV) → visible → infrared (IR) → microwaves → radio waves

• About 99% of the Sun's energy reaching the Earth is UV, visible and IR.
• Gamma rays and X-rays are mostly filtered out high in the atmosphere; microwaves and radio waves carry too little energy to warm the Earth much.
• UV (100 to 400 nm): mostly absorbed by the ozone layer; too much damages skin and eyes and can cause cancer (use sunglasses and sunscreen); also used to kill germs in water purifiers.
• Visible light reaches the surface, powers photosynthesis and warms land and water.
• IR warms the surface; the surface then gives out IR, part of which greenhouse gases (CO${}_2$, CH${}_4$, water vapour) trap.

Insolation = solar radiation reaching the Earth's surface. Solar constant ≈ **1.4 kW m${}^{-2}$** (1400 J s${}^{-1}$ m${}^{-2}$) at the top of the atmosphere, on a surface facing the Sun. At the ground on a clear day, at most about **1 kW m${}^{-2}$. Anna Mani** mapped India's sunshine and published *Solar Radiation Over India* (1982, with S. Rangarajan).

Worked example

A 2 m${}^2$ solar panel receives 1 kW m${}^{-2}$ for 5 hours. How much solar energy falls on it in joules and in 'units' (kWh)? If it converts 20% to electricity, how many units does it produce?

Try it first, then show the solutionSolution

1. Power on panel $=1000\times 2=2000$ W.
2. Energy $=2000\times 5\times 3600=3.6\times 10^7$ J $=10$ kWh (1 unit $=3.6\times 10^6$ J).
3. Electricity $=0.2\times 10=2$ units.

Answer: $3.6\times 10^7$ J (10 units); 2 units of electricity.

Uneven heating: albedo and urban heat islands

NCERT section 13.1 Uneven Heating of the Earth

Different surfaces absorb sunlight differently. Land heats (and cools) faster than water; dark surfaces absorb more than light ones (dark roads and dark clothes get hotter).

Albedo (Latin for 'whiteness') is the fraction of sunlight a surface reflects.

Surface	Albedo (approx.)
fresh snow	0.80 to 0.90
ice	0.50 to 0.70
crushed rock	0.25 to 0.30
light soil	about 0.3
black soil, ocean water	low (about 0.05 to 0.15)

• High albedo → reflects more → stays cooler (polar ice keeps poles cold).
• Low albedo → absorbs more → gets warmer.
• A feedback: when ice melts, darker water or land is exposed, which absorbs more heat and melts more ice.

All objects re-radiate heat. Concrete houses stay hot at night because concrete gives back stored heat; thick mud and wood walls stay cooler.

Urban heat island: cities (concrete, brick, steel, asphalt; little greenery) absorb and re-radiate more heat, so they are warmer than nearby villages, especially at night. This raises air-conditioning demand. Trees cool through shade and transpiration.

Worked example

Sunlight of 1000 W m${}^{-2}$ falls on fresh snow (albedo 0.85) and on black soil (albedo 0.10). How much power per square metre does each absorb? Explain why melting Arctic ice speeds up warming.

Try it first, then show the solutionSolution

1. Snow absorbs $1000\times (1-0.85)=150$ W m${}^{-2}$.
2. Black soil absorbs $1000\times (1-0.10)=900$ W m${}^{-2}$.
3. When ice melts, it exposes dark ocean with low albedo, which absorbs far more energy, warming the water and melting more ice.

Answer: 150 and 900 W m${}^{-2}$; melting replaces a reflector with an absorber, so warming speeds up.

Latitude and the atmosphere

NCERT section 13.1 Uneven Heating of the Earth

Latitude and shape: the Earth is a sphere, so the Sun's rays strike the equator nearly straight on, concentrating energy on a small area, but strike the poles at a slant, spreading the same energy over a much larger area. So the equator stays warm and the poles cold. The tilt of the axis adds seasons and changing day length.

The atmosphere is held by gravity: about 78% nitrogen, 21% oxygen, with argon, CO${}_2$, water vapour and others.

Layer	Height	Features
Troposphere	0 to about 12 km (highest over the equator, lowest over the poles)	almost all weather; heated from the ground; temperature falls about 6.5 °C per km
Stratosphere	12 to 50 km	ozone layer absorbs UV; temperature rises with height; little vertical mixing, so weather stays below
Mesosphere, thermosphere, exosphere	above	small role in surface climate; space begins at about 100 km

Two protective roles of the atmosphere:
1. It absorbs part of the incoming radiation; ozone blocks harmful UV.
2. It traps outgoing IR (the greenhouse effect), keeping the Earth warm enough for life. Extra CO${}_2$ strengthens this, causing global warming. Venus, with a thick CO${}_2$ atmosphere, is hotter than Mercury though farther from the Sun.

Ozone hole: CFCs from old refrigerators and aerosols thinned ozone over Antarctica; the Montreal Protocol cut CFCs and the layer is slowly recovering. K. R. Ramanathan measured ozone in the Himalayas in 1934.

Worked example

The ground temperature at a hill station is 30 °C. Using a drop of 6.5 °C per km, estimate the temperature 8 km above it. Why does this pattern reverse in the stratosphere?

Try it first, then show the solutionSolution

1. Drop $=6.5\times 8=52$ °C.
2. Temperature $\approx 30-52=-22$ °C.
3. In the stratosphere, ozone absorbs UV and heats the air, so temperature rises with height.

Answer: About −22 °C; ozone heating makes the stratosphere warmer higher up.

Local winds: valley and mountain breezes

NCERT section 13.2 Uneven Heating Causes Wind and Ocean Currents

Wind is air moving from high pressure to low pressure. Pressure differences come mainly from uneven heating: warm air expands, becomes less dense and rises, leaving low pressure; cool, dense air sinks, making high pressure. Land and sea breezes work this way.

Valley breeze (day): Sun-facing slopes heat up faster than the valley floor. Warm air over the slopes rises (low pressure), and cooler air from the valley flows up the slopes.

Mountain breeze (night): slopes lose heat faster and become cooler than the valley floor. Cool, dense air on the slopes flows down into the valley.

These daily reversals are felt in places like Shimla and Dehradun; they affect temperature, moisture, frost and crops.

A grass-covered slope (more moisture, transpiration, higher albedo than bare dark rock) heats less by day, so its valley breeze is gentler; bare rock heats and cools more strongly.

Worked example

In a Himalayan village, smoke from cooking fires drifts up the hillside at noon but settles down into the valley late at night. Explain both.

Try it first, then show the solutionSolution

1. At noon, slopes are warmer than the valley floor; air over them rises and valley air flows up the slope (valley breeze), carrying smoke upwards.
2. At night, slopes cool faster; cold dense air slides down into the valley (mountain breeze), carrying smoke down where it collects.

Answer: Valley breeze by day (upslope), mountain breeze by night (downslope).

Planetary winds and ocean currents

NCERT section 13.2 Uneven Heating Causes Wind and Ocean Currents

Pressure belts and planetary winds:

• Equatorial low (0°): strong heating; warm air rises.
• The risen air moves towards the poles high up, cools and sinks near 30° N and S: sub-tropical high pressure belts. Surface air flows back to the equator, completing a loop.
• Some surface air from 30° moves poleward and rises near 60° N and S: sub-polar low, where it meets cold polar air.
• Poles (90°): cold, dense air sinks: polar high; it flows towards 60°, completing another loop.
• The Earth's rotation deflects winds: to the right in the Northern Hemisphere, to the left in the Southern Hemisphere, so they curve.

Ocean currents: continuous movements of large masses of sea water.

• Planetary winds drag surface water by friction.
• Differences in temperature and salinity (density) matter: warm water flows poleward at the surface; cold, salty, denser water sinks and flows back at depth.
• Rotation deflects currents into great loops called gyres: clockwise in the Northern Hemisphere, anticlockwise in the Southern. Continents redirect them.
• Currents carry heat from the equator to the poles. The Gulf Stream and its extension, the North Atlantic Drift, keep north-west European ports ice-free in winter. Currents also carry nutrients that support fisheries.

The Indian Institute of Tropical Meteorology (IITM), Pune models how air, ocean, land and ice interact to forecast the monsoon.

Worked example

Why does air sink around 30° latitude, and why do winds blowing from there towards the equator not move straight north–south?

Try it first, then show the solutionSolution

1. Air rising at the equator moves poleward at height, cools and becomes denser.
2. Around 30° it sinks, creating the sub-tropical high-pressure belt.
3. As it flows back towards the equatorial low, the Earth's rotation deflects it (right in the north, left in the south), so the winds curve.

Answer: Cooled, dense air sinks at 30°; the Earth's rotation bends the return flow.

Biogeochemical cycles: water and carbon

NCERT section 13.3 Biogeochemical Cycles

Matter cycles between living (biotic) and non-living (abiotic) parts of the Earth. These biogeochemical cycles keep carbon, nitrogen, oxygen and water available for life and help ecosystems recover from disturbance.

Water cycle: evaporation (oceans, lakes) and transpiration (plants) → condensation into clouds → precipitation (rain, hail, snow) → runoff to rivers and oceans and infiltration into groundwater. Water dissolves minerals and carries nutrients to oceans. Climate change: warmer air holds more moisture, so some areas get heavier rain and others drought; melting glaciers raise rivers and, over time, sea level (threatening Mumbai and Chennai); intense bursts of rain cause erosion and less groundwater recharge.

Carbon cycle: carbon is in every protein, carbohydrate, fat and DNA molecule; it moves among atmosphere (CO${}_2$), biosphere, geosphere (carbonate rocks, fossil fuels) and hydrosphere (dissolved CO${}_2$, shells).

• Fast cycle (days to years): photosynthesis takes in CO${}_2$: $6{\text{CO}}_2+6{\text{H}}_2\text{O}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$; respiration and decomposition return it.
• Slow cycle (millions of years): buried remains become coal, oil and gas; burning them releases CO${}_2$ quickly: ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}$.
• Oceans hold about 71% of the world's carbon and exchange CO${}_2$ with the air; plankton and shell-builders use dissolved carbon. The atmosphere holds only about 1%.
• The Keeling curve shows CO${}_2$ rising from about 315 ppm (1960) to 420 ppm (2025), with yearly dips as northern plants grow.

Worked example

Atmospheric CO${}_2$ rose from about 315 ppm in 1960 to about 420 ppm in 2025. Find the percentage increase. Why does the curve rise and fall a little every year?

Try it first, then show the solutionSolution

1. Increase $=420-315=105$ ppm.
2. Percentage $=\frac{105}{315}\times 100\approx 33\%$, about one-third.
3. Most land plants are in the Northern Hemisphere; in its summer they absorb more CO${}_2$ by photosynthesis, so levels dip, then rise again in winter.

Answer: About a one-third rise; seasonal plant growth causes the yearly wiggle.

Nitrogen and oxygen cycles

NCERT section 13.3 Biogeochemical Cycles

Nitrogen is needed for proteins and nucleic acids. The air is the largest store (78% N${}_2$), but N${}_2$ is unreactive, so organisms cannot use it directly.

Step	What happens	Organisms or agents
Nitrogen fixation	N${}_2$ → ammonia	*Rhizobium* in root nodules of legumes; *Azotobacter* in soil; lightning (oxides); industry (Haber-Bosch)
Nitrification	ammonia → nitrite → nitrate	*Nitrosomonas* (to nitrite), *Nitrobacter* (to nitrate)
Assimilation	plants absorb nitrates and ammonium and make proteins; animals eat plants	plants, animals
Ammonification	dead remains and wastes → ammonia	decomposer bacteria and fungi
Denitrification	nitrates → N${}_2$ back to air	*Pseudomonas*

The Haber-Bosch process makes ammonia from air: ${\text{N}}_2+3{\text{H}}_2\to 2{\text{NH}}_3$. It provides most fertiliser ('bread from air'), enabled the Green Revolution, but uses 1 to 2% of world energy, and fertiliser overuse harms soil and water.

Oxygen cycle: about 21% of air is O${}_2$; oxygen is also in water, CO${}_2$, rocks and biological molecules.

• Used in respiration, combustion and forming oxides (rusting).
• Restored mainly by photosynthesis of land plants and ocean phytoplankton.

The balance between these keeps oxygen levels steady.

Worked example

Farmers grow a pulse crop (such as moong) between two wheat crops. How does this help the soil? Name the bacteria involved and the step of the nitrogen cycle.

Try it first, then show the solutionSolution

1. Pulses are legumes with root nodules containing *Rhizobium*.
2. These bacteria fix atmospheric N${}_2$ into ammonia (nitrogen fixation), which is converted to compounds the plant and later crops can use.
3. When pulse roots and remains decay, ammonification and nitrification add more usable nitrogen to the soil, reducing the need for fertiliser.

Answer: Rhizobium in legume nodules fixes nitrogen, enriching the soil for the next crop.

Human impact and solutions

NCERT section 13.4 Human Impact on Earth’s Processes

Human activities disturb the cycles across all spheres:

• Burning fossil fuels and deforestation add CO${}_2$ faster than forests and oceans can absorb it, strengthening the greenhouse effect: global warming, melting ice, rising seas, extreme weather and changing monsoons.
• Ocean acidification: oceans absorb extra CO${}_2$, becoming more acidic, which harms plankton, shell-builders and coral reefs. Warmer water also absorbs less CO${}_2$.
• Eutrophication: excess nitrate and phosphate fertiliser washes into lakes and rivers, causing algal blooms; when the algae die, decomposers use up oxygen, and fish die.
• Deforestation: less photosynthesis and transpiration, lower local rainfall, changed albedo, soil erosion, habitat and biodiversity loss.
• Smog: vehicle exhaust reacts in sunlight to form smog and ground-level ozone, which is harmful to breathe (unlike protective stratospheric ozone).

Solutions:

• Global: the Montreal Protocol (ozone recovering); the Kyoto Protocol and Paris Agreement on CO${}_2$ have been less successful.
• National: India's rapid growth of solar and other renewables, tree planting, sustainable farming.
• Personal (Mission LiFE, Lifestyle for Environment): save energy, water and food; reduce, reuse, recycle; use public transport; plant trees.

Even part of the Thar desert covered in solar panels could, in principle, meet India's electricity needs.

Worked example

After heavy fertiliser use on nearby fields, a village pond turns bright green and fish start dying. Explain the chain of events and suggest two remedies.

Try it first, then show the solutionSolution

1. Rain washes excess nitrates (and phosphates) from fields into the pond.
2. Algae grow explosively (algal bloom), covering the surface.
3. When algae die, decomposers use up dissolved oxygen, so fish suffocate (eutrophication).
4. Remedies: use fertiliser in the right amount and at the right time, plant grass or tree buffer strips around the pond, and use organic manure or legumes.

Answer: Fertiliser runoff → algal bloom → oxygen loss → fish deaths; apply less fertiliser and keep buffer vegetation.

Science in real life: Earth as a System: Energy, Matter, and Life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Solar Energy, Albedo and the Atmosphere
• Water, Carbon and Nitrogen Cycles and Human Impact

Earth as a System: Energy, Matter, and Life: formulas and key facts

Speed of light

$c=3\times 10^8$ m s⁻¹ (EM waves travel in vacuum).

EM spectrum (high to low frequency)

$Gamma\to X-ray\to UV\to visible\to IR\to microwave\to radio.$

Solar constant and insolation

About 1.4 kW m⁻² at the top of the atmosphere; about 1 kW m⁻² at the surface (clear sky).

Solar energy received

$E=\text{intensity}\times \text{area}\times \text{time}$; 1 kWh (1 unit) $=3.6\times 10^6$ J.

Albedo

Fraction reflected; absorbed fraction $=1-\text{albedo}$.

Troposphere

0 to about 12 km; temperature falls about 6.5 °C per km.

Pressure belts

$0°low;30°N/Shigh;60°N/Slow;90°high.$

Photosynthesis

$6{\text{CO}}_2+6{\text{H}}_2\text{O}\to {\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2$

Haber-Bosch

${\text{N}}_2+3{\text{H}}_2\to 2{\text{NH}}_3$

Nitrogen cycle steps

$Fixation\to nitrification\to assimilation\to ammonification\to denitrification.$

Air composition

Nitrogen about 78%, oxygen about 21%, rest argon, CO₂, water vapour.

Earth as a System: Energy, Matter, and Life: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Earth as a System: Energy, Matter, and Life

The question patterns to expect, and the method that works for each.

Tricks to remember Earth as a System: Energy, Matter, and Life

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Gentle X-rays Usually Visit In My Room

  Gamma, X-ray, UV, Visible, IR, Microwave, Radio (high to low frequency).

  Use it when: EM spectrum order.

• Bright stays cool

  High albedo (white, bright) reflects and stays cool; dark absorbs and heats.

  Use it when: Albedo questions.

• Low at 0 and 60, High at 30 and 90

  Pressure belts alternate: L-H-L-H from equator to pole.

  Use it when: Planetary winds.

• Fix, Nitrify, Absorb, Rot, Release

  Fixation, nitrification, assimilation, ammonification (rotting), denitrification (release to air).

  Use it when: Order of nitrogen cycle steps.

• Valley by day, Mountain by night

  VD and MN: winds climb in daylight, slide down at night.

  Use it when: Local winds.

Earth as a System: Energy, Matter, and Life: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Earth as a System: Energy, Matter, and Life

The slips examiners see most often, and what to write instead.

• ✗Mistake: Confusing the ozone hole with global warming

  Both are 'atmosphere problems'.

  Wrong working

  Global warming is caused by the hole in the ozone layer.

  Right working

  Global warming is caused by extra greenhouse gases trapping infrared; the ozone hole (from CFCs) lets more UV reach the ground.

  ✓Do this: Ozone = UV shield; greenhouse gases = heat trap.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Saying the atmosphere is heated mainly from above

  The Sun is above us.

  Wrong working

  The troposphere is warmest at the top because it is nearer the Sun.

  Right working

  The ground absorbs sunlight and heats the air from below, so temperature falls with height in the troposphere.

  ✓Do this: Troposphere heated from the ground.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up nitrification and nitrogen fixation

  Both have 'nitr' and involve bacteria.

  Wrong working

  Nitrification converts N₂ gas into ammonia.

  Right working

  Fixation turns N₂ into ammonia (Rhizobium, Azotobacter); nitrification turns ammonia into nitrites and nitrates (Nitrosomonas, Nitrobacter).

  ✓Do this: Fix from air; nitrify in soil.

  Usually costs: 1 mark per step.

• ✗Mistake: Reversing valley and mountain breezes

  Students guess from the names.

  Wrong working

  A mountain breeze blows up the slopes during the day.

  Right working

  By day, the valley breeze blows up the slopes; at night, the mountain breeze blows down into the valley.

  ✓Do this: Name the breeze after where it comes from.

  Usually costs: 1 mark.

• ✗Mistake: Calling all ozone helpful

  The ozone layer protects us.

  Wrong working

  Ozone in city smog protects people from UV rays.

  Right working

  Ground-level ozone in smog is a harmful pollutant; only stratospheric ozone shields us from UV.

  ✓Do this: Good up high, bad nearby.

  Usually costs: 1 mark.

NCERT exercise guide: Earth as a System: Energy, Matter, and Life

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Earth as a System: Energy, Matter, and Life

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which part of sunlight is mostly absorbed by the ozone layer?
   (a) infrared
   (b) visible
   (c) ultraviolet
   (d) radio waves

   Hint

   Causes sunburn.

   Answer

   (c) ultraviolet

2. Q2 · 1 mark · MCQExam level

   The bacterium that converts nitrates back to nitrogen gas is
   (a) Rhizobium
   (b) Nitrobacter
   (c) Pseudomonas
   (d) Azotobacter

   Hint

   Denitrification.

   Answer

   (c) Pseudomonas

3. Q3 · 1 mark · Assertion-reasonExam level

   Assertion (A): Fresh snow keeps polar regions cold.
   Reason (R): Snow has a high albedo.
   (a) A and R are both true, and R explains A correctly.
   (b) A and R are both true, but R does not explain A.
   (c) A is true, R is false.
   (d) A is false, R is true.

   Hint

   Reflect, don't absorb.

   Answer

   (a)

4. Q4 · 2 marks · Very short answerExam level

   Name the five spheres of the Earth and give one example of each.

   Hint

   Rock, water, ice, air, life.

   Answer

   Geosphere (Deccan plateau), hydrosphere (Ganga), cryosphere (Himalayan glaciers), atmosphere (air), biosphere (mangroves, coral reefs).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why is the equator warmer than the poles even though both get sunlight during the year?

   Hint

   Angle of rays.

   Answer

   Because the Earth is a sphere, sunlight strikes the equator almost head-on, concentrating energy on a small area; at the poles it strikes at a slant, spreading the same energy over a larger area (and passing through more air).

6. Q6 · 3 marks · Short answerHarder exam level

   Explain how the atmosphere keeps the Earth at a temperature suitable for life.

   Hint

   Two roles.

   Answer

   It absorbs part of the incoming radiation (ozone blocks harmful UV, clouds absorb some), preventing overheating and damage. It traps part of the infrared given out by the warm surface (greenhouse effect), so the Earth does not lose heat too fast and freeze at night. Without it, temperatures would swing to extremes, as on the Moon.

7. Q7 · 3 marks · Short answerHarder exam level

   On a clear, cloudless winter night the ground becomes much colder than on a cloudy night. Explain this using how the Earth's surface loses heat.

   Hint

   Infrared and clouds.

   Answer

   The ground loses heat by giving out infrared radiation. Clouds and water vapour absorb part of this infrared and send some back down, so a cloudy night stays warmer. On a clear night most of the infrared escapes to space, so the ground cools quickly and can become very cold.

8. Q8 · 3 marks · Short answerHarder exam level

   A district plants a large forest on degraded, bare land. Describe its likely effects on the carbon cycle, the water cycle and local temperature.

   Hint

   Photosynthesis, transpiration, shade.

   Answer

   Carbon: growing trees take in CO₂ by photosynthesis and store carbon in wood and soil, and release oxygen. Water: roots help rain soak into the ground and recharge groundwater, reduce runoff and erosion, and transpiration adds water vapour, which can increase local rainfall. Temperature: shade and transpiration cool the area compared with bare land.

9. Q9 · 3 marks · Short answerHarder exam level

   Explain how planetary winds and differences in temperature and salinity produce ocean currents. How do they affect climate?

   Hint

   Friction and density.

   Answer

   Planetary winds drag surface water by friction. Warm equatorial water moves poleward at the surface; cold, salty, dense water sinks and returns at depth. The Earth's rotation deflects these into gyres. Currents carry heat poleward, moderating climate (the Gulf Stream keeps north-west European ports ice-free) and carry nutrients for fisheries.

10. Q10 · 5 marks · Long answerHarder exam level

    Draw and explain the carbon cycle, including the fast and slow parts and the role of oceans.

    Hint

    Photosynthesis, respiration, decomposition, combustion, absorption.

    Answer

    Diagram: CO₂ in air ↔ plants (photosynthesis), plants → animals (food), plants and animals → air (respiration), dead matter → air (decomposition), dead matter → fossil fuels (burial, millions of years), fossil fuels → air (combustion), air ↔ ocean (absorption/release), ocean → plankton and shells. Fast cycle: days to years through living things; slow cycle: millions of years through fossil fuels and carbonate rocks; oceans hold about 71% of carbon and absorb CO₂, which plankton use.

11. Q11 · 3 marks · Short answerHOTS / competency

    Every one-degree rise in sea surface temperature can increase evaporation. Use this to explain why a warmer Arabian Sea may cause floods in some parts of India and drought in others.

    Hint

    More moisture, uneven delivery.

    Answer

    A warmer sea evaporates more water, so the south-west monsoon winds carry more moisture. This can fall as very heavy bursts of rain in some places (floods), while changes in wind patterns and the timing of the monsoon leave other areas with long dry spells (drought). The total may rise, but its distribution becomes more uneven.

12. Q12 · 3 marks · Short answerHOTS / competency

    Ground-level ozone is harmful but stratospheric ozone is protective. Explain this difference and how vehicle use and the Montreal Protocol connect to each.

    Hint

    Location matters.

    Answer

    High in the stratosphere, ozone absorbs harmful UV, protecting life; CFCs destroyed it, and the Montreal Protocol, by banning CFCs, is letting it recover. Near the ground, ozone forms when vehicle exhaust reacts in sunlight to make smog; breathing it irritates lungs, so cutting vehicle emissions reduces it. Same gas, opposite effects because of where it is.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Name the five spheres of the Earth.Show answer

   Geosphere, hydrosphere, cryosphere, atmosphere, biosphere.

2. 2.What is the speed of light in vacuum?Show answer

   $3\times 10^8$ m s⁻¹.

3. 3.Which three parts of the spectrum carry about 99% of the Sun's energy reaching the Earth?Show answer

   Ultraviolet, visible and infrared.

4. 4.What is the solar constant?Show answer

   About 1.4 kW m⁻²: solar energy per second per square metre facing the Sun at the top of the atmosphere.

5. 5.Define albedo.Show answer

   The fraction of incoming sunlight that a surface reflects.

6. 6.In which layer does almost all weather occur?Show answer

   The troposphere.

7. 7.Which treaty helped the ozone layer recover?Show answer

   The Montreal Protocol.

8. 8.In which direction are winds deflected in the Northern Hemisphere?Show answer

   To the right.

9. 9.Name a nitrogen-fixing bacterium in legume roots.Show answer

   Rhizobium.

10. 10.What is eutrophication?Show answer

    Excess nutrients (from fertilisers) in water cause algal blooms that use up oxygen when they decay, killing fish.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give an Indian example of each sphere?
• ☐Can you list the EM spectrum in order and say what each solar part does?
• ☐Can you calculate solar energy on a panel in joules and kWh?
• ☐Can you explain albedo and the ice-melting feedback?
• ☐Can you explain why the equator is warmer than the poles?
• ☐Can you compare the troposphere and stratosphere?
• ☐Can you explain valley and mountain breezes?
• ☐Can you draw the pressure belts and explain deflection?
• ☐Can you describe the fast and slow carbon cycles and the nitrogen cycle steps with organisms?
• ☐Can you explain eutrophication, acidification and three solutions?

Earth as a System: Energy, Matter, and Life: frequently asked questions