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Earth as a System: Energy, Matter, and Life – Class 9 Science notes, important questions and MCQs

NCERT Class 9 Science, chapter 13. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Earth works as a system of interacting spheres — atmosphere, hydrosphere, lithosphere and biosphere. Energy from the sun drives it, and matter like water, carbon and nitrogen is recycled.

Earth's spheres

Atmosphere (air): about 78% nitrogen, 21% oxygen; ozone layer blocks UV.
Hydrosphere (water): oceans, rivers, ice, groundwater.
Lithosphere (rocks and soil) and biosphere (all living things).

Energy flow

The sun is the main source of energy; plants capture it by photosynthesis.
Energy flows one way through food chains and is finally lost as heat.
The greenhouse effect (CO₂, methane, water vapour) keeps Earth warm; too much causes global warming.

Biogeochemical cycles

Water cycle: evaporation → condensation → precipitation → runoff.
Carbon cycle: photosynthesis takes CO₂ in; respiration, burning and decay release it.
Nitrogen cycle: nitrogen-fixing bacteria (Rhizobium in legume roots) and lightning make nitrates; denitrifying bacteria return N₂ to air.
Soil forms from weathering of rocks and decaying organic matter (humus).

Key terms

Biosphere: the part of Earth where life exists
Greenhouse effect: trapping of heat by gases in the atmosphere
Nitrogen fixation: conversion of atmospheric N₂ into usable compounds
Humus: decayed organic matter in soil

Formulas

Air ≈ 78% N₂, 21% O₂, 0.04% CO₂

Important questions with answers

What is the greenhouse effect?2 marks

Gases like CO₂, methane and water vapour trap heat radiated by the Earth's surface and keep the planet warm. Too much of these gases increases the effect and causes global warming.

Explain the water cycle in three steps.3 marks

Evaporation: the sun heats water in oceans, rivers and leaves (transpiration), turning it into vapour. Condensation: the vapour cools at height to form clouds. Precipitation: water falls as rain or snow and returns to water bodies or soaks into the ground.

Describe the nitrogen cycle. Why is it important for living things?5 marks

Plants cannot use atmospheric N₂ directly. Nitrogen-fixing bacteria (e.g. Rhizobium in legume root nodules) and lightning convert N₂ into compounds such as ammonia and nitrates. Plants absorb nitrates to make proteins and nucleic acids; animals get nitrogen by eating plants. Decomposers turn dead remains and wastes back into ammonia, nitrifying bacteria convert it to nitrates, and denitrifying bacteria return some nitrogen to the air as N₂. The cycle keeps nitrogen, an essential part of proteins and DNA, available to life.

Quick revision

Sun drives Earth's systems.
Carbon cycle links photosynthesis and respiration.
Ozone protects life from UV.

Exam tips

Legumes improve soil fertility because of Rhizobium in root nodules.
Matter cycles; energy flows (does not cycle).

Practice MCQs

The most abundant gas in air is
1. oxygen
2. nitrogen
3. carbon dioxide
4. argon
Show answer
Answer: (b) nitrogen. About 78%.
The part of Earth with living things is the
1. lithosphere
2. hydrosphere
3. atmosphere
4. biosphere
Show answer
Answer: (d) biosphere. Biosphere.
Which process adds water vapour to the air?
1. evaporation
2. condensation
3. precipitation
4. runoff
Show answer
Answer: (a) evaporation. Evaporation.
Nitrogen-fixing bacteria in root nodules of legumes are
1. Rhizobium
2. Lactobacillus
3. E. coli
4. Yeast
Show answer
Answer: (a) Rhizobium. Rhizobium.
Which gas is mainly linked to global warming from burning fuels?
1. carbon dioxide
2. oxygen
3. nitrogen
4. helium
Show answer
Answer: (a) carbon dioxide. CO₂ traps heat.
Plants remove CO₂ from the air through
1. photosynthesis
2. respiration
3. transpiration
4. decay
Show answer
Answer: (a) photosynthesis. Photosynthesis.
Why is energy said to flow but matter to cycle?
1. energy is lost as heat at each step while elements are reused
2. matter disappears
3. energy is recycled by decomposers
4. both cycle equally
Show answer
Answer: (a) energy is lost as heat at each step while elements are reused. Energy cannot be reused; nutrients are recycled.
Returning nitrogen gas to the atmosphere is done by
1. nitrogen-fixing bacteria
2. denitrifying bacteria
3. lightning
4. plants
Show answer
Answer: (b) denitrifying bacteria. Denitrification.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.