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NCERT Class 9 Science Important Chapter 13 Earth as a System: Energy, Matter, and Life
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Earth as a System: Energy, Matter, and Life
Chapter – 13
| IMPORTANT QUESTION ANSWER |
Short Questions & Answers:
1. What are the five interacting ‘spheres’ that make up the Earth system?
Ans: The five interacting spheres are the geosphere, hydrosphere, cryosphere, atmosphere, and biosphere.
2. What does the cryosphere consist of?
Ans: The cryosphere consists of the solid form of water, such as ice, snow, Himalayan glaciers, and polar ice caps.
3. Which three regions of the electromagnetic spectrum account for about 99% of the Sun’s energy reaching the Earth?
Ans: About 99% of the Sun’s energy falls within the ultraviolet (UV), visible, and infrared (IR) regions.
4. What is the speed of light in a vacuum?
Ans: The speed of light in a vacuum is 3 × 108 m s–1.
5. What is meant by the albedo of a surface?
Ans: Albedo is the fraction of solar radiation reflected by a surface; high albedo surfaces stay cool by reflecting more light, while low albedo surfaces heat up quickly.
6. What is the approximate value of the solar constant at the top of the Earth’s atmosphere?
Ans: The solar constant is approximately 1.4 kilowatts per square metre (1.4 kWm–2).
7. Why do cities generally experience the “Urban Heat Island Effect”?
Ans: Cities are warmer than rural areas because built-up areas with concrete, brick, and asphalt absorb solar radiation and retain more heat.
8. In which layer of the atmosphere do nearly all weather phenomena take place?
Ans: Nearly all weather phenomena, such as winds and storms, take place in the troposphere.
9. Why does the temperature increase with height in the stratosphere?
Ans: Temperature increases with height in the stratosphere because the ozone layer located there absorbs harmful UV rays and heats up the atmosphere.
10. Name two major greenhouse gases that absorb re-radiated infrared heat and keep the Earth warm.
Ans: Carbon dioxide (CO2) and methane (CH4) are major greenhouse gases.
11. When does a valley breeze typically occur?
Ans: A valley breeze occurs during the day when mountain slopes heat up faster than the valley floor, causing warm air to rise and cooler air from the valley to move up.
12. How does the Earth’s rotation affect the direction of planetary winds?
Ans: The Earth’s rotation deflects planetary winds towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere.
13. What are ocean gyres?
Ans: Ocean gyres are large circular patterns of moving ocean water masses formed due to the deflection caused by the Earth’s rotation.
14. What is a biogeochemical cycle?
Ans: The continuous cyclic movement of matter and energy between the abiotic (non-living) and biotic (living) components of the Earth is called a biogeochemical cycle.
15. Which part of the Earth holds the largest reservoir of the total global carbon?
Ans: The oceans hold the largest reservoir, containing about 71% of the total global carbon.
16. What is the role of Rhizobium bacteria in the nitrogen cycle?
Ans: Rhizobium bacteria live in the root nodules of legumes and fix atmospheric nitrogen by converting it into ammonia.
17. What is nitrification in the nitrogen cycle?
Ans: Nitrification is the process where bacteria like Nitrosomonas and Nitrobacter convert ammonia into nitrites and then into nitrates.
18. What is the significance of the Haber-Bosch process?
Ans: The Haber-Bosch process artificially fixes atmospheric nitrogen to produce ammonia, which is used to make most of the agricultural fertilisers used today.
19. What causes eutrophication in water bodies?
Ans: The overuse of agricultural fertilisers adds excessive nitrates to water bodies, causing explosive algal growth (algal blooms) that deplete oxygen and kill aquatic life.
20. What is Mission LiFE?
Ans: Mission LiFE (Lifestyle for Environment) is an India-led global initiative that encourages people to adopt mindful, eco-friendly lifestyles to build a sustainable future.
Fill in the Blanks:
1. The five interacting parts of the Earth are called __________.
Ans: Spheres.
2. The layer of the atmosphere where most weather phenomena occur is the __________.
Ans: Troposphere.
3. The fraction of solar radiation reflected by a surface is called __________.
Ans: Albedo.
4. Rhizobium bacteria convert atmospheric nitrogen into __________.
Ans: Ammonia.
5. Mission LiFE stands for Lifestyle for __________.
Ans: Environment.
True or False:
1. The cryosphere consists of ice, snow, glaciers, and polar ice caps.
Ans: True.
2. Carbon dioxide and methane are greenhouse gases.
Ans: True.
3. Valley breeze usually occurs during the night.
Ans: False.
4. Oceans hold the largest reservoir of global carbon.
Ans: True.
5. Eutrophication improves the oxygen level in water bodies.
Ans: False.
Long Questions & Answers:
1. Describe the five interacting spheres of the Earth system and explain with an example how a disturbance in one sphere can affect the others.
Ans: The Earth functions as an interconnected system made up of five major spheres: The geosphere (solid rocks, soil, and landforms), the hydrosphere (liquid surface and groundwater), the cryosphere (solid water like glaciers and snow), the atmosphere (the air surrounding Earth), and the biosphere (all living organisms and their habitats),. Because these spheres are constantly exchanging matter and energy, a disturbance in one sphere can trigger widespread changes across the others. For example, an increase in global temperatures can lead to warmer Arabian Sea water (hydrosphere), which causes more evaporation and fluctuations in the southwest monsoon winds (atmosphere). This erratic rainfall can bring floods to some regions and droughts to others (geosphere), ultimately causing habitat loss and disrupting the ecosystems where plants and animals live (biosphere). Similarly, rising atmospheric temperatures accelerate the melting of glaciers (cryosphere), which can raise sea levels and threaten coastal cities.
2. Explain the concept of albedo and describe how the uneven heating of the Earth’s surface leads to the formation of valley and mountain breezes.
Ans: Albedo is the fraction of solar radiation that is reflected by a surface. Surfaces with a high albedo, such as snow and ice, reflect most of the incoming sunlight and remain very cold, whereas surfaces with a low albedo, such as dark soil or ocean water, absorb more solar radiation and heat up quickly. This uneven heating of the Earth’s surface directly drives local wind patterns, such as mountain and valley breezes. During the day, mountain slopes heat up much faster than the valley floor; the warm air over the slopes rises to create a low-pressure region, causing cooler air from the valley to move up the slopes in what is called a valley breeze. After sunset, the process reverses as the mountain slopes lose heat faster and become cooler than the valley floor; the cold, dense air then flows downward into the valley, creating a mountain breeze.
3. Describe the main layers of the Earth’s atmosphere and explain the two crucial roles the atmosphere plays in protecting life on Earth.
Ans: The Earth’s atmosphere has a distinct layered structure that regulates climate and energy flow. The lowest layer is the troposphere (up to about 12 km), where temperature decreases with height and nearly all weather phenomena, such as winds and storms, take place. Above this is the stratosphere (12–50 km), where temperature increases with height because it contains the ozone layer, which absorbs harmful ultraviolet (UV) radiation from the Sun. The atmosphere protects life on Earth through two crucial roles: first, it filters incoming solar radiation, with the ozone layer blocking dangerous UV rays and clouds absorbing or scattering some sunlight. Second, it traps outgoing heat; as the Earth’s surface absorbs sunlight and re-radiates it as infrared heat, greenhouse gases in the atmosphere (like carbon dioxide, methane, and water vapour) absorb this heat, preventing it from escaping into space and keeping the planet warm enough to sustain life.
4. Explain the difference between the fast and slow carbon cycles, and discuss how human activities are currently disrupting this balance.
Ans: The carbon cycle continuously circulates carbon across the Earth’s spheres, operating on very different time scales. In the fast cycle, which occurs over days to years, plants absorb atmospheric carbon dioxide (CO2) to produce glucose through photosynthesis, and this carbon is rapidly returned to the atmosphere when organisms respire or when decomposers break down dead organic matter. In the slow cycle, which takes millions of years, dead plants and animals get buried deep underground and slowly convert into fossil fuels like coal, oil, and gas. Today, human activities—primarily the burning of these fossil fuels and widespread deforestation—have drastically disrupted this natural balance, raising atmospheric CO2 levels by about 35% since 1960. This sudden excess of CO2 intensifies the greenhouse effect, leading to severe global warming, melting glaciers, extreme weather events, and ocean acidification, which threatens delicate marine ecosystems like coral reefs.
5. Describe the key steps involved in the nitrogen cycle and explain the environmental consequences of artificially fixing nitrogen for agriculture.
Ans: Nitrogen is essential for building proteins and DNA, but atmospheric nitrogen gas (N2) must be converted into usable forms through the nitrogen cycle. The cycle begins with nitrogen fixation, where bacteria like Rhizobium in legume roots convert N2 into ammonia. Next, in nitrification, other bacteria convert this ammonia into nitrites and then nitrates, which plants can absorb through a process called assimilation. When organisms die, decomposers return ammonia to the soil (ammonification), and finally, denitrifying bacteria like Pseudomonas convert nitrates back into nitrogen gas (denitrification), completing the cycle. In the modern era, humans artificially fix massive amounts of nitrogen using the Haber-Bosch process to manufacture agricultural fertilisers, which has revolutionised food production. However, the overuse of these fertilisers leads to heavy nitrate runoff into rivers and lakes, causing eutrophication—a process where massive algal blooms deplete the water’s oxygen, suffocating and killing fish and other marine life.
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