NCERT Class 9 Science (Exploration) Solutions
Chapter 13: Earth as a System Energy Matter and Life
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Class 9 Science Exploration Chapter 13 Question Answer
Class 9 Science Ch 13 Earth as a System Energy Matter and Life Question Answer
Earth as a System Energy Matter and Life Class 9 Questions and Answers (Exercise)
Revise, Reflect, Refine (NCERT Textbook Page No. 267)
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.
Answer:
(ii) To recycle essential nutrients between biotic and abiotic components.
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.
Answer:
(iii) The Earth’s surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
The Sun’s energy heats the Earth’s surface first. The surface then sends that heat back out as infrared radiation, which is trapped by greenhouse gases in the atmosphere, keeping the planet warm.
Explain how climate change affects the water cycle. Illustrate with examples.
Answer:
Climate change significantly disrupts the natural functioning of the water cycle by altering temperature and moisture patterns. A rise in global temperature increases evaporation, allowing the atmosphere to hold more moisture. This results in heavier and more intense rainfall in some regions, leading to floods, while other regions may experience prolonged droughts. Melting of glaciers due to rising temperatures adds excess water to rivers and oceans, contributing to rising sea levels and threatening coastal areas.
Additionally, intense rainfall increases surface runoff, reducing groundwater recharge. This imbalance affects agriculture, especially in regions dependent on groundwater.
Describe how albedo affects the Earth’s surface temperature and its climate.
Answer:
Albedo is the fraction of solar radiation reflected by a surface. High-albedo surfaces like snow (0.80 – 0.90) and ice (0.50 – 0.70) reflect most incoming sunlight and therefore absorb little heat, keeping polar regions cold. Low-albedo surfaces like black soil and ocean water absorb more radiation and become warmer. This differential heating drives winds and ocean currents. Climate is affected as Arctic ice melts due to warming. It exposes darker ocean water with lower albedo, which absorbs more heat, causing further warming and leading to a dangerous cycle known as the ice-albedo feedback.
How are mountain and valley breezes formed? Suppose there are two mountains, one covered with grass and another covered with barren rocks; would the temperature of the two mountain breezes be different? If so, how?
Answer:
Mountain and valley breezes are local winds formed due to uneven heating of slopes and valleys. During the day, mountain slopes heat up faster than the valley floor. The air over the slopes becomes warm and rises, creating a low-pressure area.
Cooler air from the valley moves upward to replace it, forming a valley breeze. At night, the slopes cool down faster than the valley. The air over the slopes becomes cooler and denser, flowing downward into the valley as a mountain breeze.
If one mountain is covered with grass and another with barren rocks, the temperatures of their mountain breezes will differ. Rocky surfaces absorb more heat and have lower albedo, producing warmer air. However, grassy surfaces remain cooler due to higher albedo and transpiration. Hence, breezes from the Rocky Mountains will generally be warmer than those from the grassy mountains.
You have witnessed weather phenomena, such as winds, storms, rainfall, etc. Which atmospheric layer is mainly responsible for such phenomena, and what is the primary reason for its occurrence?
Answer:
The layer of the atmosphere responsible for these phenomena is the Troposphere.
Primary Reasons for its Occurrence:
- Presence of water vapour: The troposphere contains almost all the water vapour and dust particles of the atmosphere, which are essential for cloud formation and rain.
- Convection currents: As the Sun heats the Earth’s surface, the air near the ground becomes warm and rises. This vertical movement of air, along with horizontal pressure differences, creates winds and storms.
- Temperature decrease: In this layer, temperature decreases with altitude. This allows moist air to cool down and condense as it rises, leading to precipitation like rain or snow.
The troposphere is the most dynamic layer of the atmosphere where the interaction of heat, moisture, and air movement creates all common weather patterns.
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Answer:
Nitrogen fixation is the first step of the nitrogen cycle. In this step, bacteria such as Rhizobium and Azotobacter convert the atmospheric nitrogen into ammonia. This is followed by nitrification, where ammonia is converted into nitrites and then nitrates by nitrifying bacteria such as Nitrosomonas and Nitrobacter. Plants absorb these compounds, and animals obtain nitrogen by consuming plants.
When organisms die or produce waste, decomposers convert organic nitrogen back into ammonia through ammonification. Finally, the denitrifying bacteria like Pseudomonas converts the nitrates back into atmospheric nitrogen through the process of denitrification.
If nitrogen were not cycled, plants would not receive usable nitrogen to form proteins and nucleic acids. This would disrupt plant growth as well as animal life, ultimately leading to the collapse of ecosystems and making life on Earth unsustainable.
What are the impacts of deforestation on the Earth’s oxygen and carbon cycles? What are the other consequences of deforestation?
Answer:
Deforestation disrupts the natural balance of gases and the environment in the following ways
Impact on Cycles:
Carbon cycle: Fewer trees mean less Carbon Dioxide (CO2) is absorbed from the air. This increases the greenhouse effect and leads to global warming.
2. Oxygen cycle: Plants are the main source of oxygen. Cutting them down reduces the amount of oxygen released into the atmosphere for living beings to breathe.
Other Consequences:
- Soil erosion: Tree roots hold the soil. Without them, fertile topsoil is easily washed away by rain or wind.
- Loss of biodiversity: Deforestation destroys the natural habitats of many animals and plants, leading to their extinction.
- Disturbed water cycle: Fewer trees mean less transpiration, which leads to reduced rainfall and drier climates.
Deforestation causes an imbalance in atmospheric gases and severely damages the Earth’s ecosystem.
Explain with a suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2 from the atmosphere.
When plants and animals die, decomposers break down their organic matter, releasing carbon into the atmosphere. Over long periods, some carbon gets buried and forms fossil fuels such as coal and petroleum. When these fuels are burned (combustion), carbon is rapidly released back into the atmosphere as CO2.
Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?
Answer:
While plants do require CO2 for photosynthesis, excess atmospheric CO2 beyond what natural systems can absorb is undesirable because of several reasons, such as:
- It intensifies the greenhouse effect, trapping more infrared radiation and causing global warming
- This leads to melting of glaciers and polar ice, raising sea levels.
- It increases ocean acidity, threatening marine life
- It causes more extreme weather events such as intense monsoons, droughts, and cyclones
- Rising temperatures threaten ecosystems and biodiversity.
Plants can only absorb a limited amount; the excess accumulates. Thus, CO2 in the right amount sustains life, but in excess it destabilises Earth’s climate.
How is heat lost from the surface of the Earth? What is its significance?
Answer:
The Earth loses heat mainly through the process of radiation. After absorbing solar energy, the Earth’s surface re-radiates it in the form of infrared radiation back into the atmosphere. Some of this heat escapes into space, while a portion is absorbed and reemitted by greenhouse gases. This process helps in regulating the Earth’s temperature.
The significance of this heat loss lies in maintaining the planet’s energy balance. If all the heat were trapped, the Earth would become excessively hot, and if too much heat escaped, it would become extremely cold. Thus, a controlled loss of heat ensures a stable climate suitable for sustaining life on Earth.
If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?
Answer:
If the Earth were a flat disc, the distribution of solar radiation would be very different from the current spherical model. On a spherical Earth, the Sun’s rays strike different latitudes at different angles, causing uneven. This variation creates temperature differences that drives winds and ocean currents.
However, on a flat Earth, sunlight would fall more uniformly across the surface, leading to less variation in temperature between regions. The absence of significant temperature differences would reduce the formation of pressure belts, winds, and ocean currents. As a result, weather patterns would be less dynamic, and the overall climate system would be drastically altered.
Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
Answer:
A rise in atmospheric temperature creates a ripple effect across all Earth systems:
- Cryosphere (Ice): Higher temperatures cause glaciers and polar ice caps to melt rapidly. This reduces the Earth’s albedo (reflectivity), causing the planet to absorb even more heat.
- Hydrosphere (Water): The melting ice from the cryosphere flows into the oceans, causing sea levels to rise. Warmer temperatures also increase the rate of evaporation, leading to more intense storms and flooding in some areas.
- Biosphere (Life): Many species may lose their natural habitats (like polar bears losing sea ice). Climate change can disrupt breeding cycles and food availability, leading to a loss of biodiversity and potential extinction of sensitive species.
Conclusion: Because the Earth is an integrated system, a change in atmospheric temperature destabilises the balance between ice, water, and living organisms.
Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
Answer:
The atmosphere plays two important roles in protecting the life on the Earth. Firstly, it partly absorbs the incoming solar radiation, while ozone layer blocks the harmful UV rays.
Clouds and other gases also absorb some sunlight and re-radiate it in the infrared region. Greenhouse gases like CO2, CH4 and water vapour absorb this re-radiated heat, preventing it from escaping into space.
Without the atmosphere, the Earth would experience extreme temperatures, becoming too cold for life to exist. Thus, by balancing incoming and outgoing energy, the atmosphere ensures stable climatic conditions necessary for survival.
Describe the interrelationship between different spheres of the Earth. illustrate with example how these spheres function in a delicate balance.
Answer:
The Earth is made up of interconnected spheres- geosphere, hydrosphere, atmosphere, cryosphere, and biosphere – that constantly interact with each other through biogeochemical cycles like carbon cycle or water cycle. Energy and matter flow between these spheres, maintaining environmental balance.
For example, in the water cycle, water evaporates from oceans (hydrosphere), forms clouds in the atmosphere, and returns as precipitation. This water flows over land (geosphere), supports plants and animals (biosphere), and may freeze in glaciers (cryosphere).
Class 9 Science Chapter 13 Earth as a System Energy Matter and Life Question Answer (InText)
Think it Over (NCERT Textbook Page No. 252)
How does the warming of Arabian Sea water affect the southwest monsoon in India?
Answer:
Warmer Arabian Sea water leads to greater evaporation from the sea surface, thus increasing atmospheric moisture and creating stronger low-pressure systems over the land, intensifying the southwest monsoon. However, excessively warm waters can make monsoon rainfall erratic, bringing floods to some regions and droughts to others. This can disrupt India’s hydrosphere significantly.
If a large forest is cleared, how can that affect the flow of a river in that area?
Answer:
Clearing forests reduces transpiration and rainfall, which decreases water availability. Without tree roots to hold soil, soil erosion increases, and more rainwater flows as surface runoff instead of infiltrating the ground. This can cause irregular river flow, including floods during rains and reduced flow during dry periods.
What might happen to coastal cities in India if glaciers and polar ice keep melting faster?
Answer:
Melting glaciers and polar ice cause large volumes of fresh water to enter the oceans, raising sea levels. Coastal cities like Mumbai, Chennai, etc. would face increased risk of flooding, especially during high tides and cyclones. In the long run, habitats near the coast could be submerged, displacing populations and disrupting ecosystems in the biosphere. Groundwater in coastal areas could also become saline as seawater intrudes inland.
How would increasing carbon dioxide levels in the atmosphere affect the ocean plankton?
Answer:
Higher carbon dioxide levels increase its absorption by oceans, making seawater more acidic. This affects tiny organisms like plankton, especially those with shells, making it difficult for them to survive, and this can disrupt the entire marine ecosystem.
Pause and Ponder (NCERT Textbook Page No. 258)
Visit the website given below and study the effect of the concentration of greenhouse gas on surface temperature:edu/ en/simulations/greenhouse-effect.
Answer:
The concentration of greenhouse gases has a direct and significant effect on the Earth’s surface temperature.
The relationship between greenhouse gases and temperature is as follows:
Heat Trapping: Greenhouse gases in the atmosphere trap the heat (infrared radiation) that reflects off the Earth’s surface instead of letting it escape into space.
Direct Relationship: As the concentration of greenhouse gases increases, more heat is trapped within the atmosphere.
2. Temperature Rise: This trapped heat causes the Earth’s average surface temperature to rise.
Conclusion: Higher concentrations of greenhouse gases lead to an increase in surface temperature, a process commonly known as the Greenhouse Effect.
Pause and Ponder (NCERT Textbook Page No. 261)
How does the cool mountain breeze benefit agricultural activity, particularly the crops and soil?
The breeze also improves air circulation, which helps in preventing plant diseases. Overall, it creates a more favourable environment for healthy crop production.
What happens to the warm surface of water from the equator as it travels toward the poles? What impact does this movement have on the area?
Answer:
As warm equatorial surface water travels poleward through ocean currents, it gradually loses heat to the surrounding cooler atmosphere and water. This warms the climate of coastal regions it passes through. For example, the North Atlantic Drift keeps northwestern Europe significantly warmer than other regions at similar latitudes, allowing ports to remain ice free in winter. As the water cools, it becomes denser and sinks, forming deep cold currents that flow back toward the equator. This global “ocean conveyor belt” redistributes heat and nutrients across the entire planet.
Pause and Ponder (NCERT Textbook Page No. 263)
The CO2 dissolved in the ocean is disturbed when the global temperature increases. What will happen to marine life?
Answer:
When global temperature increases, the ocean’s ability to absorb carbon dioxide decreases, and more CO2 remains in the atmosphere. At the same time, increased CO2 absorption makes seawater more acidic.
This affects marine life in several ways:
- Tiny organisms like plankton and shell- forming organisms find it difficult to build and maintain their shells.
- Coral reefs may get damaged, leading to loss of marine habitats.
- Since plankton form the base of the marine food chain, their decline can affect fish and larger marine animals.
Thus, marine ecosystems may become imbalanced and less productive.
Pause and Ponder (NCERT Textbook Page No. 265)
What would happen to plants and animals on Earth if the biogeochemical cycles were disrupted and stopped? Explain by giving a few examples.
Answer:
Biogeochemical cycles ensure the continuous supply of essential nutrients like water, carbon, nitrogen, and oxygen. If these cycles are disrupted and stopped, it affects plants and animals by reducing the availability of essential nutrients, disturbing food chains, and harming ecosystems.
For example,
Water cycle disruption → Irregular rainfall, droughts or floods → affects crops and water availability. Carbon cycle disruption → Excess CO2 → global warming → climate change → affects plants and animals.
Nitrogen cycle disruption → Lack of usable nitrogen → Poor plant growth → affects food chains. Oxygen cycle disruption → Imbalance in Oxygen levels → difficulty in respiration for living organisms.
Pause and Ponder (NCERT Textbook Page No. 266)
Discuss how human activities increase the concentration of greenhouse gases in the atmosphere. What would you do as an individual to reduce the emission of greenhouse gases?
Answer:
Human activities significantly increase greenhouse gases, leading to global warming.
Human activities: effects of increased greenhouse gases
- Burning fossil fuels: Using coal, petrol, and diesel for vehicles and factories releases large amounts of Carbon Dioxide (CO2).
- Deforestation: Cutting down forests removes trees that naturally absorb CO2, leading to higher levels of the gas in the air.
- Industrialisation: Industries release various harmful gases, including methane and nitrous oxide, which trap more heat in the atmosphere.
Individual Actions to Reduce Emissions:
- Save energy: Switching off electrical appliances when not needed to reduce the power demand.
- Green transport: Walking, cycling, or using public transport instead of personal motor vehicles.
- Plantation: Planting more trees to help clean the air and absorb CO2.
- Waste reduction: Following the 3Rs (Reduce, Reuse, Recycle) to minimise industrial pollution and waste.
Small individual efforts can help maintain the balance of the Earth system and control the effects.
What if… (NCERT Textbook Page No. 265)
If photosynthesis stopped, what would happen to the Earth?
Answer:
If photosynthesis stopped, plants would no longer produce food (glucose) or oxygen. As a result, plants would die first, and animals and humans that depend on them for food would also eventually die.
The level of carbon dioxide would increase, while oxygen levels would decrease rapidly. This would disturb the balance of gases in the atmosphere. Food chains would collapse, and life on Earth would not be possible.
Class 9 Science Chapter 13 Question Answer (Activities)
Activity 13.1:Let Us Explore (NCERT Textbook Page No. 253)
Observe the features of the Earth as shown in Fig. 13.1. Identify and circle one example representing each of the geosphere, hydrosphere, cryosphere, atmosphere, and biosphere.
Answer:
- Geosphere (land, soil, mountains)
- Hydrosphere (lake)
- Cryosphere (frozen water, ice, snow)
- Atmosphere (air surrounding the Earth)
- Biosphere (all living things like herder, his sheep, grass, or any plant).
How does snow (cryosphere) eventually become part of the lake (hydrosphere)?
Answer:
Snow from the mountains (cryosphere) melts when temperatures rise. The melted water flows downhill over the land (geosphere) and eventually joins the lake, becoming a part of the water bodies (hydrosphere).
If there is less snowfall during winters for a few years, how would this affect the lake’s level and the grass available for the sheep?
Answer:
If there is less snowfall during winters for a few years, the cryosphere stores less water. When spring and summer come, less snow will melt, so the lake’s level will be lower than usual. With less water in the lake and reduced soil moisture, the grass for the sheep will also grow less, reducing food availability for them.
Discuss with your classmates and write down how all the spheres are interconnected, and how a disturbance in one can lead to changes in others.
Answer:
All the spheres are interconnected, so a change in one will impact the others. For example, less snowfall (cryosphere) reduces lake water (hydrosphere), which lowers soil moisture and the growth of grass (biosphere). Similarly, changes in the atmosphere, like higher temperatures, can speed up the melting of the snow, thereby affecting the water level of the lake and the population of the biotic components living in this ecosystem. Thus, a small disturbance in any sphere can affect the entire ecosystem.
Activity 13.2:Let Us Find Out (NCERT Textbook Page No. 256)
Aim: To complete the following table using information from authentic sources like websites and books.
Table 13.1: Reflection of solar radiation by surfaces of materials
Material | Albedo |
Question 3 (1)
Snow | 0.80 – 0.90 |
2. Ice | 0.50 – 0.70 |
3. Crushed rock | 0.25 – 0.30 |
4. Light coloured soil | |
5. Black soil | |
6. Ocean water |
Materials | Albedo |
1. Snow | 0.80 – 0.90 |
2. Ice | 0.50 – 0.70 |
3. Crushed rock | 0.25 – 0.30 |
4. Light coloured soil | 0.30 – 0.40 |
5. Black soil | 0.10 – 0.20 |
6. Ocean water | 0.05 – 0.10 |
(Answer may vary as these ranges are approximate.)