Exploration - Science solution
Class 9 - Chapter 13: Earth as a System: Energy, Matter, and Life
Question 1
Choose the most appropriate option to describe the role of biogeochemical cycles in an ecosystem.
Correct Answer: (ii) To recycle essential nutrients between biotic and abiotic components.
Explanation:
Biogeochemical cycles are natural processes through which essential nutrients such as water, carbon, nitrogen, and oxygen continuously move between the biotic (living) and abiotic (non-living) components of the Earth. These cycles ensure that nutrients remain available for all living organisms and help maintain ecological balance.
| Option | Correct/Incorrect | Reason |
|---|---|---|
| (i) To provide food directly to all organisms. | Incorrect | Biogeochemical cycles recycle nutrients; they do not directly provide food. |
| (ii) To recycle essential nutrients between biotic and abiotic components. | Correct | These cycles continuously recycle nutrients between living organisms and the environment. |
| (iii) To create new elements for use by living things. | Incorrect | Biogeochemical cycles recycle existing elements; they do not create new ones. |
| (iv) To remove pollutants and toxins from the organism. | Incorrect | The primary function of these cycles is nutrient recycling, not detoxification. |
Question 2
Which of the following is primarily responsible for warming of the Earth?
Correct Answer: (iii) The Earth's surface absorbs solar radiation, which is then re-radiated and trapped by greenhouse gases.
Explanation:
Most of the Sun's visible radiation reaches the Earth's surface, where it is absorbed and converted into heat. The heated Earth emits this energy as infrared radiation. Greenhouse gases such as carbon dioxide (CO2), methane (CH4), and water vapour absorb part of this outgoing infrared radiation and trap heat in the atmosphere. This natural greenhouse effect keeps the Earth warm enough to support life.
| Option | Correct/Incorrect | Reason |
|---|---|---|
| (i) | Incorrect | Carbon dioxide does not immediately absorb most incoming solar radiation. It mainly absorbs outgoing infrared radiation. |
| (ii) | Incorrect | Dust and tiny particles absorb only a small portion of solar radiation and are not the main cause of Earth's warming. |
| (iii) | Correct | The Earth's surface absorbs sunlight, re-radiates it as infrared radiation, and greenhouse gases trap part of this heat. |
| (iv) | Incorrect | Clouds reflect some sunlight but reflected radiation is not the primary source of Earth's warming. |
Question 3
Explain how climate change affects the water cycle. Illustrate with examples.
Answer:
Climate change disturbs the natural water cycle by increasing the Earth's temperature. A warmer atmosphere can hold more water vapour, leading to changes in rainfall patterns and the frequency of extreme weather events.
The major effects are:
- Higher temperatures increase evaporation from oceans, rivers and lakes.
- Warmer air holds more moisture, causing heavier rainfall and floods in some regions.
- Other regions receive less rainfall, resulting in droughts.
- Rapid melting of glaciers increases river flow initially and contributes to sea-level rise in the long term.
- Heavy rainfall increases surface runoff, causing soil erosion.
- Reduced infiltration decreases groundwater recharge, making water shortages more severe.
Examples:
- Intensified monsoon rainfall causing floods in many parts of India.
- Melting Himalayan glaciers increasing river discharge and threatening coastal cities through rising sea levels.
| Climate Change | Effect on Water Cycle |
|---|---|
| Increase in global temperature | More evaporation from water bodies. |
| Warmer atmosphere | More moisture in the air and heavier rainfall. |
| Melting glaciers | Higher river flow and rising sea levels. |
| Heavy rainfall | More runoff, soil erosion and less groundwater recharge. |
| Irregular rainfall | Floods in some areas and droughts in others. |
Question 4
Describe how albedo affects the Earth's surface temperature and its climate.
Answer:
Albedo is the fraction of incoming solar radiation that is reflected by a surface. Different surfaces have different albedo values, which determine how much solar energy they reflect or absorb.
Surfaces with a high albedo reflect most of the incoming sunlight and remain cooler. Surfaces with a low albedo absorb more solar radiation and become warmer.
This difference in heating influences local temperatures, weather conditions and the Earth's climate.
| Surface | Albedo | Effect |
|---|---|---|
| Snow and Ice | High | Reflect most sunlight and remain cold. |
| Light-coloured Soil | Moderately High | Reflects more sunlight and heats slowly. |
| Black Soil | Low | Absorbs more sunlight and becomes warmer. |
| Ocean Water | Low | Absorbs a large amount of solar radiation. |
Importance of Albedo:
- Controls the amount of solar energy absorbed by the Earth's surface.
- Helps regulate local and global temperatures.
- Influences weather patterns and climate.
- Melting of snow and ice reduces Earth's albedo, causing even more heat absorption and accelerating global warming.
Question 5
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:
Formation of Valley Breeze
During the day, the mountain slopes receive more solar radiation than the valley floor and heat up more quickly. The air above the slopes becomes warm, expands and rises, creating a region of low pressure. Cooler air from the valley moves upward to replace the rising warm air. This movement of air from the valley towards the mountain slopes is called a valley breeze.
Formation of Mountain Breeze
During the night, the mountain slopes lose heat more rapidly than the valley floor. The air over the slopes becomes cooler and denser, causing it to flow downhill into the valley. This movement of cool air from the mountains towards the valley is called a mountain breeze.
Comparison of Mountain Breezes
Yes, the temperatures of the two mountain breezes would be different.
A mountain covered with grass remains comparatively cooler because vegetation provides shade and cools the surroundings through transpiration. On the other hand, barren rocks absorb more solar radiation during the day and release more heat at night.
Therefore, the mountain breeze from the grass-covered mountain will be cooler, while the breeze from the barren rocky mountain will be comparatively warmer.
| Feature | Valley Breeze | Mountain Breeze |
|---|---|---|
| Time | Day | Night |
| Direction | Valley → Mountain | Mountain → Valley |
| Cause | Warm air rises over mountain slopes. | Cool, dense air flows downhill. |
| Effect | Helps maintain local weather. | Cools the valley during night. |
Question 6
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 troposphere is the atmospheric layer mainly responsible for weather phenomena such as winds, storms, clouds and rainfall.
This is because the troposphere is heated from the Earth's surface. Solar radiation first warms the land and water, which then heat the air above them. The uneven heating of the Earth's surface creates differences in temperature and air pressure. Warm air rises while cooler air moves in to replace it, producing winds, cloud formation and rainfall.
| Atmospheric Layer | Role |
|---|---|
| Troposphere | Formation of clouds, rainfall, winds, storms and other weather events. |
| Reason | Uneven heating of the Earth's surface creates temperature and pressure differences. |
| Additional Feature | Temperature decreases with increasing altitude. |
Question 7
Explain the processes involved in the nitrogen cycle. How would life on Earth be affected if nitrogen were not cycled?
Answer:
The nitrogen cycle is the continuous movement of nitrogen between the atmosphere, soil, water and living organisms. Since atmospheric nitrogen (N2) cannot be directly used by most plants and animals, it must be converted into usable forms through several biological processes.
Processes of the Nitrogen Cycle
| Process | Description |
|---|---|
| Nitrogen Fixation | Nitrogen-fixing bacteria such as Rhizobium and Azotobacter convert atmospheric nitrogen into ammonia. |
| Nitrification | Nitrosomonas converts ammonia into nitrite, while Nitrobacter converts nitrite into nitrate. |
| Assimilation | Plants absorb nitrates from the soil. Animals obtain nitrogen by eating plants or other animals. |
| Ammonification | Decomposers break down dead organisms and wastes, returning ammonia to the soil. |
| Denitrification | Denitrifying bacteria such as Pseudomonas convert nitrates back into atmospheric nitrogen. |
Effect if Nitrogen Were Not Cycled
- Plants would not receive usable nitrogen for growth.
- Proteins, enzymes and nucleic acids could not be formed properly.
- Crop production would decline significantly.
- Animals would suffer due to a shortage of plant food.
- Food chains and ecosystems would become unstable.
- Life on Earth would eventually be severely affected because nitrogen is essential for all living organisms.
Question 8
What are the impacts of deforestation on the Earth's oxygen and carbon cycles? What are the other consequences of deforestation?
Answer:
Forests play a vital role in maintaining the balance of the Earth's oxygen and carbon cycles. Deforestation disturbs these natural cycles and affects the entire Earth system.
Impact on Oxygen and Carbon Cycles
| Cycle | Impact of Deforestation |
|---|---|
| Oxygen Cycle | Fewer trees mean less photosynthesis, resulting in reduced oxygen production. |
| Carbon Cycle | Less carbon dioxide is absorbed by plants, increasing the concentration of CO2 in the atmosphere and strengthening the greenhouse effect. |
Other Consequences of Deforestation
- Increase in global warming due to higher atmospheric CO2.
- Reduction in transpiration, leading to lower local rainfall.
- Increase in soil erosion because tree roots no longer hold the soil firmly.
- Loss of wildlife habitats and decline in biodiversity.
- Disturbance of the water cycle.
- Greater risk of floods and droughts.
- Loss of ecological balance and degradation of ecosystems.
| Aspect | Effect of Deforestation |
|---|---|
| Photosynthesis | Decreases |
| Atmospheric CO2 | Increases |
| Oxygen Production | Decreases |
| Rainfall | May decrease locally |
| Soil | More erosion and loss of fertility |
| Biodiversity | Habitat destruction and species decline |
| Climate | Global warming becomes more severe |
Question 9
Explain with suitable diagram the path that carbon takes to go back to the atmosphere. You may start from plants using CO2 from the atmosphere.
Answer:
The carbon cycle is the continuous movement of carbon between the atmosphere, living organisms, oceans and the Earth. Plants absorb carbon dioxide (CO2) from the atmosphere during photosynthesis. This carbon then moves through food chains and finally returns to the atmosphere through various natural and human activities.
Path of Carbon Back to the Atmosphere
Atmospheric CO₂
│
▼
Plants absorb CO₂
(Photosynthesis)
│
▼
Plants prepare food
│
▼
Animals eat plants
│
▼
Respiration by plants
and animals
│
▼
CO₂ released into
the atmosphere
▲
│
Decomposition of
dead plants and animals
│
▼
CO₂ released
Dead organisms buried
for millions of years
│
▼
Fossil Fuels
│
▼
Combustion of fossil fuels
│
▼
CO₂ released into
the atmosphere
| Process | Role in Carbon Cycle |
|---|---|
| Photosynthesis | Plants absorb atmospheric CO2 to prepare food. |
| Respiration | Plants and animals release CO2 back into the atmosphere. |
| Decomposition | Microorganisms decompose dead organisms and release carbon dioxide. |
| Combustion | Burning fossil fuels releases stored carbon into the atmosphere. |
| Ocean Exchange | Oceans absorb and release carbon dioxide continuously. |
Question 10
Why is an excess of CO2 in the atmosphere considered undesirable even though it is required by plants?
Answer:
Carbon dioxide is essential for photosynthesis, through which plants prepare food and release oxygen. However, an excessive amount of CO2 disturbs the natural balance of the atmosphere.
CO2 is a greenhouse gas. Excess CO2 traps more infrared radiation, strengthening the greenhouse effect and increasing the Earth's average temperature.
This leads to several environmental problems such as:
- Global warming.
- Melting of glaciers and polar ice.
- Rise in sea level.
- More frequent floods, droughts and heat waves.
- Disturbance of rainfall patterns.
- Ocean acidification, which harms plankton and coral reefs.
- Loss of biodiversity and agricultural productivity.
| Importance of CO2 | Problems Caused by Excess CO2 |
|---|---|
| Required for photosynthesis. | Causes global warming. |
| Helps maintain the carbon cycle. | Enhances the greenhouse effect. |
| Supports plant growth. | Leads to climate change and sea-level rise. |
Question 11
How is heat lost from the surface of the Earth? What is its significance?
Answer:
The Earth's surface absorbs solar radiation during the day and becomes warm. It then loses heat by emitting infrared radiation into the atmosphere and space.
Greenhouse gases such as carbon dioxide, methane and water vapour absorb part of this outgoing infrared radiation and trap some heat, while the remaining heat escapes into space.
Significance
- Maintains the Earth's energy balance.
- Prevents excessive heating of the Earth's surface.
- Keeps the Earth's average temperature suitable for life.
- Helps regulate weather and climate.
- Supports the natural greenhouse effect.
| Stage | Explanation |
|---|---|
| Absorption | The Earth's surface absorbs incoming solar radiation. |
| Re-radiation | The heated surface emits infrared radiation. |
| Heat Trapping | Greenhouse gases absorb part of the infrared radiation. |
| Heat Loss | The remaining heat escapes into space. |
Question 12
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 Sun's rays would strike almost all places at nearly the same angle. As a result, solar radiation would be distributed much more uniformly than on the present spherical Earth.
The present spherical shape causes the Sun's rays to strike the equatorial region more directly and the polar regions at a slanting angle. This uneven heating creates temperature differences that drive winds, ocean currents and weather systems.
If the Earth were flat:
- There would be much less variation in temperature from place to place.
- The equator and poles would not have large temperature differences.
- Pressure belts would be weak or absent.
- Global winds and ocean currents would be greatly altered.
- Climate zones would become less distinct.
- Weather patterns would be very different from those observed today.
| Spherical Earth | Hypothetical Flat Earth |
|---|---|
| Uneven heating occurs. | Heating would be nearly uniform. |
| Distinct climate zones exist. | Climate differences would reduce. |
| Strong pressure belts form. | Pressure belts would weaken. |
| Global winds and ocean currents develop. | Wind and ocean circulation patterns would change greatly. |
| Equator is much warmer than the poles. | Temperature differences would be much smaller. |
Question 13
Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?
Answer:
A rise in the Earth's atmospheric temperature affects all the Earth's spheres because they are closely interconnected. The cryosphere, hydrosphere and biosphere are directly influenced by global warming.
Effects on Different Spheres
| Earth Sphere | Effect of Rising Atmospheric Temperature |
|---|---|
| Cryosphere | Glaciers, ice sheets and polar ice melt rapidly, reducing snow cover and increasing sea level. |
| Hydrosphere | Sea level rises due to melting ice and thermal expansion of seawater. Evaporation increases, leading to changes in rainfall patterns, floods and droughts. |
| Biosphere | Plants and animals face habitat loss, biodiversity decreases, food chains are disturbed and many species may become endangered or extinct. |
Overall Impact:
- Increase in extreme weather events.
- Reduction in freshwater availability due to glacier melting.
- Disturbance of ecosystems and ecological balance.
- Negative effects on agriculture and human health.
Question 14
Explain how the Earth's atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.
Answer:
The Earth's atmosphere acts like a protective blanket around the planet. It regulates temperature by controlling the amount of heat entering and leaving the Earth.
During the day, the Earth's surface absorbs solar radiation and becomes warm. At night, it releases this heat as infrared radiation. Greenhouse gases such as carbon dioxide (CO2), water vapour and methane (CH4) absorb part of this outgoing infrared radiation and retain heat in the atmosphere. This natural process is called the greenhouse effect.
Without the atmosphere and the natural greenhouse effect, the Earth's average temperature would be too low to support life.
| Role of Atmosphere | Importance |
|---|---|
| Allows most solar radiation to reach the Earth's surface. | Provides energy needed for life. |
| Absorbs part of the outgoing infrared radiation. | Prevents excessive heat loss. |
| Maintains the natural greenhouse effect. | Keeps the Earth's average temperature suitable for living organisms. |
| Reduces extreme day and night temperature differences. | Creates a stable environment for plants, animals and humans. |
Conclusion:
The atmosphere maintains a balanced temperature by allowing sunlight to enter while preventing all the heat from escaping into space. This makes the Earth a habitable planet.
Question 15
Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.
Answer:
The Earth functions as a single integrated system in which the geosphere, hydrosphere, atmosphere, cryosphere and biosphere continuously interact with one another. A change in one sphere influences the others. This interaction maintains the balance necessary for sustaining life on Earth.
Interrelationship among Earth's Spheres
| Earth Sphere | Interaction with Other Spheres |
|---|---|
| Geosphere | Provides land, minerals and soil that support plants and animals. |
| Hydrosphere | Supplies water required for life and participates in the water cycle. |
| Atmosphere | Provides gases such as oxygen and carbon dioxide and regulates climate. |
| Cryosphere | Stores freshwater as snow and ice and influences global climate. |
| Biosphere | Includes all living organisms that exchange matter and energy with all other spheres. |
Example of Delicate Balance
Forests provide an excellent example of the interaction among Earth's spheres.
- Trees (biosphere) absorb carbon dioxide from the atmosphere during photosynthesis.
- Their roots anchor the soil of the geosphere and reduce erosion.
- Plants release water vapour into the atmosphere through transpiration.
- This water vapour contributes to cloud formation and rainfall, connecting the atmosphere and hydrosphere.
- Rainwater replenishes rivers, lakes and groundwater, supporting all living organisms.
If forests are removed, carbon dioxide increases, rainfall patterns change, soil erosion becomes severe, biodiversity declines and the balance among all the Earth's spheres is disturbed.
| Change in One Sphere | Effect on Other Spheres |
|---|---|
| Deforestation in the Biosphere | Less carbon dioxide is absorbed from the atmosphere. |
| Increase in Atmospheric CO2 | Global temperature rises. |
| Higher Temperature | Glaciers in the cryosphere melt faster. |
| Melting Glaciers | Sea level rises in the hydrosphere. |
| Sea-Level Rise and Climate Change | Habitats are affected and biodiversity in the biosphere declines. |
Conclusion:
All the Earth's spheres are closely interconnected. Maintaining the balance among them is essential for a stable climate, healthy ecosystems and the survival of life on Earth.