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The biosphere represents the global ecological system integrating all living beings and their relationships. It is the specific life-supporting zone where the solid crust (lithosphere), water systems (hydrosphere), and air envelope (atmosphere) meet and interact to sustain life.

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Concept Explanation

Earth's clean, dry atmosphere is a mix of distinct gases. Nitrogen is the most abundant gas, making up about 78% of the air volume. Oxygen accounts for roughly 21%, while trace gases like argon and carbon dioxide make up the remaining 1%.

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The outermost solid shell of our planet, which includes the crust and upper mantle, is called the lithosphere. It forms the landmasses, mountains, and ocean floors we see today.

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Carbon dioxide fixation happens through two main pathways: green plants convert gas into glucose using solar photosynthesis, and certain marine organisms convert dissolved carbon dioxide into carbonate shells. Respiration and combustion release carbon dioxide back into the air.

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Concept Explanation

Air is a bad conductor of heat. The atmosphere acts as a protective blanket, preventing intense solar radiation from overheating the Earth's surface during the day, and trapping heat at night to keep the planet warm.

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During the day, land heats up faster than water. The air above the land warms and rises, creating a low-pressure zone. Cooler, denser air over the sea then rushes inland to fill this gap, generating a daytime sea breeze that blows from sea to land.

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Concept Explanation

At night, land cools down much faster than water due to its lower specific heat capacity. The air above the warmer sea water rises, creating a low-pressure area over the ocean. The cooler, denser air over the land then moves out toward the sea, creating a land breeze.

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Concept Explanation

As water vapor rises, it expands and cools. This cooling causes the gas to transition back into liquid droplets through condensation. These tiny droplets form around airborne dust particles, clustering together to create clouds.

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The water cycle maintains the global circulation of moisture. Water evaporates from oceans and lakes, transitions into clouds via condensation, precipitates back down as rain or snow, and returns to the seas through river channels.

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The term smog is a blend of the words "smoke" and "fog." It forms when unburnt fossil fuel particulates and pollutants mix with ambient water vapor in cold weather, reducing visibility and indicating air pollution.

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All cellular processes take place in a fluid environment. Water acts as a universal solvent, dissolving chemical reactants so they can interact during cellular metabolic processes, while also helping transport nutrients across membranes.

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Concept Explanation

When synthetic chemicals like fertilizers dissolve into rivers and ponds, they cause water pollution. Fertilizers spur rapid algal blooms that deplete dissolved oxygen levels, which can lead to the suffocation of fish and other aquatic organisms.

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Soil formation is a slow, long-term process driven by the mechanical and chemical weathering of rocks. Environmental forces like solar heating, water freezing expansion, wind friction, and pioneer organisms like lichens gradually pulverize solid stone into fine sand and silt grains.

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Concept Explanation

Humus is the dark, organic material in soil created by the decomposition of plant leaves and animal matter by soil microorganisms. It improves soil structure, aeration, and water retention while supplying nutrients to vegetation.

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Concept Explanation

The loss of topsoil through wind or water action is called soil erosion. Vegetative roots bind soil particles together, meaning deforestation greatly accelerates erosion by leaving topsoil exposed to rain and wind runoff.

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Concept Explanation

Plants cannot absorb atmospheric nitrogen gas directly. Leguminous plants (pulses, peas, lentils) house symbiotic *Rhizobium* bacteria inside specialized root nodules. These microorganisms fix inert nitrogen gas into soluble ammonium ions that the host plant can easily absorb.

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Concept Explanation

The high temperatures and pressures generated by lightning strikes provide the energy needed for atmospheric nitrogen ($N_2$) to react with oxygen ($O_2$), producing oxides of nitrogen. These oxides dissolve in rainwater to form **nitrites and nitrates**, falling to earth where plants can absorb them.

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Concept Explanation

Respiration, combustion, and the formation of nitrogen oxides consume atmospheric oxygen. Photosynthesis by green vegetation and phytoplankton is the only major natural process that releases molecular oxygen back into the atmosphere by splitting water molecules.

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Carbon dioxide prevents infrared heat reflected from the Earth's surface from escaping back into space. As emissions increase, this trapped heat intensifies the greenhouse effect, leading to global warming and climate disruptions.

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Concept Explanation

While standard elemental oxygen gas is diatomic ($O_2$), ozone is an elemental allotrope with the formula O₃. Located in the upper stratosphere, this triatomic gas shield absorbs harmful solar ultraviolet (UV) radiation, protecting life on Earth.

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Concept Explanation

Water requires more thermal energy input to raise its temperature by one degree than soil or rock does because water has a high **specific heat capacity**. Consequently, during the day, the land absorbs heat and warms up much faster than the ocean, making the reason a correct explanation for the assertion.

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Concept Explanation

Lichens absorb water and nutrients directly from the air across their entire surface. Because they are highly sensitive to industrial **sulphur dioxide**, they cannot survive in polluted cities or industrial zones. Their absence serves as a reliable biological indicator of poor air quality.

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Concept Explanation

The global oxygen cycle balances consumption and production. Animals and plants consume oxygen during respiration, and fires consume it during combustion. However, green vegetation returns an equivalent amount of oxygen to the atmosphere through photosynthesis, keeping global levels stable.

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Concept Explanation

While chemical fertilizers supply immediate macronutrients (NPK) to crops, their prolonged use alters soil pH and salinity. This hostile chemical environment kills off the natural decomposers and earthworms responsible for recycling organic matter into humus, which degrades the soil structure over time.

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Concept Explanation

Carbon dioxide is transparent to incoming shortwave sunlight but absorbs longwave infrared heat bouncing back off the planet. By blocking this heat from escaping into cold space, it raises the temperature of the lower troposphere, driving the greenhouse effect.

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Concept Explanation

Greenhouse gases are atmospheric gases that absorb and emit infrared thermal radiation. Carbon dioxide and water vapour are highly effective at trapping this infrared heat bounced off the Earth's surface, keeping our planet's temperature regulated. Diatomic oxygen and nitrogen do not absorb infrared radiation and are not greenhouse gases.

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Concept Explanation

Oxygen is removed from the atmosphere by three main processes: biological respiration, fuel combustion, and the formation of nitrogen oxides during high-energy lightning strikes. Photosynthesis (iii) is the unique biological process that does the opposite—it generates and releases oxygen back into the atmosphere.

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Concept Explanation

The lithosphere is the solid crust. The hydrosphere encompasses all liquid water, ice, and vapor reservoirs. The atmosphere forms the gaseous insulating envelope around Earth, and the biosphere is the overlapping interaction zone where conditions allow life to thrive.

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Concept Explanation

All four factors drive soil formation. The **Sun** cracks rocks through uneven thermal expansion. **Water** enters these cracks, breaking stone when it freezes or carving paths as a river. **Wind** erodes rock surfaces via friction, and **living organisms** like lichens secrete organic acids to chemically break down rock into fine soil grains.

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Concept Explanation

Evaporation changes surface water into gas. Condensation happens as this vapor rises and cools into liquid drops to create clouds. Precipitation releases this water as rain, and collection gathers the runoff back into major bodies of water.

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Water pollution manifests in three main ways: dumping toxic chemicals (pesticides, sewage), depleting dissolved oxygen (often caused by fertilizer runoff fueling algal blooms), or dumping hot water from industrial cooling towers, which causes thermal pollution. A simple seasonal increase in rain (iv) is a natural weather event, not a form of pollution.

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Concept Explanation

Nitrogen is an essential nutrient for life. It forms the amine groups in **proteins**, the nitrogenous bases in **nucleic acids** (DNA/RNA), and is a core component of many metabolic **vitamins**. Glucose (iv) is a carbohydrate built out of carbon, hydrogen, and oxygen ($C_6H_{12}O_6$), containing no nitrogen.

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Concept Explanation

Biological respiration, deforestation fires, fossil fuel combustion, and industrial activities release carbon dioxide into the air. Photosynthesis (iii) is the primary counter-process that absorbs carbon dioxide to synthesize organic compounds.

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Concept Explanation

While oceans hold about 97% of Earth's water, it is highly saline. Fresh water is found in rivers, streams, lakes, underground aquifers, and locked away as ice sheets inside polar glaciers and **ice caps**, satisfying option C.

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Concept Explanation

Humus is decayed organic matter. Soil erosion is the loss of topsoil due to wind or water action. Smog is a visible air pollutant formed by a mix of smoke and fog, and lichens are organisms whose presence or absence indicates local air pollution levels.

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Concept Explanation

Inhaling suspended particulates, hydrocarbons, and sulfur oxides damages the respiratory lining, causing allergies and cancer, while also putting strain on the cardiovascular system. Air pollution does not increase soil fertility (iv); instead, it can lead to acid rain, which reduces fertility.

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Concept Explanation

The nitrogen cycle relies on specific biochemical steps: fixing inert gas into usable forms, converting ammonia into nitrates (nitrification), and converting soil nitrates back into nitrogen gas (denitrification) via specialized microbes. Carbon fixation (iv) belongs exclusively to the carbon cycle.

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Concept Explanation

Vegetative roots anchor soil particles against wind and water action. Afforestation, preserving plant covers, and terrace farming on slopes reduce topsoil erosion. Deforestation (iii) strips away this root network, increasing topsoil erosion.

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Concept Explanation

Nitrogen fixation converts atmospheric gas into nitrogen compounds. Lightning provides the energy to fuse nitrogen with oxygen into nitrates. Root nodules house symbiotic *Rhizobium* bacteria, and denitrification converts nitrates back into nitrogen gas.

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Concept Explanation

Photosynthesis absorbs carbon dioxide to synthesize carbohydrates. Respiration breaks down glucose, releasing carbon dioxide back into the atmosphere. Combustion releases carbon dioxide through burning fossil fuels, and the greenhouse effect describes the trapping of thermal energy by these gases.

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Concept Explanation

The oxygen cycle controls atmospheric gas balance. The carbon cycle tracks carbon through life forms and structural sinks. The water cycle drives global precipitation, and the nitrogen cycle converts atmospheric gas into soluble nitrates.

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Concept Explanation

Fertilizers introduce excess nutrients that fuel algal blooms. Pesticides introduce toxic compounds that kill aquatic life. Sewage adds domestic pathogens to ecosystems, and industrial discharge raises water temperatures, causing thermal pollution.

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Concept Explanation

Ozone absorbs harmful ultraviolet rays. Chlorofluorocarbons (CFCs) are stable synthetic compounds that release chlorine atoms in the upper atmosphere, breaking down ozone molecules. High-energy UV rays can cause skin cancers, and the stratospheric ozone hole was famously discovered over Antarctica.

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Concept Explanation

Sea breezes blow inland during the day. Land breezes blow out to sea at night. Low pressure forms when warm air rises, and high pressure forms when cool, dense air sinks. Air moves from high-pressure to low-pressure areas, generating wind.

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Concept Explanation

Topsoil is the nutrient-rich outermost layer of soil. Humus supplies organic content and enhances fertility. Deforestation removes protective forest canopies, increasing soil erosion, while maintaining an active vegetative cover uses root systems to anchor topsoil against wind and rain.

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Concept Explanation

Water is an absolute requirement for cellular metabolic processes. Because areas with high moisture availability and regular rainfall can support a larger volume and variety of vegetation, they attract diverse animal populations, directly validating why rainfall patterns explain regional biodiversity.

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Concept Explanation

Humus is crumbly and organic, improving soil crumb structure. By creating small, crumbly spaces between mineral grains, it increases soil porosity so roots can breathe and absorb water easily, while also preventing nutrient runoff. This directly explains the high fertility of the soil.

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Concept Explanation

Fossil fuels (coal, petroleum) contain high concentrations of ancient stored carbon. When burned in the presence of oxygen (combustion), this carbon undergoes oxidation, forming carbon dioxide gas ($C + O_2 \rightarrow CO_2$) which enters the air. The chemical equation of combustion directly supports the assertion.

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Concept Explanation

Both statements are factually correct. Ground-level ozone is a highly reactive, toxic gas that damages human lung tissue and crops, acting as a pollutant. Conversely, stratospheric ozone is beneficial because it filters out harmful UV radiation. However, its protective role in the stratosphere does not explain why it is toxic at ground level, which makes option B correct.

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Concept Explanation

• Ecosystem Stability: Aquatic life depends on dissolved oxygen for aerobic respiration. Consistent oxygen levels are vital to support biodiversity and prevent mass die-offs.
• Suffocation Risk: When oxygen levels drop (hypoxia), aerobic organisms—including most fish species—lack the necessary oxygen to survive, leading to suffocation. Because this mortality directly threatens ecosystem stability, the reason provides a perfect explanation for the assertion.