CBSE Class 9 Science Chapter 13 Earth As A System Energy, Matter, And Life MCQs Set 01

Answer: (B) Geosphere

Explanation:
The geosphere is explicitly defined in the chapter as comprising solid rocks, soil, landforms and the Earth's interior. The other spheres each have distinct compositions: the hydrosphere contains liquid water, the biosphere contains living organisms, and the atmosphere is the surrounding air.

Answer: (B) 3 × 10⁸ m s⁻¹

Explanation:
The chapter explicitly states that electromagnetic waves travel through a vacuum at the speed of light, which is 3 × 10⁸ m s⁻¹. This value is fundamental to understanding solar radiation's properties.

Answer: (C) About 99%

Explanation:
According to the chapter, approximately 99 per cent of the Sun's energy falls within the ultraviolet (UV), visible and infrared (IR) wavelengths. These three regions are responsible for shaping Earth's climate and supporting life.

Answer: (B) Reflects

Explanation:
The chapter defines albedo as the fraction of solar radiation reflected by a surface. High albedo surfaces like snow reflect more light and stay cooler, while low albedo surfaces like black soil absorb more light and heat up faster.

Answer: (B) Stratosphere

Explanation:
The stratosphere, located between 12–50 km altitude, contains the ozone layer which absorbs UV rays and heats the atmosphere, causing temperature to rise with height. This is distinct from the troposphere below it where temperature decreases with height.

Answer: (B) Low pressure

Explanation:
During the day, mountain slopes heat faster than valley floors. The warm air over the slopes becomes less dense and rises, creating a low pressure region. Cooler air from the valley then moves up the slopes to replace this rising warm air, producing the valley breeze.

Answer: (B) Warm equatorial waters move toward poles at the surface while colder, denser waters flow back toward the equator at depth

Explanation:
The chapter explains that warm equatorial waters travel over the ocean surface toward the poles, while colder and denser waters slowly flow back toward the equator through deeper ocean levels. This creates a continuous circulation pattern driven by temperature differences.

Answer: (C) Gyres

Explanation:
The chapter defines gyres as large circular patterns formed when the Earth's rotation deflects moving ocean water masses. Continents further modify these paths by blocking and redirecting the currents.

Answer: (C) Nitrogen-fixing bacteria like Rhizobium

Explanation:
• Nitrogen-fixing bacteria such as Rhizobium in root nodules of legumes and Azotobacter in soil convert atmospheric N₂ into ammonia (NH₃)
• Nitrifying bacteria convert ammonia to nitrite and nitrate in a later step called nitrification
• Denitrifying bacteria do the opposite, converting nitrates back to nitrogen gas

Answer: (B) It artificially fixed nitrogen from the atmosphere to produce fertilisers on a large scale

Explanation:
The chapter describes the Haber-Bosch process as a method of artificially making ammonia from atmospheric nitrogen, which revolutionised agriculture by enabling the production of most fertilisers used today. More than half the nitrogen atoms in the human body come from this process.

Answer: (B) Increased by 35% (from 315 ppm to 420 ppm), primarily from burning fossil fuels and deforestation

Explanation:
Figure 13.14 and the accompanying text in the chapter show that atmospheric CO₂ has risen about 35% since 1960 (from 315 ppm to 420 ppm). The chapter explicitly attributes this unprecedented rise to human activities like burning fossil fuels and deforestation, which have disrupted the carbon cycle.

Answer: (B) The widespread growth of algae caused by excessive nitrogen from fertiliser runoff, depleting oxygen and killing fish

Explanation:
Eutrophication occurs when overuse of fertilisers in agriculture adds excessive nitrogen via nitrates to rivers and lakes, causing rapid algal growth. These algal blooms deplete oxygen in the water and kill fish, threatening aquatic ecosystems and coastal fisheries.

Answer: (C) Fast cycle; over days to years

Explanation:
The fast cycle of the carbon cycle operates over days to years, during which plants use photosynthesis to convert atmospheric CO₂ into glucose. The slow cycle, by contrast, takes millions of years and involves the burial and conversion of dead organisms into fossil fuels.

Answer: (B) Oxygen would gradually deplete as it is consumed through respiration and combustion but not replaced

Explanation:
Photosynthesis is the primary mechanism by which oxygen is restored to the atmosphere. If it stopped, organisms would continue consuming oxygen through respiration, fuel combustion would continue, and the atmosphere would lose its main source of oxygen replenishment, eventually depleting atmospheric O₂ supplies.

Answer: (B) The Montreal Protocol successfully recovered the ozone layer through global cooperation, while the Kyoto Protocol and Paris Agreement have been less successful in reducing CO₂ emissions

Explanation:
The chapter contrasts these international agreements directly. The Montreal Protocol has shown success in recovering the ozone layer through global cooperation, but the Kyoto Protocol and Paris Agreement, which aimed to reduce CO₂ emissions, have been less successful, highlighting the varying effectiveness of different environmental agreements.

Answer: (B) Cryosphere and hydrosphere, as ice transforms into flowing water

Explanation:
Glaciers exist within the cryosphere as solid ice, and when they melt, the water enters the hydrosphere as rivers and runoff. The chapter emphasizes how disturbances in one sphere trigger changes across others—in this case, the transition from frozen to liquid water directly involves both the cryosphere losing ice and the hydrosphere gaining water.

Answer: (B) Dark roads have lower albedo, so they absorb more sunlight and heat up more rapidly

Explanation:
The chapter defines albedo as the fraction of solar radiation reflected by a surface. Dark surfaces have low albedo—they reflect very little light and absorb most of it, converting the absorbed energy into heat. Light-coloured surfaces have high albedo and reflect more light, remaining cooler by comparison.

Answer: (B) Ozone in the stratosphere absorbs ultraviolet radiation and heats the air in that layer

Explanation:
The chapter explicitly states that in the stratosphere, ozone absorbs UV rays and heats up the atmosphere, causing temperature to rise with height. This is opposite to the troposphere, where heat comes from the Earth's surface below, making that layer warmer near the ground.

Answer: (B) Monsoon rainfall patterns will become more intense and variable across Indian regions

Explanation:
• The chapter connects warmer Arabian Sea water to increased evaporation and atmospheric moisture
• This leads to fluctuations in the southwest monsoon, causing variable rainfall patterns
• Some Indian regions receive floods while others experience drought—demonstrating the intensification and unpredictability of monsoon conditions when ocean temperatures rise.

Answer: (B) Earth's rotation on its axis causes moving air masses to curve away from their straight paths

Explanation:
The chapter explains that planetary winds would naturally flow directly from high to low pressure regions, but Earth's rotation deflects them from their straight paths—rightward in the Northern Hemisphere and leftward in the Southern Hemisphere. This deflection is a fundamental consequence of how motion is affected by a rotating reference frame.

Answer: (B) Mountain slopes heat and cool at different rates than the valley floor, reversing pressure patterns

Explanation:
During the day, mountain slopes facing the Sun heat faster than the valley floor, causing warm air to rise and creating a valley breeze. After sunset, slopes cool faster and become denser, causing cooler air to sink down into the valley as a mountain breeze. This daily cycle of unequal heating and cooling reverses the pressure gradient and wind direction.

Answer: (B) Oceans act as a massive reservoir that regulates atmospheric CO₂, but warming reduces their absorption capacity

Explanation:
The oceans absorb atmospheric CO₂, making them a crucial carbon sink. However, the chapter notes that warmer ocean water reduces its capacity to absorb CO₂ effectively. This feedback loop—where warming reduces the ocean's ability to absorb carbon, which then intensifies warming—illustrates why the large oceanic carbon reservoir is critical to climate regulation.

Answer: (B) Nitrification by Nitrobacter bacteria in the soil

Explanation:
The chapter clearly defines nitrification as the process where Nitrobacter converts nitrite (NO₂⁻) into nitrate (NO₃⁻). While Nitrosomonas begins this series by converting ammonia to nitrite, Nitrobacter completes the nitrification step, producing the form of nitrogen that plants can assimilate from soil.

Answer: (B) Fewer trees means reduced photosynthesis, leading to decreased oxygen production and increased atmospheric CO₂

Explanation:
Plants restore oxygen to the atmosphere through photosynthesis. The chapter explains the balance between oxygen consumption (respiration and combustion) and production (photosynthesis). Deforestation reduces photosynthesis directly, lowering oxygen production while human activities and animal respiration continue to consume atmospheric oxygen, shifting the balance toward oxygen depletion.

Answer: (B) Nitrates from fertiliser runoff stimulate algal blooms that consume oxygen and deplete it in the water

Explanation:
The chapter explains that excessive nitrogen from fertilisers causes widespread algal growth (algal blooms). These algae eventually die and decompose, and their decomposition consumes dissolved oxygen in the water, creating oxygen-depleted zones where fish cannot survive. This process is called eutrophication and represents a disruption of the nitrogen cycle affecting the hydrosphere and biosphere.