CBSE Class 9 Science Chapter 13 Earth As A System Energy, Matter, And Life MCQs Set 02

Answer: (B) Hydrosphere

Explanation:
The hydrosphere encompasses all forms of liquid water on Earth's surface and underground, including oceans, rivers, lakes, and groundwater. The geosphere contains solid rock and soil, the cryosphere is frozen water, and the biosphere includes living organisms.

Answer: (B) The Sun's rays strike the equator at a steeper angle and are spread over a smaller area

Explanation:
The spherical shape of Earth means the Sun's rays hit the equator more directly and concentrate over a smaller surface area, while they strike the poles at a shallow angle and spread over a larger area. This geometry, not distance from the Sun, causes the uneven heating.

Answer: (B) It absorbs short-wavelength ultraviolet radiation that would otherwise harm living organisms

Explanation:
The ozone layer in the stratosphere absorbs harmful UV radiation, protecting life from skin damage and other harmful effects. Greenhouse gases trap infrared heat; the ozone's role is specifically UV protection.

Answer: (B) The valley floor loses heat rapidly while slopes remain warm, causing dense air to flow downhill

Explanation:
After sunset, mountain slopes cool faster than the valley floor and become denser. This cool, dense air sinks and flows downhill into the valley, creating the mountain breeze. This is the reverse of the daytime valley breeze pattern.

Answer: (B) They transport heat toward higher latitudes, moderating temperatures and keeping ports ice-free

Explanation:
Warm ocean currents carry heat from equatorial regions poleward, reducing temperature differences across the planet. The Gulf Stream keeps northwestern European ports ice-free even at high latitudes. Their effect is specifically thermal, not mechanical barrier-based or salinity-driven.

Answer: (A) • Fast cycle: photosynthesis and respiration over days to years • Slow cycle: fossil fuel formation and combustion over millions of years • Both cycles release CO₂, but at vastly different rates and storage durations

Explanation:
The fast cycle operates on short timescales through photosynthesis and respiration, while the slow cycle involves the burial of organic matter over millions of years, its conversion to fossil fuels, and eventual combustion—releasing stored carbon very quickly. This contrast is key to understanding CO₂ imbalance today.

Answer: (A) Excess nitrate runoff stimulates algal blooms that consume oxygen, killing fish and depleting the ecosystem

Explanation:
Eutrophication occurs when runoff containing excess nitrogen (as nitrates) enters water bodies, causing rapid algae growth. When these algae die and decompose, oxygen is depleted, killing fish and disrupting the ecosystem. This is a well-documented consequence of agricultural pollution.

Answer: (A) The Montreal Protocol targeted a single, chemically simple problem with clear substitutes available

Explanation:
The Montreal Protocol successfully reduced ozone-damaging CFCs through global cooperation and available alternatives, with the ozone layer now recovering. The Kyoto Protocol's goal of reducing CO₂ emissions proved less successful because replacement energy sources took longer to develop. The chapter notes this contrast explicitly.

Answer: (C) Ammonification

Explanation:
Ammonification is the decomposition step where bacteria and fungi break down organic matter and release ammonia (NH₃) back into the soil. Nitrogen fixation converts atmospheric N₂ to ammonia; nitrification converts ammonia to nitrite and nitrate; denitrification converts nitrate back to N₂.

Answer: (B) The capacity of greenhouse gases to trap outgoing infrared radiation and warm the Earth

Explanation:
More greenhouse gases directly trap more of the infrared radiation re-radiated by Earth's surface, intensifying the greenhouse effect and causing warming. Photosynthesis rates, albedo changes, and salinity shifts are secondary or long-term consequences rather than direct short-term effects.

Answer: (B) At different latitudes, the Sun's rays strike the surface at different angles, concentrating heating near the equator and spreading it over larger areas toward the poles

Explanation:
Earth's curvature means the Sun's rays are perpendicular to the equatorial surface (concentrated heating) but strike the poles at shallow angles (spread over larger areas). This geometric fact, independent of atmospheric effects, is the primary cause of latitude-dependent heating differences.

Answer: (B) Precipitation in the form of rain, snow, or hail

Explanation:
Precipitation is the process by which water returns to Earth's surface from clouds. Infiltration, transpiration, and groundwater flow occur after water reaches the surface. The cycle shown in the chapter moves from evaporation to condensation to precipitation as the primary pathway.

Answer: (B) Large quantities of meltwater flowing into oceans will raise sea levels over time

Explanation:
As glaciers and polar ice melt, the added freshwater enters the oceans, raising sea levels. The chapter explicitly mentions this as a threat to low-lying coastal cities. While melting does affect ocean ecosystems, the primary hazard to coastal cities is sea level rise.

Answer: (C) Electromagnetic waves can travel through a vacuum at the speed of light, while sound waves require a medium

Explanation:
A key distinction made in the chapter is that electromagnetic waves travel through a vacuum at 3 × 10⁸ m/s, unlike sound waves (mechanical waves) which require a medium. This is fundamental to understanding how solar energy reaches Earth from the Sun.

Answer: (A) Burning fossil fuels releases stored carbon (geosphere) into the atmosphere, intensifying the greenhouse effect and warming oceans (hydrosphere), which affects rainfall patterns (atmosphere) and threatens agricultural productivity (biosphere)

Explanation:
This scenario directly illustrates how disturbing one cycle (carbon) by tapping geosphere reserves creates cascading effects across the hydrosphere, atmosphere, and biosphere. The chapter emphasises this interconnectedness and how human disruption of one cycle affects all others.

Answer: (C) Warm air rises at the equator and moves poleward at higher altitudes, cools and sinks around 30° latitudes, creating high pressure belts from which air flows back toward the equator

Explanation:
The chapter explains that intense heating causes warm air to rise at the equator, forming a low pressure belt. As this air moves poleward at higher altitudes and cools, it becomes denser and sinks around 30° North and South latitudes, establishing high pressure belts. From these regions, air flows back toward the equator along the surface, completing the circulation cycle that drives planetary winds.

Answer: (B) Ozone absorption of UV rays heats the stratosphere and causes temperature to increase with height, preventing vertical mixing of air and confining weather to the troposphere

Explanation:
In the stratosphere, the ozone layer absorbs UV radiation and heats the atmosphere, causing temperature to rise with altitude. This temperature inversion prevents the vertical circulation and convection that would otherwise create weather disturbances. In contrast, the troposphere is heated from the Earth's surface below, driving upward air movement and weather formation.

Answer: (C) Snow (0.80–0.90) and ocean water

Explanation:
Snow with an albedo of 0.80–0.90 reflects most incoming solar radiation and heats up very slowly. Ocean water, having lower albedo, absorbs more radiation and becomes warmer. This extreme difference between high and low albedo materials creates the most pronounced contrast in heating rates, which is why polar regions with high-albedo snow and ice remain very cold despite receiving the same amount of solar radiation as other areas.

Answer: (C) Atmospheric nitrogen gas cannot be used by plants and must first be converted to ammonia, which can then be further transformed by nitrifying bacteria into nitrite and nitrate that plants absorb

Explanation:
• Nitrogen gas (N₂) is non-reactive and cannot be directly used by plants or animals
• Rhizobium bacteria convert N₂ to ammonia (NH₃), making it available in a usable form
• Nitrifying bacteria then transform ammonia into nitrite and finally nitrate (NO₃⁻)
• Plants assimilate these nitrogen compounds from soil, completing the crucial step that transfers atmospheric nitrogen into the biosphere

Answer: (B) The Montreal Protocol recovery shows that international agreements can reverse environmental damage, whereas excess CO₂ from human activities continues to intensify the greenhouse effect despite some policy efforts

Explanation:
The chapter highlights that the Montreal Protocol has successfully begun recovery of the ozone layer through global cooperation, demonstrating the power of coordinated international action. In contrast, despite the Kyoto Protocol and Paris Agreement, CO₂ levels continue rising because fossil fuel burning and deforestation saturate natural carbon sinks. This comparison shows that while some environmental crises can be reversed through collective effort, the failure to adequately reduce greenhouse gas emissions shows how persistent certain human-driven impacts can be when mitigation efforts are insufficient.

Answer: (B) Planetary wind belts would still form, but without continents to block and redirect currents, ocean circulation patterns would form simpler, more symmetrical gyres around the globe without the complex current modifications caused by landmasses

Explanation:
While uneven heating between equator and poles would still create planetary wind belts and pressure systems on an all-ocean world, the chapter notes that continents further modify ocean current paths by blocking and redirecting them. Without this blocking effect, ocean gyres would likely follow simpler, more symmetrical circular patterns driven solely by the atmospheric pressure belts and the Coriolis effect from Earth's rotation, rather than the irregular patterns we observe due to continental interference.

Answer: (B) Warming of the Arabian Sea increases evaporation, putting more moisture into the atmosphere; when planetary winds drive this moisture-rich air inland during monsoon season, it produces heavier rainfall in some areas while other regions experience drought due to disrupted circulation patterns

Explanation:
Warmer Arabian Sea water leads to more evaporation, introducing extra moisture into the atmosphere. When this moisture-laden air encounters the planetary wind systems during monsoon season, it intensifies rainfall in areas along the monsoon path. However, because the warming disrupts the delicate balance of atmospheric circulation, other regions may experience drought as moisture is redirected. The chapter emphasizes that this illustrates how changes in one sphere (hydrosphere warming) disrupt processes across multiple spheres, affecting the atmosphere and biosphere through altered precipitation patterns.

Answer: (B) The fast cycle allows plants and animals to exchange carbon daily through photosynthesis and respiration, keeping carbon available in the biosphere; the slow cycle stores excess carbon in fossil fuels and ocean sediments, preventing runaway greenhouse warming. If the slow cycle reversed and released all stored carbon rapidly, atmospheric CO₂ would spike catastrophically

Explanation:
The chapter explains that plants use atmospheric CO₂ through photosynthesis in the fast cycle, while respiration and decomposition return it, maintaining a balance suitable for life. The slow cycle removes carbon from the atmosphere by burying dead organisms and storing carbon as fossil fuels, preventing excessive greenhouse heating. If geological processes suddenly released all stored carbon (reversing the slow cycle), it would flood the atmosphere with CO₂, intensifying the greenhouse effect far beyond what human fossil fuel burning alone has achieved, making the climate inhospitable.

Answer: (B) Excess nitrogen from agricultural runoff stimulates massive algal blooms; when these algae die and decompose, bacterial decomposition consumes available oxygen, creating anoxic zones where fish cannot survive

Explanation:
Excess nitrogen reaches rivers and lakes through agricultural runoff, fertilising rapid algal growth (algal blooms). When these blooms die, decomposing bacteria consume dissolved oxygen to break down the organic matter, depleting the oxygen that fish and other aquatic organisms need. This creates dead zones where life cannot persist, demonstrating how a nutrient excess in one part of the biogeochemical cycle (nitrogen) disrupts the oxygen cycle and harms aquatic ecosystems.

Answer: (B) Concrete and brick absorb more solar radiation and retain and re-radiate heat throughout the day and night, while mud and wood absorb less radiation and re-radiate less heat, keeping interior temperatures lower

Explanation:
The chapter explains that different materials absorb and re-radiate solar radiation differently. Concrete and brick have lower albedo, absorbing significant solar energy and retaining it as heat, which is then re-radiated into the building's interior, especially at night. Mud and wood absorb and re-radiate less heat, maintaining cooler conditions inside even during hot summers. This demonstrates how material properties directly influence local temperature through albedo and heat retention.