UPSC CSE Prelims
Science & Technology Previous Year Questions (PYQs)

Detailed Explanation:

CL-20, HMX, and LLM-105 are advanced high-energy explosive compounds used in military applications.

• CL-20 is one of the most powerful non-nuclear explosives developed.

• HMX is widely used in missiles, warheads, and other military munitions.

• LLM-105 is a modern explosive known for its high performance and improved thermal stability.

• Therefore, all three are explosives used in military weapons, making Option B the correct answer.

Why Other Options Are Wrong

Detailed Explanation:

In most Lithium-ion Electric Vehicle (EV) batteries, the cathode is made using combinations of Lithium, Cobalt, and Nickel.

• Statement I (Cobalt) ✅ Correct – Used in cathodes such as NMC and LCO batteries.

• Statement II (Graphite) ❌ Incorrect – Graphite is generally used in the anode, not the cathode.

• Statement III (Lithium) ✅ Correct – A key component of all lithium-ion battery cathodes.

• Statement IV (Nickel) ✅ Correct – Commonly used in NMC and NCA cathode chemistries.

• Therefore, only three elements (Cobalt, Lithium, and Nickel) usually make up battery cathodes, making Option C the correct answer.

Element-wise Analysis

EV Battery Components

Common Cathode Chemistries

• LCO – Lithium Cobalt Oxide

• NMC – Nickel Manganese Cobalt

• NCA – Nickel Cobalt Aluminum

• LFP – Lithium Iron Phosphate

Important Facts

• Cathode materials largely determine a battery's energy density and cost.

• Graphite remains the most widely used anode material in EV batteries.

• Demand for Lithium, Nickel, and Cobalt has increased significantly due to the global EV transition.

Detailed Explanation:

Correct Answer: ✅ Option 3 (I, II and III)

Direct Air Capture (DAC) is an emerging technology that removes carbon dioxide (CO₂) directly from the atmosphere. The captured CO₂ can either be permanently stored underground (carbon sequestration) or utilized in various industrial applications, making it an important tool in climate change mitigation.

✅ Statement I is Correct: DAC can capture atmospheric CO₂ and store it permanently underground, helping in carbon sequestration.

✅ Statement II is Correct: Captured CO₂ can be used in the production of plastics, chemicals, beverages, and food processing industries.

✅ Statement III is Correct: In aviation, captured CO₂ can be combined with green hydrogen to produce synthetic low-carbon aviation fuels (e-fuels).

Short Notes: Direct Air Capture (DAC)

• DAC removes CO₂ directly from ambient air.

• It is considered a Negative Emission Technology (NET).

• Captured CO₂ can be stored underground or utilized industrially.

• DAC supports carbon sequestration and climate mitigation efforts.

• CO₂ from DAC can be used to manufacture plastics, chemicals, and synthetic fuels.

• Synthetic aviation fuels are produced by combining captured CO₂ with green hydrogen.

• Major challenges include high energy requirements and operational costs.

Detailed Explanation:

Correct Answer: ✅ Long Duration Energy Storage (Option C)

Pumped-Storage Hydropower (PSH) is often called a "water battery." It stores excess electricity by pumping water from a lower reservoir to a higher reservoir and releases it later to generate electricity when demand increases.

This technology is increasingly important for integrating renewable energy sources like solar and wind, which are intermittent in nature.

❌ Irrigation of terraced crop fields: PSH is designed for energy storage, not agricultural irrigation.

❌ Lift irrigation of cereal crops: Although pumps are used, the primary objective of PSH is electricity storage and grid balancing.

✅ Long Duration Energy Storage: PSH is one of the world's most widely used and mature technologies for storing electricity over long periods.

❌ Rainwater harvesting system: Rainwater harvesting focuses on water conservation, not electricity storage.

Short Notes: Pumped-Storage Hydropower (PSH)

• PSH is the largest grid-scale energy storage technology in the world.

• It uses two reservoirs at different elevations.

• During low demand, water is pumped uphill using surplus electricity.

• During peak demand, water flows downhill through turbines to generate power.

• Round-trip efficiency is generally around 70–80%.

• PSH helps integrate solar and wind energy into the power grid.

• It provides long-duration energy storage and grid stability.

• India is promoting PSH projects to support its renewable energy transition.

Detailed Explanation:

Correct Answer: ✅ 1 Only (Option A)

Hydrogels are three-dimensional polymer networks that can absorb and retain large amounts of water. Their biocompatibility and controlled swelling properties make them highly useful in medical applications.

✅ Statement I is Correct: Hydrogels are widely used in controlled drug delivery because they can absorb water and release medicines gradually at a desired rate.

❌ Statement II is Incorrect: Mobile air-conditioning systems use refrigerants, compressors, and heat exchangers. Hydrogels are not standard components in these cooling systems.

❌ Statement III is Incorrect: Industrial lubricants are generally oil-based or synthetic formulations. Hydrogels do not possess the required lubrication properties for industrial use.

Short Notes: Hydrogels

• Hydrogels are water-absorbing polymer networks capable of holding large amounts of water.

• They can contain up to 90–99% water while maintaining their structure.

• Widely used in controlled drug delivery systems.

• Used in contact lenses, wound dressings, and tissue engineering.

• Most hydrogels are biocompatible and non-toxic.

• They respond to changes in pH, temperature, and moisture.

• Hydrogels are important materials in biomedical engineering and regenerative medicine.

Detailed Explanation:

Correct Answer: ✅ Water Vapour (Option D)

Fuel Cell Electric Vehicles (FCEVs) use hydrogen as fuel. Inside the fuel cell, hydrogen reacts with oxygen from the air to generate electricity that powers the vehicle.

The by-products of this reaction are water (H₂O) and heat. Therefore, the only exhaust emission released from the vehicle's tailpipe is water vapour, making FCEVs a clean-energy transportation technology.

❌ Hydrogen Peroxide: Not produced as a normal exhaust product in fuel cells.

❌ Hydronium (H₃O⁺): Exists in aqueous solutions and is not emitted from vehicles.

❌ Oxygen: Oxygen is consumed in the reaction, not emitted.

✅ Water Vapour: The primary and only exhaust emission from hydrogen fuel cell vehicles.

Short Notes: Fuel Cell Electric Vehicles (FCEVs)

• FCEVs use hydrogen (H₂) as fuel to generate electricity.

• Electricity is produced through an electrochemical reaction, not combustion.

• Main reaction: Hydrogen + Oxygen → Electricity + Water + Heat.

• The exhaust emission is only water vapour (H₂O).

• Fuel cells are considered zero tailpipe emission technology.

• Hydrogen is stored in high-pressure tanks inside the vehicle.

• Examples include the Toyota Mirai and Hyundai Nexo.

• FCEVs offer fast refuelling and longer driving ranges compared to many battery EVs.

Detailed Explanation:

Correct Answer: ✅ Only Two (Option B)

Radar works by transmitting radio waves and detecting their reflected signals from objects. It is widely used in weather monitoring and wildlife tracking, but not for identifying narcotics on passengers.

❌ Statement I is Incorrect: Narcotics detection at airports is generally done using X-ray scanners, trace detectors, sniffer dogs, and chemical analysis, not radar.

✅ Statement II is Correct: Weather radars are extensively used to monitor precipitation such as rain, snow, and storms.

✅ Statement III is Correct: Radar technology is used to track the migration of birds, bats, and other animals for ecological studies.

Short Notes: Radar Applications

• RADAR stands for Radio Detection And Ranging.

• It uses radio waves to detect the location, speed, and direction of objects.

• Weather radars monitor rainfall, thunderstorms, and cyclones.

• Doppler Radar can measure the velocity of moving objects.

• Radar is widely used in aviation, defense, navigation, and meteorology.

• It helps track bird and animal migration over long distances.

• Radar works effectively in darkness, fog, and cloudy conditions.

Detailed Explanation:

Correct Answer: ✅ Option 4 (None)

Fifth-generation fighter aircraft are characterized by stealth technology, sensor fusion, supercruise capability, advanced avionics, and network-centric warfare systems. None of the aircraft listed belong to the fifth-generation category.

❌ Rafale – Incorrect: The Dassault Rafale is a 4.5-generation multirole fighter aircraft developed by France.

❌ MiG-29 – Incorrect: The Mikoyan MiG-29 is a fourth-generation fighter aircraft.

❌ Tejas MK-1 – Incorrect: The HAL Tejas Mk1 is generally classified as a fourth-generation light combat aircraft with some modern upgrades.

Therefore, none of the above are fifth-generation fighter aircraft.

Short Notes: Fighter Aircraft Generations

• 4th Generation: High maneuverability, fly-by-wire systems, pulse-Doppler radar (e.g., MiG-29).

• 4.5 Generation: Advanced avionics, AESA radar, limited stealth features (e.g., Rafale).

• 5th Generation: Full stealth, sensor fusion, supercruise, network-centric warfare.

• Examples of 5th-generation fighters: F-22 Raptor, F-35 Lightning II, Sukhoi Su-57.

• India currently operates Rafale, Su-30MKI, MiG-29, and Tejas, but none are fifth-generation fighters.

• India is developing the Advanced Medium Combat Aircraft (AMCA).

• Stealth technology reduces radar detection.

• Sensor fusion combines data from multiple sensors into a single operational picture.

Detailed Explanation:

Correct Answer: ✅ Option 1 (Nitric Oxide)

Nitric Oxide (NO) is a signaling molecule naturally produced in the human body from the amino acid L-arginine by the enzyme nitric oxide synthase (NOS). It relaxes the smooth muscles of blood vessels, causing vasodilation (widening of blood vessels), which increases blood flow and helps regulate blood pressure.

✅ Nitric Oxide – Correct: Causes vasodilation and improves blood circulation.

❌ Nitrous Oxide (N₂O) – Incorrect: Commonly known as "laughing gas"; used as an anesthetic and analgesic.

❌ Nitrogen Dioxide (NO₂) – Incorrect: A harmful air pollutant that can damage the respiratory system.

❌ Nitrogen Pentoxide (N₂O₅) – Incorrect: A reactive chemical compound with no role in human vasodilation.

Short Notes: Nitric Oxide (NO)

• Discovered as an important biological messenger; awarded the 1998 Nobel Prize in Physiology or Medicine.

• Produced from L-arginine by nitric oxide synthase (NOS).

• Causes vasodilation, increasing blood flow and lowering blood pressure.

• Plays a key role in cardiovascular health.

• Involved in neurotransmission and immune responses.

• Drugs like Nitroglycerin act by releasing nitric oxide.

• Deficiency of NO may contribute to hypertension and vascular diseases.

• Acts as a short-lived gaseous signaling molecule.

Detailed Explanation:

Correct Answer: ✅ Option 2 (Statements 2 and 3 only)

Radioisotope Thermoelectric Generators (RTGs) generate electricity from the heat released during the natural radioactive decay of isotopes. They are widely used in deep-space missions where solar power is insufficient.

❌ Statement I is Incorrect: RTGs are not miniature fission reactors. They do not use nuclear fission; instead, they generate power from the natural decay of radioactive isotopes.

✅ Statement II is Correct: RTGs are used to power spacecraft, satellites, and planetary rovers, especially in deep-space missions where sunlight is weak.

✅ Statement III is Correct: RTGs commonly use Plutonium-238, which can be produced during nuclear weapons-related processes and is specifically manufactured for civilian space applications.

Short Notes: Radioisotope Thermoelectric Generators (RTGs)

• Convert heat from radioactive decay directly into electricity using thermocouples.

• Do not involve a nuclear chain reaction or fission process.

• Common fuel: Plutonium-238 (Pu-238).

• Pu-238 has a half-life of about 87.7 years, providing long-term power.

• Ideal for missions far from the Sun where solar panels are ineffective.

• Used in missions such as Voyager 1, Voyager 2, and Curiosity Rover.

• Reliable, maintenance-free power source for decades.

• Important for deep-space exploration and planetary missions.

Detailed Explanation:

Correct Answer: ✅ Option 3 (Metaverse)

The Metaverse refers to a persistent, interconnected network of 3D virtual worlds where millions of users can interact, work, socialize, create content, and own digital assets. A key feature is interoperability, allowing movement and interaction across different virtual environments.

❌ Big Data Analytics – Incorrect: It involves analyzing large datasets to derive insights and patterns, not creating virtual worlds.

❌ Cryptography – Incorrect: It is the science of securing communication and digital transactions. While important for virtual assets, it does not describe the virtual ecosystem itself.

✅ Metaverse – Correct: It is an interoperable network of virtual worlds where users can own, buy, sell, and transfer digital assets and experiences.

❌ Virtual Matrix – Incorrect: This is not a recognized term for the concept described in the question.

Short Notes: Metaverse

• A shared 3D virtual environment accessible through the internet.

• Combines Virtual Reality (VR), Augmented Reality (AR), AI, and Blockchain technologies.

• Allows users to interact through digital avatars.

• Supports ownership of virtual assets through NFTs and digital property rights.

• Key feature: Interoperability between virtual platforms.

• Can be used for gaming, education, business meetings, shopping, and entertainment.

• Often associated with the development of Web 3.0.

• Major technology companies are investing in metaverse ecosystems.

Detailed Explanation:

Correct Answer: ✅ Option 4 (All Four)

Distributed Energy Resources (DERs) are small-scale energy generation, storage, or management systems located close to consumers rather than large centralized power plants. They help improve grid reliability, energy efficiency, and renewable energy integration.

✅ Battery Storage – Correct: Stores electricity locally and supplies power when needed, making it a key DER.

✅ Biomass Generators – Correct: Small-scale biomass-based power plants can generate electricity near consumption points.

✅ Fuel Cells – Correct: Fuel cells produce electricity in a decentralized manner and are widely recognized as DERs.

✅ Rooftop Solar Photovoltaic Units – Correct: One of the most common DERs, generating electricity directly at homes, offices, and industries.

Short Notes: Distributed Energy Resources (DERs)

• DERs are small-scale energy resources located near end-users.

• Include both energy generation and energy storage technologies.

• Common examples: rooftop solar, battery storage, fuel cells, biomass plants.

• Reduce transmission and distribution losses.

• Improve grid resilience and reliability.

• Support integration of renewable energy sources.

• Can operate independently or alongside the main grid.

• Important for the development of smart grids and decentralized energy systems.

Detailed Explanation:

Answer: Option 3 — Statement-I is correct but Statement-II is incorrect

India indeed has domestic uranium deposits but relies heavily on coal for electricity generation (around 70% of total capacity). However, Statement-II is factually wrong because commercial nuclear power plants do not require uranium enriched to 60%.

✅ Statement-I – Correct: India has uranium deposits in Jharkhand (Singhbhum), Andhra Pradesh (Tummalapalle), and other regions, yet coal remains the dominant source for electricity production due to its abundance, lower cost, and established infrastructure.

❌ Statement-II – Incorrect: Uranium enriched to 60% or more is classified as Highly Enriched Uranium (HEU) and is used for research reactors or nuclear weapons, not commercial electricity production. India's PHWRs use natural uranium (0.7% U-235), while most commercial Light Water Reactors worldwide use Low Enriched Uranium (LEU) enriched to only 3-5% U-235.

📝 Short Notes: Nuclear Fuel Enrichment Levels

• India's Nuclear Program: Uses indigenous three-stage program starting with natural uranium in PHWRs
• Coal Dominance: India's coal reserves (4th largest globally) and lower capital costs make it preferred despite environmental concerns
• Uranium Sources: Jharkhand (Jaduguda), Andhra Pradesh (Tummalapalle - one of world's largest), Meghalaya, Rajasthan
• Nuclear Capacity: Only ~3% of India's total electricity comes from nuclear power (as of 2023)

Detailed Explanation:

Answer: Option 4 — None

All three statements regarding coal-based thermal power plants in India are incorrect. Some plants use seawater for cooling (like Mundra), many are located in water-stressed districts, and a significant number are privately owned.

❌ Statement 1 – Incorrect: The Mundra Thermal Power Plant uses seawater from the Gulf of Kutch in a closed-cycle cooling system, and also employs purified seawater from reverse osmosis plants for auxiliary systems.

❌ Statement 2 – Incorrect: According to WRI research, 40% of India's thermal power plants are situated in water-stressed regions, with 14 out of 20 largest thermal utilities experiencing shutdowns between 2013-2016 due to water shortages.

❌ Statement 3 – Incorrect: Out of India's 269 thermal power plants, 138 are publicly owned while 131 are privately owned, demonstrating significant private sector participation.

📝 Short Notes: Coal-Based Thermal Power Plants in India

• Cooling Water Sources: While most plants use freshwater from rivers or reservoirs, coastal plants like Mundra (Gujarat) utilize seawater cooling systems to conserve freshwater resources.
• Water Stress Challenge: 40% of thermal plants are in water-stressed regions, leading to operational disruptions and forced shutdowns during water scarcity periods.
• Ownership Structure: Total 269 plants - 138 public sector (Central/State utilities like NTPC, State Gencos) and 131 private sector (Adani Power, Tata Power, Reliance Power, etc.).
• Capacity Distribution: Thermal power contributes approximately 50-55% of India's total installed electricity generation capacity.
• Environmental Concerns: High water consumption (3-5 liters per kWh), thermal pollution, and emissions make water management critical for sustainability.

Detailed Explanation:

Answer: Option 3 — All three

All three statements are correct as extremophiles (microorganisms adapted to extreme conditions) can survive and thrive in environments that would be lethal to most life forms. These include extreme temperatures both above boiling and below freezing, as well as highly acidic conditions.

✅ Statement 1 – Correct: Hyperthermophiles like Pyrolobus fumarii can thrive in temperatures above 100°C (boiling point of water), often found in hydrothermal vents at temperatures up to 113°C.

✅ Statement 2 – Correct: Psychrophiles (cryophiles) are cold-loving microorganisms capable of growth in temperatures ranging from -20°C to +10°C, well below the freezing point of water (0°C).

✅ Statement 3 – Correct: Acidophiles such as Picrophilus oshimae thrive in highly acidic environments with pH below 3, with some species surviving at pH levels near 0.

📝 Short Notes: Extremophiles