Physics and chemistry in daily life; energy and nuclear technology: previous-year questions
46 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 13 of them came in the last five papers.
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In the context of electric vehicle batteries, consider the following elements :
- I. Cobalt
- II. Graphite
- III. Lithium
- IV. Nickel
How many of the above usually make up battery cathodes?
Answer (c) & explanation
Three of the four usually make up cathodes (c). A lithium-ion cathode is a lithium metal oxide, such as nickel-manganese-cobalt (NMC), nickel-cobalt-aluminium (NCA), or lithium iron phosphate (LFP). So lithium (III), nickel (IV) and cobalt (I) go into cathodes. Item II is wrong: graphite is the usual anode material, not the cathode. So options (a), (b) and (d) fail.
Notice the shift to LFP batteries, which avoid costly cobalt and are safer, now common in EVs. Sodium-ion batteries are an emerging cheaper option. India's PLI scheme for Advanced Chemistry Cell batteries aims to make cells at home, and the Kolar and Jammu lithium finds made news.
Answer as per the official UPSC answer key.
Consider the following substances :
- I. Ethanol
- II. Nitroglycerine
- III. Urea
Coal gasification technology can be used in the production of how many of them?
Answer (b) & explanation
Coal gasification can produce two of them (b). Gasification turns coal into syngas, a mix of carbon monoxide and hydrogen. Item III is right: syngas can be made into ammonia and then urea, as at the revived Talcher fertiliser plant. Item I is right: syngas can be made into methanol and ethanol, which is why India's coal gasification mission names these products. Item II is wrong: nitroglycerine is made by nitrating glycerol, which comes from fats, not from syngas.
Notice the National Coal Gasification Mission, aiming to gasify 100 million tonnes of coal by 2030, to cut imports of gas, ammonia and methanol. Gasification is cleaner than burning coal directly, but still emits CO₂.
Answer as per the official UPSC answer key.
What is the common characteristic of the chemical substances generally known as CL-20, HMX and LLM-105, which are sometimes talked about in media?
Answer (b) & explanation
They are explosives used in military weapons (b). CL-20 (hexanitrohexaazaisowurtzitane), HMX (octogen) and LLM-105 are high explosives used in warheads and munitions. CL-20 is among the most powerful non-nuclear explosives. LLM-105 is valued because it is insensitive, so it does not go off easily by accident. Option (a) is wrong: they are not refrigerants. Options (c) and (d) are wrong: they are not fuels for cruise missiles or rockets, which burn steadily rather than detonate.
Notice India's interest: DRDO's High Energy Materials Research Laboratory in Pune works on such compounds. Other common military explosives include RDX and TNT. Some rocket propellants do add HMX for extra energy.
Answer as per the official UPSC answer key.
Consider the following statements :
Statement I : Some rare earth elements are used in the manufacture of flat television screens and computer monitors.
Statement II : Some rare earth elements have phosphorescent properties.
Which one of the following is correct in respect of the above statements?
Answer (a) & explanation
Both are correct and Statement II explains Statement I (a). Statement I is right: some rare earth elements are used in flat TV screens and monitors. Statement II is right and is the reason: rare earths such as europium, terbium and yttrium are used as phosphors, the materials that glow red, green and blue in screens and LED lighting. Their light-giving property is why screens use them. So options (b), (c) and (d) fail.
Notice their other uses: neodymium magnets in EV motors and wind turbines. China controls most rare earth processing, and its export curbs in 2025 hit carmakers worldwide. India's IREL mines rare earths from beach sands in Kerala and Odisha, and a 2025 scheme backs rare-earth magnet making.
Answer as per the official UPSC answer key.
Consider the following types of vehicles :
- I. Full battery electric vehicles
- II. Hydrogen fuel cell vehicles
- III. Fuel cell-electric hybrid vehicles
How many of the above are considered as alternative powertrain vehicles?
Answer (c) & explanation
All three are alternative powertrain vehicles (c). An alternative powertrain is any drive system other than the conventional petrol or diesel engine. Item I is right: full battery electric vehicles run only on a battery and motor. Item II is right: hydrogen fuel cell vehicles make electricity from hydrogen. Item III is right: fuel cell–electric hybrid vehicles combine a fuel cell with a battery. So options (a), (b) and (d) fail. CNG and petrol-electric hybrid cars also count.
Notice India's push: the PM E-DRIVE scheme (2024) for electric vehicles, FAME before it, and the National Green Hydrogen Mission (2023). Flex-fuel cars running on ethanol blends are another option, with E20 petrol now sold across India.
Answer as per the official UPSC answer key.
Consider the following :
- 1. Battery storage
- 2. Biomass generators
- 3. Fuel cells
- 4. Rooftop solar photovoltaic units
How many of the above are considered “Distributed Energy Resources” ?
Answer (d) & explanation
All four are Distributed Energy Resources (d). Distributed Energy Resources (DERs) are small units that generate or store power close to where it is used, often behind the consumer's meter, rather than at a big central plant. Battery storage (item 1) stores power for later. Biomass generators (item 2) make power from crop waste locally. Fuel cells (item 3) make electricity from hydrogen on site. Rooftop solar (item 4) is the most common DER. Micro-wind turbines and EV chargers that feed back power also count.
Notice the benefits: less power lost in long lines, more resilience during outages, and more renewables. India's PM Surya Ghar scheme (2024) aims at rooftop solar for one crore homes. Grouping many DERs together creates a virtual power plant.
Answer as per the official UPSC answer key.
With reference to radioisotope thermoelectric generators (RTGs), consider the following statements :
- 1. RTGs are miniature fission reactors.
- 2. RTGs are used for powering the onboard systems of spacecrafts.
- 3. RTGs can use Plutonium-238, which is a by-product of weapons development.
Which of the statements given above are correct ?
Answer (b) & explanation
Statements 2 and 3 are correct (b). Statement 2 is right: RTGs power the onboard systems of spacecraft, especially deep-space probes such as Voyager and New Horizons, and Mars rovers like Curiosity and Perseverance, where sunlight is too weak or dust blocks solar panels. Statement 3 is right: they use plutonium-238, made in reactors linked to weapons programmes. Statement 1 is wrong: an RTG is not a reactor and has no chain reaction. It turns the heat of natural radioactive decay into electricity through thermocouples.
Notice that India's Chandrayaan-3 lander carried small radioisotope heater units from BARC to test survival through the cold lunar night. Pu-238 has a half-life of about 88 years, giving decades of steady power.
Answer as per the official UPSC answer key.
In which of the following are hydrogels used ?
- 1. Controlled drug delivery in patients
- 2. Mobile air-conditioning systems
- 3. Preparation of industrial lubricants
Select the correct answer using the code given below :
Answer (a) & explanation
Hydrogels are used in controlled drug delivery (a). Hydrogels are networks of polymer chains that can soak up and hold large amounts of water while keeping their shape, like jelly. Item 1 is right: they are used for controlled drug release, letting medicine leak out slowly in the body. They are also used in soft contact lenses, wound dressings, nappies and to hold moisture in farm soil. Items 2 and 3 are not accepted by UPSC's key: hydrogels are not used in air-conditioning systems or in making industrial lubricants.
Notice the farm use: hydrogel granules mixed in soil soak up rain and release it to roots slowly, helping crops survive dry spells. India's Pusa Hydrogel, from IARI, is one such product.
Answer as per the official UPSC answer key.
Recently, the term “pumped-storage hydropower” is actually and appropriately discussed in the context of which one of the following ?
Answer (c) & explanation
Pumped-storage hydropower is discussed for long-duration energy storage (c). Pumped storage uses spare power, often cheap solar power at midday, to pump water up to a higher reservoir. At peak demand, the water is released through turbines to make electricity again. It works like a giant battery for long-duration storage, and helps fit variable renewables like solar and wind into the grid. Options (a), (b) and (d), about terrace farming, lift irrigation and rainwater harvesting, have nothing to do with it.
Notice India's push: the Power Ministry issued guidelines in 2023 to promote pumped storage projects. India has large potential, over 100 GW. Pumped storage makes up most of the world's grid storage today.
Answer as per the official UPSC answer key.
Consider the following statements :
- 1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
- 2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
Answer (a) & explanation
Only statement 1 is correct (a). Statement 1 is right: carbon fibre composites are very strong and light, so they are used in aircraft such as the Boeing 787 and Airbus A350, in racing and electric cars, wind turbine blades and sports gear like rackets and bicycles. Lighter vehicles burn less fuel. Statement 2 is wrong: carbon fibres can be recycled. Processes such as pyrolysis, which burns away the resin, and solvolysis, which dissolves it, recover the fibres, though these are costly and the fibres come out shorter.
Notice India's push: carbon fibre is a strategic material for missiles and space vehicles, and India still imports most of it. ISRO and DRDO have built their own, and private firms are setting up plants.
Answer as per the official UPSC answer key.
In a pressure cooker, the temperature at which the food is cooked depends mainly upon which of the following?
- 1. Area of the hole in the lid
- 2. Temperature of the flame
- 3. Weight of the lid
Select the correct answer using the code given below.
Answer (c) & explanation
The hole area and lid weight set the temperature (c). Water boils at a higher temperature when the pressure is higher. In a pressure cooker, the pressure inside is set by the weight of the whistle on the vent divided by the area of the hole. Item 3 is right: a heavier weight holds more pressure. Item 1 is right: a smaller hole means the same weight holds more pressure. Item 2 is wrong: a hotter flame only makes the cooker reach that pressure sooner and release more steam; it does not raise the cooking temperature.
Notice the reverse case: at high altitude, low air pressure makes water boil below 100 °C, so food cooks slowly. That is why pressure cookers are vital in the mountains. Typical cookers reach about 120 °C.
Answer as per the official UPSC answer key.
Water can dissolve more substances than any other liquid because
Answer (a) & explanation
Water dissolves so much because it is dipolar (a). A water molecule is polar, or dipolar: the oxygen end carries a slight negative charge, and the two hydrogen ends carry slight positive charges. These partial charges surround ions and other polar molecules and pull them apart, so water dissolves more substances than any other liquid. That is why it is called the "universal solvent". Options (b) and (c) are true of water but do not explain dissolving. Option (d) is true, but being an oxide of hydrogen is not the reason.
Notice water's other special traits: high specific heat, which moderates climate; hydrogen bonding; and ice being less dense than liquid water, so it floats.
Answer as per the official UPSC answer key.
With reference to street-lighting, how do sodium lamps differ from LED lamps?
- 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
- 2. As street-lights, sodium lamps have longer life span than LED lamps.
- 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.
Select the correct answer using the code given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Statement 1 is right: a sodium lamp glows in all directions (360°), so much light is wasted unless reflectors are used, while LEDs are directional. Statement 3 is right: sodium lamps give a nearly monochromatic yellow light, so colours are hard to tell apart, while LEDs render colours far better. Statement 2 is wrong: LEDs last longer than sodium lamps, often 50,000 hours or more, and use much less power.
Notice India's programmes: UJALA (2015) for LED bulbs in homes, and the Street Lighting National Programme (SLNP), run by EESL, which replaced over 1.3 crore street lights with LEDs. The 2014 Nobel Prize in Physics went for the blue LED.
Answer as per the official UPSC answer key.
In India, why are some nuclear reactors kept under “IAEA Safeguards” while others are not ?
Answer (b) & explanation
Some reactors use imported uranium, others domestic (b). After the 2008 India–US civil nuclear deal and the Nuclear Suppliers Group waiver, India separated its civilian and military facilities. Reactors fuelled by imported uranium (from Russia, Kazakhstan, Canada and others) are placed under IAEA safeguards, so suppliers can check the fuel is not used for weapons. Reactors on domestic uranium and the strategic programme stay outside. Option (a) is wrong because thorium is not the dividing line. Options (c) and (d) are wrong because all Indian power reactors are run by state-owned firms such as NPCIL.
Notice India's three-stage nuclear programme, planned by Homi Bhabha: PHWRs, then fast breeder reactors, then thorium reactors. India is not a member of the NPT.
Answer as per the official UPSC answer key.
With reference to solar water pumps, consider the following statements :
- 1. Solar power can be used for running surface pumps and not for submersible pumps.
- 2. Solar power can be used for running centrifugal pumps and not the ones with piston.
Which of the statements given above is/are correct ?
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong: solar panels can run both surface pumps, which lift water from ponds and canals, and submersible pumps, which sit inside a borewell. Statement 2 is wrong: solar power can run both centrifugal pumps and positive-displacement pumps, such as piston pumps. Each statement wrongly rules out one type; the pump type depends on the water depth and flow needed, not on the power source.
Notice PM-KUSUM (2019), which subsidises standalone solar pumps, solarisation of grid-connected pumps, and small solar plants on farmland. Solar pumps cut diesel use and power subsidies, but critics warn that free daytime power can lead to over-extraction of groundwater.
Answer as per the official UPSC answer key.
With reference to carbon nanotubes, consider the following statements :
- 1. They can be used as carriers of drugs and antigens in the human body.
- 2. They can be made into artificial blood capillaries for an injured part of human body.
- 3. They can be used in biochemical sensors.
- 4. Carbon nanotubes are biodegradable.
Which of the statements given above are correct ?
Answer (c) & explanation
Statements 1, 3 and 4 are correct (c). Carbon nanotubes are tiny tubes of carbon atoms, like rolled sheets of graphene. They are very strong, light and good conductors. Statement 1 is right because they are studied as carriers of drugs and antigens (for vaccines) into cells. Statement 3 is right because they are used in biochemical sensors, for example to detect glucose or disease markers. Statement 4 is right because research has shown that enzymes such as myeloperoxidase, found in white blood cells, can break them down. Statement 2 is wrong in UPSC's key, as artificial capillaries made from them are not established.
Notice the concerns: some long nanotubes behave like asbestos in the lungs. Sumio Iijima described them in 1991.
Answer as per the official UPSC answer key.
Consider the following phenomena :
- 1. Light is affected by gravity.
- 2. The Universe is constantly expanding.
- 3. Matter warps its surrounding space-time.
Which of the above is/are the prediction/predictions of Albert Einstein’s General Theory of Relativity, often discussed in media ?
Answer (d) & explanation
All three are predictions of general relativity (d). Einstein's General Theory of Relativity (1915) says that mass and energy curve space-time (3), and that this curve is what we feel as gravity. So light bends as it passes a massive body (1). This was confirmed by Eddington during the 1919 solar eclipse, and it causes gravitational lensing. The theory's equations also describe a universe that is not fixed but expanding (2). Einstein first resisted this, but Hubble observed it in 1929. So options that leave one out are incomplete.
Notice the news links: LIGO detected gravitational waves in 2015, and India is building LIGO-India at Hingoli, Maharashtra. Also, GPS clocks must correct for relativity.
Answer as per the official UPSC answer key.
With reference to solar power production in India, consider the following statements :
- 1. India is the third largest in the world in the manufacture of silicon wafers used in photovoltaic units.
- 2. The solar power tariffs are determined by the Solar Energy Corporation of India.
Which of the statements given above is/are correct ?
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong because, in 2018, India made almost no silicon wafers. It imported most cells and modules, mainly from China, and only assembled modules at home. Statement 2 is wrong because solar tariffs are set by competitive bidding (reverse auctions) or by electricity regulators, the CERC and SERCs. SECI only runs the auctions and acts as an intermediary buyer; it does not fix tariffs.
Notice the change since then: the PLI scheme for high-efficiency solar modules supports building the full chain, from polysilicon to wafers to modules. ALMM lists and import duties favour local makers. India crossed about 100 GW of solar capacity in 2025, aiming for 500 GW of non-fossil capacity by 2030.
Answer as per the official UPSC answer key.
Consider the following pairs :
| Commonly used/consumed materials | Unwanted or controversial chemicals likely to be found in them
| 1. Lipstick | Lead
| 2. Soft drinks | Brominated vegetable oils
| 3. Chinese fast food | Monosodium glutamate
Which of the pairs given above is/are correctly matched ?
Answer (d) & explanation
All three pairs are correct (d). Tests in several countries have found traces of lead in some lipsticks (1), a heavy metal that is toxic even in small doses. Brominated vegetable oil (BVO) was used to keep citrus flavours evenly mixed in soft drinks (2). It is banned in India, the EU and Japan, and the US FDA banned it in 2024 over thyroid concerns. Monosodium glutamate (MSG), sold as Ajinomoto, is the flavour enhancer that gives the umami taste in Chinese fast food (3). It is widely seen as safe but remains controversial.
Notice the related news: the Maggi row of 2015, when FSSAI questioned its MSG labelling and lead levels. Other food additives to know are potassium bromate (banned in bread) and trans fats.
Answer as per the official UPSC answer key.
‘Net metering’ is sometimes seen in the news in the context of promoting the
Answer (a) & explanation
Net metering promotes the production and use of solar energy by households and consumers (a). A bidirectional meter records power a rooftop solar owner exports to the grid and power drawn from it, and the consumer pays only for the net amount. It has no link to piped natural gas (b), CNG kits in cars (c) or water meters (d).
Gross metering is different: all solar power is sold to the grid at a fixed tariff. Net metering is central to India's rooftop solar programme, including PM Surya Ghar: Muft Bijli Yojana (2024). Notice how it turns consumers into "prosumers", who both produce and consume power.
Answer as per the official UPSC answer key.