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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On which of the following can you find the Bureau of Energy Efficiency Star Label?
- 1. Ceiling fans
- 2. Electric geysers
- 3. Tubular fluorescent lamps
Select the correct answer using the code given below.
Answer (d) & explanation
All three carry the BEE star label (d). The Bureau of Energy Efficiency (a statutory body under the Energy Conservation Act, 2001, under the Ministry of Power) runs the Standards & Labelling programme from 2006. It rates appliances from 1 to 5 stars, with more stars meaning more efficient. Labels are mandatory for items like air conditioners, frost-free refrigerators, tubular fluorescent lamps (3), distribution transformers, electric geysers (2), colour TVs and LED lamps. Ceiling fans (1) were voluntary at first and became mandatory in 2023.
Notice the link with UJALA, which spread cheap LED bulbs, and with the Energy Conservation Building Code. The 2022 amendment to the Act also brought in a domestic carbon credit trading framework.
Answer as per the official UPSC answer key.
India is an important member of the ‘International Thermonuclear Experimental Reactor’. If this experiment succeeds, what is the immediate advantage for India?
Answer (d) & explanation
India could build fusion reactors (d). ITER, being built at Cadarache, France, is testing whether a tokamak can produce net energy from fusion, aiming for ten times more energy out than goes into heating its plasma. As one of seven members (EU, India, China, Japan, Korea, Russia, USA), India gets full access to the know-how in return for its share, including the cryostat built by L&T. Thorium (a) belongs to India's separate three-stage fission programme. Satellite navigation (b) is NavIC's field. ITER does nothing for fission reactors (c).
Fusion joins light nuclei, such as deuterium and tritium, and leaves little long-lived waste. Notice India's own tokamaks, Aditya and SST-1, at the Institute for Plasma Research, Gandhinagar.
Answer as per the official UPSC answer key.
In the context of modern scientific research, consider the following statements about ‘IceCube’, a particle detector located at South Pole, which was recently in the news :
- 1. It is the world’s largest neutrino detector, encompassing a cubic kilometre of ice.
- 2. It is a powerful telescope to search for dark matter.
- 3. It is buried deep in the ice.
Which of the statements given above is/are correct?
Answer (d) & explanation
All three are correct (d). IceCube, at the Amundsen–Scott South Pole station, has thousands of light sensors spread through a cubic kilometre of ice, making it the world's largest neutrino detector (1). It also searches for dark matter particles (2). Its sensors are buried 1.5–2.5 km deep in the ice (3), where the ice is very clear and shielded from other radiation.
Neutrinos are nearly massless particles that pass through matter almost untouched, so they carry information from distant cosmic events. India's proposed India-based Neutrino Observatory planned to study them under a mountain in Tamil Nadu.
Answer as per the official UPSC answer key.
With reference to ‘fuel cells’ in which hydrogen-rich fuel and oxygen are used to generate electricity, consider the following statements :
- 1. If pure hydrogen is used as a fuel, the fuel cell emits heat and water as by-products.
- 2. Fuel cells can be used for powering buildings and not for small devices like laptop computers.
- 3. Fuel cells produce electricity in the form of Alternating Current (AC).
Which of the statements given above is/are correct?
Answer (a) & explanation
Only statement 1 is correct (a). A hydrogen fuel cell combines hydrogen and oxygen electrochemically and gives off only water and heat (1). Statement 2 is wrong: fuel cells come in all sizes, from micro cells for laptops to megawatt units for buildings. Statement 3 is wrong: their output is direct current, which an inverter converts to AC.
Fuel cells are clean at the point of use, but their green credentials depend on how the hydrogen is made. India's National Green Hydrogen Mission (2023) aims for 5 million tonnes a year by 2030. Notice how this links chemistry to clean energy policy.
Answer as per the official UPSC answer key.
With reference to the Indian Renewable Energy Development Agency Limited (IREDA), which of the following statements is/are correct?
- 1. It is a Public Limited Government Company.
- 2. It is a Non-Banking Financial Company.
Select the correct answer using the code given below.
Answer (c) & explanation
Both are correct (c). IREDA (1987), under the Ministry of New and Renewable Energy, is a public limited government company (1). It is registered with the RBI as a Non-Banking Financial Company that lends to renewable energy and energy-efficiency projects (2).
IREDA was listed on the stock exchanges in 2023 and became a Navratna company in 2024. It helps finance India's goal of 500 GW of non-fossil capacity by 2030. Notice how specialised lenders bridge the gap between policy targets and actual projects on the ground.
Answer as per the official UPSC answer key.
With reference to technologies for solar power production, consider the following statements :
- 1. ‘Photovoltaics’ is a technology that generates electricity by direct conversion of light into electricity, while ‘Solar Thermal’ is a technology that utilizes the Sun’s rays to generate heat which is further used in electricity generation process.
- 2. Photovoltaics generates Alternating Current (AC), while Solar Thermal generates Direct Current (DC).
- 3. India has manufacturing base for Solar Thermal technology, but not for Photovoltaics.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statement 1 is correct: photovoltaic cells convert light directly into electricity, while solar thermal (concentrated solar power) uses mirrors to heat a fluid whose steam drives a turbine. Statement 2 has it reversed: PV gives direct current (DC), which an inverter turns into AC, while a turbine-driven generator produces AC. Statement 3 is wrong because in 2014 India already had a PV manufacturing base for cells and modules, though small.
Since then the PLI scheme and the ALMM list have pushed domestic module making. Notice how a basic physics idea (AC vs DC) turns into an energy-policy question.
Answer as per the official UPSC answer key.
With reference to two non-conventional energy sources called ‘coalbed methane’ and ‘shale gas’, consider the following statements :
- 1. Coalbed methane is the pure methane gas extracted from coal seams, while shale gas is a mixture of propane and butane only that can be extracted from fine-grained sedimentary rocks.
- 2. In India, abundant coalbed methane sources exist, but so far no shale gas sources have been found.
Which of the statements given above is/are correct?
Answer (d) & explanation
Neither statement is correct (d). Statement 1 is wrong because shale gas is mostly methane, like other natural gas, not just propane and butane; and coalbed methane is mainly methane but not pure. Statement 2 is wrong because India has identified shale basins, including Cambay, Krishna–Godavari, Cauvery and Damodar, and ONGC drilled its first shale well in 2013.
Shale gas is extracted by hydraulic fracturing, which needs huge volumes of water. That has limited its spread in water-stressed India. Notice how the next question on guar gum connects to this same fracking process.
Answer as per the official UPSC answer key.
Which of the following is/are the example/examples of chemical change?
- 1. Crystallization of sodium chloride
- 2. Melting of ice
- 3. Souring of milk
Select the correct answer using the code given below.
Answer (b) & explanation
Only the souring of milk is a chemical change (b): bacteria turn lactose into lactic acid, which curdles the milk protein casein, and the change cannot be reversed. Crystallisation of sodium chloride (1) and melting of ice (2) change only the physical state; the substance stays the same and the change can be undone.
Signs of a chemical change include a new substance, a change in colour or smell, gas given off, or heat released or absorbed. Rusting, burning and digestion are other examples. Notice how curd-making is everyday microbiology.
Answer as per the official UPSC answer key.
With reference to the usefulness of the by-products of sugar industry, which of the following statements is/are correct?
- 1. Bagasse can be used as biomass fuel for the generation of energy.
- 2. Molasses can be used as one of the feedstocks for the production of synthetic chemical fertilizers.
- 3. Molasses can be used for the production of ethanol.
Select the correct answer using the codes given below.
Answer (c) & explanation
Statements 1 and 3 are correct (c). Bagasse, the fibrous residue left after crushing cane, is burnt in co-generation plants to produce steam and electricity (1). Molasses, the thick syrup left after sugar crystallises, is fermented into ethanol for blending with petrol and for alcohol (3). Statement 2 is wrong: synthetic fertilisers such as urea are made from natural gas or naphtha, not molasses, though press mud from sugar mills is used as organic manure. So (a), (b) and (d) fail.
Notice the ethanol drive: India reached E20 (20% ethanol in petrol) in 2025, five years ahead of the original target, with sugarcane juice, B-heavy molasses and grains as feedstock. Bagasse is also used to make paper. Trap: organic manure is not synthetic fertiliser.
Answer as per the official UPSC answer key.
Ball bearings are used in bicycles, cars, etc., because
Answer (c) & explanation
The effective area of contact is reduced is correct (c). Ball bearings replace sliding with rolling. The balls touch the races only at tiny points, so the effective area of contact is reduced, and rolling friction is far less than sliding friction. Less friction means less heat, wear and energy loss. (a) and (b) are wrong: more contact area would increase friction. (d) fails because (c) is correct.
Notice the related ideas: lubricants such as oil and grease add a thin film to cut friction further, and roller bearings handle heavier loads. Friction is not always bad: tyres need grip, and treads increase it on roads. The invention of the wheel was the first use of rolling over sliding. Trap: 'actual' versus 'effective' area is the subtle distinction.
Answer as per the official UPSC answer key.
Consider the following phenomena :
- 1. Size of the sun at dusk
- 2. Colour of the sun at dawn
- 3. Moon being visible at dawn
- 4. Twinkle of stars in the sky
- 5. Polestar being visible in the sky
Which of the above are optical illusions?
Answer (c) & explanation
Items 1, 2 and 4 are correct (c). The sun looks larger near the horizon at dusk, a perception effect called the moon illusion, which also applies to the sun (1). Its reddish colour at dawn comes from scattering of shorter blue wavelengths as light travels through more air (2). The twinkling of stars comes from refraction by moving layers of air of different density (4). Items 3 and 5 are wrong: the moon at dawn and the pole star are really there, so they are not illusions. So (a), (b) and (d) fail.
Notice the related effects: atmospheric refraction lets us see the sun about two minutes before it actually rises and after it sets. Planets do not twinkle much because they appear as discs, not points. Trap: 'colour' is a real scattering effect, but UPSC groups it with appearances.
Answer as per the official UPSC answer key.
Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?
- 1. Dispersion
- 2. Refraction
- 3. Internal reflection
Select the correct answer using the codes given below.
Answer (d) & explanation
All three are correct (d). Sunlight is refracted, or bent, as it enters a raindrop (2), and split into its colours because each wavelength bends differently: dispersion (1). The light is then reflected internally at the back of the drop (3) and refracted again as it leaves. Together, these make a rainbow at about 42° from the point opposite the sun. So options (a), (b) and (c) leave out real processes.
Notice the related facts: a rainbow is seen only with the sun behind you; a secondary rainbow, from two internal reflections, has reversed colours and lies outside the main bow. Red is on the outside of the primary rainbow. Newton used a prism to show dispersion. Trap: internal reflection is often forgotten.
Answer as per the official UPSC answer key.
The known forces of nature can be divided into four classes, viz., gravity, electromagnetism, weak nuclear force and strong nuclear force. With reference to them, which one of the following statements is *not* correct?
Answer (a) & explanation
Gravity is the strongest is the statement not correct (a). Gravity is the weakest of the four forces; it dominates only at large scales because masses always attract and add up, while electric charges cancel. The strong nuclear force is the strongest. (b) is correct: electromagnetism acts on charged particles. (c) is correct: the weak nuclear force causes beta decay, a form of radioactivity. (d) is correct: the strong force holds protons and neutrons in the nucleus despite the protons' repulsion.
Notice the ranking: strong (1) > electromagnetic (about 1/137) > weak > gravity (about 10⁻³⁸). Electromagnetism and the weak force are unified as the electroweak force (Nobel 1979). Gravity is explained by general relativity, not yet joined with quantum theory. Trap: gravity feels powerful in daily life but is the weakest.
Answer as per the official UPSC answer key.
The efforts to detect the existence of Higgs boson particle have become frequent news in the recent past. What is/are the importance/importances of discovering this particle?
- 1. It will enable us to understand as to why elementary particles have mass.
- 2. It will enable us in the near future to develop the technology of transferring matter from one point to another without traversing the physical space between them.
- 3. It will enable us to create better fuels for nuclear fission.
Select the correct answer using the codes given below.
Answer (a) & explanation
Statement 1 only is correct (a). The Higgs boson, discovered at CERN's Large Hadron Collider in July 2012, confirmed the Higgs field, which explains why elementary particles have mass (1). Statement 2 is wrong: it has no link to teleporting matter. Statement 3 is wrong: it does not help make nuclear fuels. So (b), (c) and (d) fail.
Notice the Indian link: the particle is a boson, a class named after Satyendra Nath Bose. Peter Higgs and François Englert won the 2013 Nobel Prize in Physics. India became an associate member of CERN in 2017. The popular name 'God particle' comes from a book title, not from physics. Trap: dramatic applications are attached to basic science.
Answer as per the official UPSC answer key.
Recently, there has been a concern over the short supply of a group of elements called ‘rare earth metals’. Why?
- 1. China, which is the largest producer of these elements, has imposed some restrictions on their export.
- 2. Other than China, Australia, Canada and Chile, these elements are not found in any country.
- 3. Rare earth metals are essential for the manufacture of various kinds of electronic items and there is a growing demand for these elements.
Which of the statements given above is/are correct?
Answer (c) & explanation
Statements 1 and 3 are correct (c). In 2010, China, which then produced over 90% of the world's rare earths, cut export quotas (1), just as demand was rising for magnets, phones, batteries, wind turbines and electric vehicles (3). Statement 2 is wrong: rare earths are found in many countries, including India (monazite sands), the USA, Brazil, Vietnam and Russia. So (a), (b) and (d) fail.
Notice the recent turn: China imposed export controls on several rare earths and magnets in April 2025 and widened them in October 2025, hurting automakers. India launched a National Critical Mineral Mission (2025) and a scheme for rare earth permanent magnets (November 2025); IREL handles domestic output. Trap: 'rare' refers to concentration, not scarcity.
Answer as per the official UPSC answer key.
Consider the following statements :
If there were no phenomenon of capillarity
- 1. it would be difficult to use a kerosene lamp
- 2. one would not be able to use a straw to consume a soft drink
- 3. the blotting paper would fail to function
- 4. the big trees that we see around would not have grown on the Earth
Which of the statements given above are correct?
Answer (b) & explanation
Statements 1, 3 and 4 are correct (b). Capillarity draws liquid up narrow spaces because of adhesion and surface tension. It lifts kerosene up a lamp wick (1), soaks ink into blotting paper (3), and helps raise water up the narrow xylem vessels of trees, along with transpiration pull and cohesion (4). Statement 2 is wrong: drinking through a straw works by air pressure; sucking lowers the pressure inside, and atmospheric pressure pushes the drink up. So (a), (c) and (d) fail.
Notice the related examples: water rising in soil and in a towel, and the meniscus in thin tubes. Mercury shows capillary fall, not rise, because cohesion exceeds adhesion. Trap: the straw is about pressure, not capillarity.
Answer as per the official UPSC answer key.
Biomass gasification is considered to be one of the sustainable solutions to the power crisis in India. In this context, which of the following statements is/are correct?
- 1. Coconut shells, groundnut shells and rice husk can be used in biomass gasification.
- 2. The combustible gases generated from biomass gasification consist of hydrogen and carbon dioxide only.
- 3. The combustible gases generated from biomass gasification can be used for direct heat generation but not in internal combustion engines.
Select the correct answer using the codes given below :
Answer (a) & explanation
Statement 1 only is correct (a). A gasifier partly burns biomass in limited air to make producer gas. Coconut shells, groundnut shells and rice husk are all good feedstocks (1). Statement 2 is wrong: the gas is mainly carbon monoxide, hydrogen and methane, with nitrogen and some CO₂; carbon dioxide does not burn. Statement 3 is wrong: the gas can be used for heat and also to run internal combustion engines for electricity, as in village power plants. So (b), (c) and (d) fail.
Notice the example: Husk Power Systems in Bihar has lit villages using rice-husk gasifiers since 2007. India's SATAT scheme promotes compressed biogas, a different, biological route. Trap: 'only' and 'but not' make statements 2 and 3 false.
Answer as per the official UPSC answer key.
What is the role of ultraviolet (UV) radiation in the water purification systems?
- 1. It inactivates/kills the harmful microorganisms in water.
- 2. It removes all the undesirable odours from the water.
- 3. It quickens the sedimentation of solid particles, removes turbidity and improves the clarity of water.
Which of the statements given above is/are correct?
Answer (a) & explanation
Statement 1 only is correct (a). Ultraviolet light, especially at about 254 nm, damages the DNA of bacteria and viruses, so they cannot multiply; this inactivates harmful microbes (1). Statement 2 is wrong: UV adds no chemicals and does not remove odours; activated carbon filters do that. Statement 3 is wrong: UV does not settle particles or remove turbidity; in fact, cloudy water shields microbes from UV, so water must be filtered first. So (b), (c) and (d) fail.
Notice the related methods: chlorination leaves a residual disinfectant, reverse osmosis removes dissolved salts, and ozonation also disinfects. UV does not remove arsenic or fluoride. Trap: UV is a disinfection step only.
Answer as per the official UPSC answer key.
Graphene is frequently in news recently. What is its importance?
- 1. It is a two-dimensional material and has good electrical conductivity.
- 2. It is one of the thinnest but strongest materials tested so far.
- 3. It is entirely made of silicon and has high optical transparency.
- 4. It can be used as ‘conducting electrodes’ required for touch screens, LCDs and organic LEDs.
Which of the statements given above are correct?
Answer (c) & explanation
Statements 1, 2 and 4 are correct (c). Graphene is a single layer of carbon atoms arranged in a honeycomb, so it is two-dimensional and conducts electricity very well (1). It is one atom thick yet about 200 times stronger than steel (2). Being nearly transparent and conductive, it suits conducting electrodes in touch screens, LCDs and OLEDs (4). Statement 3 is wrong: it is made of carbon, not silicon. So (a), (b) and (d) fail.
Notice the history: Andre Geim and Konstantin Novoselov won the 2010 Nobel Prize in Physics for isolating it with sticky tape. India set up the India Graphene Engineering and Innovation Centre in Kerala. Trap: 'silicon' comes from the link with electronics.
Answer as per the official UPSC answer key.
A team of scientists at Brookhaven National Laboratory including those from India created the heaviest anti-matter (anti-helium nucleus). What is/are the implication/implications of the creation of anti-matter?
- 1. It will make mineral prospecting and oil exploration easier and cheaper.
- 2. It will help probe the possibility of the existence of stars and galaxies made of anti-matter.
- 3. It will help understand the evolution of the universe.
Select the correct answer using the codes given below :
Answer (b) & explanation
Statements 2 and 3 are correct (b). The anti-helium-4 nucleus, the heaviest antimatter nucleus then made, was detected at the RHIC collider at Brookhaven (2011), with Indian scientists in the STAR team. Such work helps search for antimatter stars and galaxies (2) and understand why the early universe ended up with more matter than antimatter (3). Statement 1 is wrong: antimatter has no use in mineral or oil exploration; it is made in tiny amounts and vanishes on contact with matter. So (a), (c) and (d) fail.
Notice the related tools: the Alpha Magnetic Spectrometer on the ISS searches for cosmic antimatter. Positrons (anti-electrons) are used in medical PET scans. Trap: practical-sounding uses are attached to basic physics.
Answer as per the official UPSC answer key.