Science and Technology: previous-year questions
185 real UPSC Prelims questions (2011–2026) with the official answer and a clear explanation. 50 of them came in the last five papers.
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Questions per year, counted from the official papers.
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What is “Virtual Private Network” ?
Answer (b) & explanation
Option (b) is correct. A Virtual Private Network builds a secure, encrypted 'tunnel' over the public internet, so a remote user can reach the office network as if sitting inside it, while the data stay private. (a) is wrong: a VPN does not need a private network; its point is to use the public internet safely. (c) is wrong: it describes cloud computing, where users share computing resources from a provider. (d) fails because (b) is correct.
Notice the policy angle: CERT-In's 2022 directions require VPN providers in India to keep user records for five years, which led some foreign providers to remove Indian servers. VPNs also let users hide their location. Trap: (a) sounds similar but misses the 'public internet' part.
Answer as per the official UPSC answer key.
A company marketing food products advertises that its items do not contain trans-fats. What does this campaign signify to the customers ?
- 1. The food products are not made out of hydrogenated oils.
- 2. The food products are not made out of animal fats/oils.
- 3. The oils used are not likely to damage the cardiovascular health of the consumers.
Which of the statements given above is/are correct ?
Answer (c) & explanation
Statements 1 and 3 are correct (c). Trans fats form mainly when vegetable oil is partially hydrogenated, as in vanaspati, so 'no trans fat' signals no such oils (1). Trans fats raise 'bad' LDL cholesterol and lower 'good' HDL, so avoiding them protects the heart (3). Statement 2 is wrong: the claim says nothing about animal fats; ghee and butter may still be used, and they carry a little natural trans fat. So (a), (b) and (d) fail.
Notice the policy: FSSAI capped industrial trans fat at 2% of total fat in oils and foods from January 2022, in line with the WHO's 'REPLACE' goal. Trap: 'no trans fat' is not the same as 'no saturated or animal fat'.
Answer as per the official UPSC answer key.
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason ?
Answer (c) & explanation
Density of water is maximum at 4°C is correct (c). As a lake cools in winter, surface water reaches 4°C, becomes densest and sinks. Colder water then stays on top and freezes, and ice floats because it is lighter than water. So the bottom stays liquid at about 4°C, and fish survive. (a) is a helping factor, since ice is a poor conductor and insulates the water below, but it is not the basic reason. (b) is wrong: heat is lost from the surface, which is why it freezes. (d) fails because (c) is correct.
Notice the wider idea: this anomalous expansion of water between 0°C and 4°C is unusual among liquids. It also makes water pipes burst in frost and helps break rocks (frost weathering).
Answer as per the official UPSC answer key.
What is the difference between Bluetooth and Wi-Fi devices ?
Answer (a) & explanation
Option (a) is correct. Bluetooth works in the 2.4 GHz band for short-range personal area networks (PAN), linking devices a few metres apart. Wi-Fi uses 2.4 GHz or 5 GHz (and newer 6 GHz in Wi-Fi 6E/7) for wireless local area networks. (b) is wrong: Bluetooth is not a WLAN technology, and Wi-Fi is a LAN, not a wide-area, technology. (c) is wrong: neither needs line of sight; radio waves pass through walls, unlike infrared remotes. (d) fails because (b) is wrong.
Notice the wider idea: India delicensed the lower 6 GHz band (5925–6425 MHz) for Wi-Fi in 2025. Bluetooth is named after the Danish king Harald Bluetooth. Trap: line of sight belongs to infrared, not Bluetooth.
Answer as per the official UPSC answer key.
Aspartame is an artificial sweetener sold in the market. It consists of amino acids and provides calories like other amino acids. Yet, it is used as a low-calorie sweetening agent in food items. What is the basis of this use ?
Answer (d) & explanation
Option (d) is correct. Aspartame, made of two amino acids, gives about 4 kcal per gram, the same as sugar, but it is about 200 times sweeter. So only a tiny amount is needed, and the food ends up with very few calories. (a) is wrong: it is not as sweet as sugar, and the body does digest and oxidise it. (b) is wrong: processing does not make it resistant to oxidation; in fact it breaks down with heat. (c) is wrong: its metabolites do yield calories.
Notice the debate: the WHO's cancer agency (IARC) classed aspartame as 'possibly carcinogenic' in 2023, but JECFA kept the safe daily intake at 40 mg/kg body weight. People with phenylketonuria must avoid it.
Answer as per the official UPSC answer key.