Chemistry Concepts MCQs for UPSC Prelims

66 practice questions on Chemistry Concepts from the Science & Technology section of the UPSC Prelims syllabus. 66 come with a written explanation and 17 are actual previous year questions. Try the sample set below - the answer stays hidden until you ask for it.

14 Easy 38 Medium 14 Hard 17 from past papers

Sample questions

Q1
Previous year question hard

Consider the following substances : I. Ethanol II. Nitroglycerine III. Urea Coal gasification technology can be used in the production of how many of them?

  1. A Only one
  2. B Only two
  3. C All the three
  4. D None
Show answer and explanation

Correct answer: B - Only two

Coal gasification produces synthesis gas, a mixture of carbon monoxide and hydrogen, which can be converted into ethanol and, via ammonia synthesis, into urea fertilizer. Nitroglycerine is produced by nitrating glycerol with acids and is not a product of coal gasification. Hence coal gasification can be used to make only two of the listed substances.

Q2
Previous year question hard

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?

  1. A Aspartame is as sweet as table sugar, but unlike table sugar, it is not readily oxidized in human body due to lack of requisite enzymes
  2. B When aspartame is used in food processing, the sweet taste remains, but it becomes resistant to oxidation
  3. C Aspartame is as sweet as sugar, but after ingestion into the body, it is converted into metabolites that yield no calories
  4. D Aspartame is several times sweeter than table sugar, hence food items made with small quantities of aspartame yield fewer calories on oxidation
Show answer and explanation

Correct answer: D - Aspartame is several times sweeter than table sugar, hence food items made with small quantities of aspartame yield fewer calories on oxidation

Aspartame is roughly 200 times sweeter than table sugar, so only a tiny quantity is needed to achieve the same sweetness. Although gram for gram it yields about the same calories as other amino acids, the minute amount used means the calories contributed are negligible, which is the basis of its use as a low-calorie sweetener; option (d) is correct. Options (a) and (b) are wrong because aspartame IS metabolised (oxidised) in the body; it is not resistant to oxidation. Option (c) is wrong because its metabolites do yield calories; it is the small quantity, not zero-calorie metabolism, that explains its low-calorie use.

Q3
Previous year question medium

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?

  1. A 1 only
  2. B 2 and 3 only
  3. C 1 and 3 only
  4. D 1, 2 and 3
Show answer and explanation

Correct answer: C - 1 and 3 only

Industrial trans-fats are formed mainly by partial hydrogenation of vegetable oils, so a 'no trans-fat' claim implies the product is not made from such hydrogenated oils, making statement 1 correct. Trans-fats raise LDL and lower HDL cholesterol, so avoiding them reduces cardiovascular risk, making statement 3 correct. Statement 2 is wrong because animal fats also naturally contain small amounts of trans-fats (from ruminants), and more importantly being free of trans-fats does not mean the product avoids animal fats altogether; the two are not equivalent. Hence only 1 and 3 are correct.

Q4
Previous year question easy

Which one of the following sets of elements was primarily responsible for the origin of life on the Earth?

  1. A Hydrogen, Oxygen, Sodium
  2. B Carbon, Hydrogen, Nitrogen
  3. C Oxygen, Calcium, Phosphorus
  4. D Carbon, Hydrogen, Potassium
Show answer and explanation

Correct answer: B - Carbon, Hydrogen, Nitrogen

Life is built on organic molecules such as proteins, nucleic acids, carbohydrates and lipids, whose backbones are made chiefly of carbon, hydrogen, nitrogen (and oxygen). Carbon's ability to form long chains, together with hydrogen and nitrogen, made the assembly of amino acids and nucleotides possible in the primitive environment, so option (b) is the best set. The other options include sodium, calcium, phosphorus or potassium, which though biologically useful are not the primary structural elements that gave rise to the first organic molecules of life, so they are incorrect.

Q5
Previous year question medium

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?

  1. A 1 only
  2. B 2 and 3 only
  3. C 1 and 3 only
  4. D 1, 2 and 3
Show answer and explanation

Correct answer: A - 1 only

Only statement 1 is correct. Ultraviolet radiation at germicidal wavelengths damages the DNA and RNA of bacteria, viruses and protozoa, inactivating or killing them, which is why UV lamps are used as a disinfection stage in water purifiers. Statement 2 is wrong because UV does not remove dissolved substances responsible for odour; that needs activated carbon or similar adsorbents. Statement 3 is wrong because UV has no effect on suspended solids, sedimentation or turbidity; clarity is improved by filtration, coagulation and settling, and turbid water actually reduces UV effectiveness by shielding microbes.

Q6
Previous year question medium

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?

  1. A Both Statement I and Statement II are correct and Statement II explains Statement I
  2. B Both Statement I and Statement II are correct but Statement II does not explain Statement I
  3. C Statement I is correct but Statement II is not correct
  4. D Statement I is not correct but Statement II is correct
Show answer and explanation

Correct answer: A - Both Statement I and Statement II are correct and Statement II explains Statement I

Rare earth elements such as europium, terbium and yttrium are used in flat television screens and computer monitors, so statement I is correct, and these elements indeed possess phosphorescent or luminescent properties, so statement II is correct. It is precisely because of their phosphorescent behaviour that they are used to produce colours in display screens, so statement II explains statement I. Hence both are correct and II explains I.

Q7
Previous year question medium

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?

  1. A These are alternatives to hydro-fluorocarbon refrigerants
  2. B These are explosives in military weapons
  3. C These are high-energy fuels for cruise missiles
  4. D These are fuels for rocket propulsion
Show answer and explanation

Correct answer: B - These are explosives in military weapons

CL-20, HMX and LLM-105 are all high-energy nitrogen-rich compounds used as powerful explosives in military munitions and warheads. They are not refrigerant substitutes, nor are they primarily fuels for cruise missiles or rocket propulsion. Hence their common characteristic is that they are explosives used in military weapons.

Q8
Previous year question medium

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?

  1. A 1 only
  2. B 2 and 3 only
  3. C 1 and 3 only
  4. D 1, 2 and 3
Show answer and explanation

Correct answer: C - 1 and 3 only

Rare earth metals are vital for electronics, magnets, batteries and many high-tech goods whose demand is rising, and China, the dominant producer, had restricted exports, so statements 1 and 3 are correct. Rare earths are actually found in several other countries including the USA and India, so statement 2 (claiming they occur only in China, Australia, Canada and Chile) is wrong. The answer is 1 and 3 only.

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