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General ScienceHardAsked in: NDA · NDA-I 2022, GAT

Which one of the following is not an example of a redox reaction?

  1. A.AlCl3 + 3H2O → Al(OH)3 + 3HCl
  2. B.2NaH → 2Na + H2
  3. C.4Fe + 3O2 → 2Fe2O3
  4. D.CuSO4 + Zn → Cu + ZnSO4

Correct answer

A. AlCl3 + 3H2O → Al(OH)3 + 3HCl

Explanation

The correct answer is A, AlCl3 + 3H2O → Al(OH)3 + 3HCl. This is a hydrolysis reaction in which no element changes its oxidation number, so nothing is oxidised or reduced.

A redox reaction is one in which one substance loses electrons (its oxidation number rises) and another gains them (its oxidation number falls). In the hydrolysis of aluminium chloride, aluminium stays at +3, chlorine at −1, hydrogen at +1 and oxygen at −2 on both sides; the ions only exchange partners. The other three reactions all show changes in oxidation number.

B is a redox reaction because in sodium hydride hydrogen is −1 and sodium +1; on decomposition hydrogen rises to 0 and sodium falls to 0.

C is a redox reaction because iron goes from 0 to +3 (oxidised) and oxygen from 0 to −2 (reduced) in rusting-like oxidation.

D is a redox reaction because zinc goes from 0 to +2 (oxidised) and copper from +2 to 0 (reduced), a displacement reaction.

Exam tip: check oxidation numbers; if none changes, as in hydrolysis, neutralisation or precipitation, the reaction is not redox.

Practice Questions

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Q1.General ScienceAsked in: UPSC CAPF · CAPF 2025, Paper IMedium

Which one among the following statements is not correct?

  1. A.The reaction of methane and chlorine in sunlight produces hydrogen gas.
  2. B.Palladium catalyzes the addition of hydrogen gas on an alkene.
  3. C.Ethanol burns in oxygen to give carbon dioxide, water and releases heat and light.
  4. D.Unsaturated hydrocarbons burn with a sooty flame.
Show answer
Correct answer: A. The reaction of methane and chlorine in sunlight produces hydrogen gas.

Methane and chlorine react in sunlight to give chloromethane and hydrogen chloride, not hydrogen gas. This is a substitution reaction: CH4 + Cl2 → CH3Cl + HCl in the presence of sunlight.

Q2.General ScienceAsked in: UPSC CAPF · CAPF 2025, Paper IMedium

Which of the following statements relating to carbon and its compounds are correct? 1. 1-Propyne has three double bonds. 2. The boiling point of acetic acid is higher than methane. 3. Ethanol reacts with sodium metal and produces hydrogen gas. 4. Esters give effervescence on reaction with sodium bicarbonate. Select the answer using the code given below.

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

Acetic acid boils far above methane, and ethanol gives hydrogen with sodium, while statements 1 and 4 are false. Propyne (CH3–C≡CH) is an alkyne: it has one carbon–carbon triple bond and no double bonds, so statement 1 is wrong.

Q3.General ScienceAsked in: UPSC CAPF · CAPF 2025, Paper IMedium

Which one among the following statements regarding gypsum is not correct?

  1. A.On heating gypsum at 373 K, it loses water molecules and becomes CaSO4 · H2O.
  2. B.On mixing with water, plaster of Paris changes to gypsum.
  3. C.Plaster of Paris is used for supporting fractured bones.
  4. D.Although gypsum possesses water of crystallization, it is not a liquid at room temperature.
Show answer
Correct answer: A. On heating gypsum at 373 K, it loses water molecules and becomes CaSO4 · H2O.

Gypsum heated at 373 K becomes calcium sulphate hemihydrate, CaSO4 · ½H2O, which is plaster of Paris, not CaSO4 · H2O as the statement says.

Q4.General ScienceAsked in: UPSC CAPF · CAPF 2026, Paper IEasy

Which of the following is/are the basic step(s) in genetically modifying an organism? 1. Identification of DNA with desirable genes 2. Introduction of the identified DNA into the host 3. Maintenance of introduced DNA in the host and transfer of the DNA to its progeny Select the correct answer using the code given below.

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

Making a genetically modified organism needs all three steps: find the useful DNA, put it into the host, and make sure the host keeps it and passes it on.

Q5.General ScienceAsked in: NDA · NDA-II 2018, GATMedium

If one set of chromosomes for a given plant is represented as N; in case of double fertilization, the zygote and the endosperm nucleus of a diploid plant would have how many sets of chromosomes respectively?

  1. A.N and 2N
  2. B.2N and 2N
  3. C.N and 3N
  4. D.2N and 3N
Show answer
Correct answer: D. 2N and 3N

The zygote is diploid (2N) and the primary endosperm nucleus is triploid (3N). Double fertilisation is a feature of flowering plants.