NDA General Studies · Chemistry · 62 questions · 2017 to 2026

NDA Redox, metals and non-metals previous year questions with solutions

Redox, metals and non-metals are about one General Studies question in thirty, two or three a sitting. The questions are one-fact recall, with oxidation numbers and three-statement sets rising since 2024. Every question UPSC has set on it since 2017, solved. Grouped by the method each needs, newest first; open a question for its answer and worked solution.

11.1 Oxidation and reduction: definitions, oxidation numbers and the electrochemical series (17)11.2 Corrosion: rusting, tarnishing and their prevention (6)11.3 Metals: the reactivity series, extraction and alloys (16)11.4 Hydrogen, water and hydrogen peroxide (7)11.5 Carbon, nitrogen and oxygen: the common gases and their oxides (10)11.6 Halogens, noble gases, sulphur and phosphorus (6)

11.1 Oxidation and reduction: definitions, oxidation numbers and the electrochemical series

1. NDA 2026 (II)

Consider the following statements regarding oxidation numbers:
I. The oxidation number of oxygen in peroxides and superoxides is assigned as –⁠1 and –⁠1/⁠2 respectively.
II. In all its compounds, fluorine has an oxidation number of –⁠1.
III. In a reduction half-reaction, the oxidation number of the reductant decreases.
Which of the statements given above are correct?

(a) I and II only(b) II and III only(c) I and III only(d) I, II and III

Answer and solution

Answer: (a) I and II only

Method. Rules for oxidation numbers

I: O is -1 in peroxides and -1/⁠2 in superoxides, correct (KC7-A41). II: F, the most electronegative element, is always -1, correct (KC7-A44). III: in a reduction half-reaction the oxidation number of the OXIDANT decreases; the reductant's increases, so III is wrong (KC7-A53). So I and II only.

2. NDA 2026 (I)

Which of the following are oxidation reactions?
I. Rusting of iron
II. Burning of fuel
III. Rancidity of oils and fats
IV. Browning of fruits
Select the correct answer using the code given below.

(a) I, II and III only(b) I, II and IV only(c) III and IV only(d) I, II, III and IV

Answer and solution

Answer: (d) I, II, III and IV

Method. Oxidation processes

Rusting of iron, burning of fuel, rancidity of oils and fats, and browning of cut fruit (enzymatic oxidation of phenolics by oxygen) are all oxidations (C3-A62, KC7-A04, KC7-A02).

3. NDA 2026 (I)

Consider the following pairs of reactions and properties:
Which of the above pairs are correct?

(blank)	Reactions	Properties
I.	Fe(s) + 2HCl(aq) → FeCl_{2}(aq) + H_{2}(g)	Fe is reductant
II.	Zn(s) + CuSO_{4}(aq) → ZnSO_{4}(aq) + Cu(s)	Cu is being reduced
III.	Br_{2}(l) + 2I^{-}(aq) → 2Br^{-}(aq) + I_{2}(s)	Br_{2} is reductant
IV.	CH_{4}(g) + 2O_{2}(g) → CO_{2}(g) + 2H_{2}O(l)	C atom in CH_{4} is oxidized

(a) I and II only(b) II, III and IV(c) I, II and IV(d) I, III and IV

Answer and solution

Answer: (c) I, II and IV

Method. Judging oxidant and reductant in each pair

I: Fe 0 -> +2, so Fe is the reductant, correct. II: Cu2+ +2 -> 0, Cu is reduced, correct. III: Br2 0 -> -1 is reduced, so Br2 is the OXIDANT; calling it reductant is wrong. IV: C -4 -> +4, oxidised, correct. So I, II and IV (KC7-A53, KC7-A68).

4. NDA 2025 (II)

In the following redox reaction, which compound is acting as a reducing agent?
N₂H₄ + 2H₂O₂ → N₂ + 4H₂O

(a) H₂O₂(b) N₂H₄(c) N₂(d) H₂O

Answer and solution

Answer: (b) N₂H₄

Method. Reducing agent by oxidation number

N in N2H4: 2x + 4 = 0, x = -2; in N2: 0. N increases (oxidation), so N2H4 is the reducing agent. In H2O2, O goes from -1 to -2 (reduction), so H2O2 is the oxidising agent (KC7-A53, KC7-A55).

5. NDA 2025 (I)

What happens when the fat and oil containing food materials are left outside for a long time?

(a) Fats and oils in the food get oxidized(b) Fats and oils in the food get reduced(c) Fats and oils in the food get ice-covered(d) No reaction takes place

Answer and solution

Answer: (a) Fats and oils in the food get oxidized

Method. Rancidity

Fats and oils left in air are oxidised to foul-smelling aldehydes and acids: rancidity is oxidation (cf. KC7-A02). Standard result.

6. NDA 2025 (I)

Which one of the following statements regarding oxidation and reduction reactions is NOT correct?

(a) If a substance loses hydrogen during reaction, it is reduced(b) If a substance loses oxygen during reaction, it is reduced(c) If a substance gains hydrogen during reaction, it is reduced(d) If a substance gains oxygen during reaction, it is oxidized

Answer and solution

Answer: (a) If a substance loses hydrogen during reaction, it is reduced

Method. Classical redox definitions

Loss of hydrogen is OXIDATION, not reduction (KC7-A06, KC7-A09); so (a) is not correct. Loss of oxygen and gain of hydrogen are reduction; gain of oxygen is oxidation (KC7-A04, KC7-A10).

7. NDA 2024 (II)

Which one among the following is an example of oxidation reaction?

(a) Freezing of water(b) Dissolving sugar in water(c) Rusting of iron(d) Boiling of petrol

Answer and solution

Answer: (c) Rusting of iron

Method. Oxidation versus physical change

Rusting is oxidation of iron (Fe -> Fe3+ with O2 and moisture) (C3-A61, C3-A62). Freezing, dissolving and boiling are physical changes.

8. NDA 2024 (I)

What is the oxidation state of Vanadium in V₂O₅?

(a) + 2(b) + 4(c) + 3(d) + 5

Answer and solution

Answer: (d) + 5

Method. Oxidation number of V in V2O5

2x + 5(-2) = 0, so x = +5 (KC7-A45).

9. NDA 2023 (II)

Consider the following reaction:
Fe₂O₃(s) + 2Al(s) → 2Fe(s) + Al₂O₃(s)
Which of the following statements about the given reaction is NOT correct?

(a) It is an example of displacement reaction(b) It is highly endothermic reaction(c) Fe₂O₃ acts as oxidizing agent(d) This reaction is applicable to joining of railway tracks

Answer and solution

Answer: (b) It is highly endothermic reaction

Method. Thermit reaction

Fe2O3 + 2Al -> 2Fe + Al2O3: Al displaces Fe (a, true); Fe +3 -> 0, so Fe2O3 is the oxidising agent (c, true); it is highly EXOTHERMIC and gives molten iron used to weld rail tracks (d, true) (C3-A56). So (b) 'highly endothermic' is the incorrect statement.

10. NDA 2022 (I)

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

(a) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl(b) 2NaH → 2Na + H₂(c) 4Fe + 3O₂ → 2Fe₂O₃(d) CuSO₄ + Zn → Cu + ZnSO₄

Answer and solution

Answer: (a) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl

Method. Redox test by oxidation number

AlCl3 + 3H2O -> Al(OH)3 + 3HCl is hydrolysis: Al +3, Cl -1, H +1, O -2 throughout, so no redox. 2NaH -> 2Na + H2: Na +1 -> 0, H -1 -> 0, redox. 4Fe + 3O2 -> 2Fe2O3: redox. CuSO4 + Zn -> Cu + ZnSO4: displacement, redox (KC7-A53, KC7-A59).

11. NDA 2021 (I)

In electrolytic refining of copper, the electrolyte is a solution of

(a) acidified copper chloride.(b) acidified copper sulphate.(c) potassium chloride.(d) sodium sulphate.

Answer and solution

Answer: (b) acidified copper sulphate.

Method. Electrolytic refining of copper

Impure copper is the anode, pure copper the cathode, and the electrolyte is acidified CuSO4 solution (C3-A59, T12 TC2-A56).

12. NDA 2019 (II)

Which one of the following compounds does not exhibit a different oxidation number of the same element?

(a) Pb₃O₄(b) Fe₃O₄(c) Fe₂O₃(d) Mn₃O₄

Answer and solution

Answer: (c) Fe₂O₃

Method. Oxidation numbers in mixed oxides

Pb3O4 = 2PbO.PbO2 (+2 and +4). Fe3O4 = FeO.Fe2O3 (+2 and +3). Mn3O4 = MnO.Mn2O3 (+2 and +3). Fe2O3: 2x − 6 = 0, Fe is +3 only (KC7-A78, KC7-A79).

13. NDA 2019 (II)

Which one of the following represents the correct order of electron releasing tendency of metals?

(a) Zn > Cu > Ag(b) Ag > Cu > Zn(c) Cu > Zn > Ag(d) Cu > Ag > Zn

Answer and solution

Answer: (a) Zn > Cu > Ag

Method. Electrochemical series

Electron-releasing tendency follows the reactivity: Zn > Cu > Ag (zinc displaces Cu2+, copper displaces Ag+; KC7-A31, KC7-A22, KC7-A27).

14. NDA 2019 (I)

Which one of the following statements is NOT correct for the given reaction?
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

(a) Iron is the reducing agent(b) The solution turns green in colour after the reaction(c) Copper is a more reactive metal than iron(d) The reaction is an example of a redox reaction

Answer and solution

Answer: (c) Copper is a more reactive metal than iron

Method. Reading a displacement reaction

Fe + CuSO4 -> FeSO4 + Cu: Fe is oxidised (0 -> +2), so it is the reducing agent (a, true); FeSO4 is pale green (b, true); it is redox (d, true). Fe displaces Cu, so iron is the MORE reactive metal; (c) is false (C3-A33, KC7-A61).

15. NDA 2018 (II)

Which one of the following is an oxidation-reduction reaction?

(a) NaOH + HCl → NaCl + H₂O(b) CaO + H₂O → Ca(OH)₂(c) 2Mg + O₂ → 2MgO(d) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Answer and solution

Answer: (c) 2Mg + O₂ → 2MgO

Method. Change of oxidation number

2Mg + O2 -> 2MgO: Mg 0 -> +2, O 0 -> -2, so a redox reaction (KC7-A56). The others (neutralisation, hydration of CaO, double decomposition) change no oxidation number.

16. NDA 2017 (II)

Why is potassium permanganate used for purifying drinking water?

(a) It kills germs(b) It dissolves the impurities(c) It is a reducing agent(d) It is an oxidizing agent

Answer and solution

Answer: (d) It is an oxidizing agent

Method. Oxidising action of KMnO4

Potassium permanganate is a strong oxidising agent (Mn +7 goes to a lower state, K11 KC7-A95); it oxidises and so destroys organic matter and germs in water. 'It kills germs' states the result, 'oxidizing agent' states the reason asked for ('why').

17. NDA 2017 (II)

Consider the following reaction:
CH₄ + 2O₂ → CO₂ + 2H₂O
Which of the following about the reaction given above is/⁠are correct?
(1) Carbon is oxidized.
(2) Hydrogen is oxidized.
(3) Hydrogen is reduced.
(4) Carbon is reduced.
Select the correct answer using the code given below:

(a) 1 only(b) 1 and 2 only(c) 2 and 3 only(d) 2 and 4 only

Answer and solution

Answer: (a) 1 only

Method. Oxidation numbers in combustion of CH4

C: -4 in CH4 -> +4 in CO2 (increase = oxidised). H: +1 in CH4 and +1 in H2O (no change). O: 0 -> -2 (reduced). Only statement 1 is correct (KC7-A52, KC7-A53).

11.2 Corrosion: rusting, tarnishing and their prevention

18. NDA 2023 (II)

Galvanization is a method of protecting iron from rusting by coating with a thin layer of

(a) Gallium(b) Aluminum(c) Zinc(d) Silver

Answer and solution

Answer: (c) Zinc

Method. Galvanisation

Galvanisation coats iron with a thin layer of zinc (C3-A64).

19. NDA 2022 (II)

Silver artefacts get tarnished in air due to the formation of

(a) silver chloride(b) silver oxide(c) silver sulphide(d) silver sulphate

Answer and solution

Answer: (c) silver sulphide

Method. Tarnishing of silver

Silver reacts with H2S/⁠sulphur in air to form black silver sulphide (C3-A61).

20. NDA 2021 (II)

To protect steel and iron from rusting, a thin layer of which one of the following metals is applied?

(a) Magnesium(b) Zinc(c) Aluminium(d) Lead

Answer and solution

Answer: (b) Zinc

Method. Galvanisation

Iron or steel is coated with zinc, which also protects by sacrificial action (C3-A64).

21. NDA 2019 (I)

On exposure to moist air, copper gains a green coat on its surface due to formation of which one of the following compounds?

(a) Copper carbonate(b) Copper oxide(c) Copper sulphate(d) Copper nitrate

Answer and solution

Answer: (a) Copper carbonate

Method. Corrosion of copper

Copper reacts with moist CO2 to give the green basic copper carbonate (verdigris) (C3-A61).

22. NDA 2017 (II)

Zinc is used to protect iron from corrosion because zinc is

(a) more electropositive than iron(b) cheaper than iron(c) a bluish white metal(d) a good conductor of heat and electricity

Answer and solution

Answer: (a) more electropositive than iron

Method. Sacrificial protection

Zinc lies above iron in the reactivity series, so it is more electropositive and loses electrons more readily; in a scratch it corrodes in place of iron (galvanisation, C3-A64, C3-A79; cf. TC2-A78). Cost, colour and conductivity are not the reason.

23. NDA 2017 (I)

Which one of the following elements corrodes rapidly?

(a) Aluminium(b) Iron(c) Zinc(d) Silver

Answer and solution

Answer: (b) Iron

Method. Corrosion of iron versus self-protecting metals

Iron in moist air gains a brown, flaky, porous rust that exposes fresh metal, so it corrodes continuously (C3-A61, C3-A62). Aluminium and zinc are covered by a thin, adherent oxide layer that stops further attack (C3-A19). Silver only tarnishes slowly with sulphur (silver sulphide, C3-A61).

11.3 Metals: the reactivity series, extraction and alloys

24. NDA 2025 (I)

Which one among the following elements is known to be discovered the earliest?

(a) Copper(b) Gold(c) Oxygen(d) Uranium

Answer and solution

Answer: (a) Copper

Method. Early use of elements

Copper was in use from about 9000 BCE (earliest metal worked), gold from about 4000 BCE, whereas oxygen was identified in 1774 and uranium in 1789. Standard historical result; so copper.

25. NDA 2023 (II)

Which one of the following alloys contains a non-metal as one of its constituent?

(a) Brass(b) Bronze(c) Amalgam(d) Steel

Answer and solution

Answer: (d) Steel

Method. Composition of alloys

Steel is iron with carbon (a non-metal). Brass = Cu-Zn, bronze = Cu-Sn, amalgam = Hg with another metal (C3-A65, C3-A66, C3-A69).

26. NDA 2023 (II)

Which one of the following metals does NOT react with cold water to liberate hydrogen gas?

(a) Potassium(b) Iron(c) Calcium(d) Sodium

Answer and solution

Answer: (b) Iron

Method. Reactivity with cold water

K, Na and Ca all react with cold water to give H2 (C3-A23, C3-A24); iron reacts only with steam (C3-A26).

27. NDA 2023 (II)

Which one of the following metal is NOT an essential component in stainless steel?

(a) Iron(b) Nickel(c) Chromium(d) Tin

Answer and solution

Answer: (d) Tin

Method. Stainless steel

Stainless steel is iron with chromium and nickel (and carbon); tin is not a component (C3-A69).

28. NDA 2023 (II)

Which one of the following metals can be extracted using carbon as reducing agent?

(a) Zinc(b) Silver(c) Gold(d) Aluminum

Answer and solution

Answer: (a) Zinc

Method. Extraction by carbon reduction

ZnO is reduced by carbon: ZnO + C -> Zn + CO (C3-A54). Ag and Au occur native or are got by other methods (C3-A46, C3-A50), and Al lies high in the series, so its oxide is reduced electrolytically (C3-A57, C3-A58).

29. NDA 2023 (I)

Which one of the following is the correct arrangement of metals in the decreasing order of their reactivity?

(a) Iron, Sodium, Silver, Copper(b) Silver, Copper, Iron, Sodium(c) Sodium, Copper, Silver, Iron(d) Sodium, Iron, Copper, Silver

Answer and solution

Answer: (d) Sodium, Iron, Copper, Silver

Method. Reactivity series

Order of decreasing reactivity: Na > Fe > Cu > Ag (C3-A34, KC7-A31).

30. NDA 2022 (II)

Which one of the following is the correct reactivity series with water?

(a) Zinc > Iron > Lead > Copper(b) Copper > Lead > Zinc > Iron(c) Copper > Zinc > Iron > Lead(d) Zinc > Copper > Iron > Lead

Answer and solution

Answer: (a) Zinc > Iron > Lead > Copper

Method. Reactivity series

The series reads Zn > Fe > Pb > Cu (Zn and Fe react with steam, Pb and Cu do not react with water at all; C3-A26, C3-A27, C3-A34).

31. NDA 2022 (II)

Which one of the following metals floats in cold water?

(a) Magnesium(b) Calcium(c) Potassium(d) Copper

Answer and solution

Answer: (b) Calcium

Method. Reaction of metals with cold water (NCERT Class 10 ch. 3)

Calcium reacts gently with cold water, Ca + 2H2O -> Ca(OH)2 + H2, and starts floating because bubbles of hydrogen stick to its surface (NCERT). Magnesium does not react with cold water; it reacts with hot water and floats then. Potassium reacts so violently that the hydrogen evolved catches fire, so it is never seen floating in cold water even though its density (0.86 g/⁠cm3) is below water's. Copper does not react with water at all. Hence calcium (b).

32. NDA 2022 (II)

Which of the following compounds undergoes/⁠undergo thermal decomposition?

(a) Zinc oxide(b) Silver oxide and zinc oxide(c) Silver oxide(d) Magnesium oxide

Answer and solution

Answer: (c) Silver oxide

Method. Thermal decomposition of oxides

Silver oxide decomposes on moderate heating: 2Ag2O -> 4Ag + O2 (oxides of metals low in the series, C3-A50). ZnO and MgO are stable to ordinary heating (reduction needs carbon, C3-A54). Only silver oxide qualifies.

33. NDA 2022 (II)

Bronze is an alloy of

(a) Cu and Zn(b) Cu and Sn(c) Zn and Mg(d) Fe and Cu

Answer and solution

Answer: (b) Cu and Sn

Method. Alloy composition

Bronze = Cu + Sn; brass = Cu + Zn (C3-A65). Standard result.

34. NDA 2021 (II)

Cinnabar is an ore of which one of the following?

(a) Copper(b) Zinc(c) Mercury(d) Manganese

Answer and solution

Answer: (c) Mercury

Method. Ores

Cinnabar is HgS, the ore of mercury (C3-A50 on metals low in the series; standard result).

35. NDA 2021 (I)

Solder is an alloy of

(a) Cu and Sn.(b) Fe and Zn.(c) Pb and Sn.(d) Ag and Zn.

Answer and solution

Answer: (c) Pb and Sn.

Method. Alloy composition

Solder is an alloy of lead and tin (C3-A65 on alloys; standard result).

36. NDA 2021 (I)

Which one of the following reactions does not result in the evolution of hydrogen gas?

(a) Reaction of zinc metal with dilute sulphuric acid solution(b) Mixing water to Plaster of Paris(c) Heating zinc metal with sodium hydroxide solution(d) Reaction of potassium metal with water

Answer and solution

Answer: (b) Mixing water to Plaster of Paris

Method. Hydrogen-evolving reactions

Zn + H2SO4 -> ZnSO4 + H2 (C3-A28); Zn + 2NaOH -> Na2ZnO2 + H2 (C3-A16, zinc oxide/⁠zinc is amphoteric); 2K + 2H2O -> 2KOH + H2 (C3-A23). CaSO4.1/⁠2H2O + 1.5H2O -> CaSO4.2H2O is hydration only, no gas.

37. NDA 2019 (I)

Which one of the following metals does NOT react with cold water?

(a) Calcium (Ca)(b) Potassium (K)(c) Magnesium (Mg)(d) Sodium (Na)

Answer and solution

Answer: (c) Magnesium (Mg)

Method. Reactivity of metals with water

K and Na react violently with cold water (C3-A23), Ca reacts less violently (C3-A24). Mg does not react with cold water; it reacts only with hot water (C3-A25).

38. NDA 2018 (I)

Which one of the following metals is alloyed with sodium to transfer heat in a nuclear reactor?

(a) Potassium(b) Calcium(c) Magnesium(d) Strontium

Answer and solution

Answer: (a) Potassium

Method. Liquid-metal coolant

An Na-K alloy is liquid at room temperature and is used as the heat-transfer coolant in nuclear reactors. Standard result.

39. NDA 2018 (I)

Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy?

(a) Tungsten(b) Copper(c) Rubidium(d) Aluminium

Answer and solution

Answer: (c) Rubidium

Method. Photoelectric metals

Rubidium (like caesium) has a very low ionisation energy, so light of modest energy ejects electrons: it is used in photoelectric cells. Tungsten, copper and aluminium have high work functions. Standard result.

11.4 Hydrogen, water and hydrogen peroxide

40. NDA 2026 (II)

Consider the following statements regarding the properties of hydrogen and water:
I. Tritium, the isotope of hydrogen, is radioactive and emits low energy alpha particles.
II. In the gas phase, the structure of water is linear.
III. The basic principle of a hydrogen economy is the transportation and storage of energy in the form of liquid or gaseous molecular hydrogen.
Which of the statements given above is/⁠are not correct?

(a) I and II only(b) II and III only(c) I and III only(d) II only

Answer and solution

Answer: (a) I and II only

Method. Properties of hydrogen and water

I is not correct: tritium (3H) is radioactive but emits low-energy beta particles, not alpha. II is not correct: gas-phase water is bent (104.5 degrees), not linear. III is correct: the hydrogen economy stores and transports energy as liquid or gaseous H2 (TC2-A79). The question asks for the statements that are not correct: I and II only.

41. NDA 2026 (II)

Consider the following statements:
I. A 30% solution of hydrogen peroxide is marketed as ‘10 volume’ H₂O₂.
II. In the compound copper sulphate pentahydrate, all the five water molecules are hydrogen bonded.
Which of the statements given above is/⁠are correct?

(a) I only(b) II only(c) Both I and II(d) Neither I nor II

Answer and solution

Answer: (d) Neither I nor II

Method. Hydrogen peroxide strength and hydrated salt

I: volume strength = litres of O2 from 1 L of solution. 30% w/⁠v = 300 g/⁠L = 300/⁠34 = 8.82 mol/⁠L; 2H2O2 -> 2H2O + O2 gives 11.2 L O2 per mole, so 8.82 x 11.2 = 98.8, i.e. about 100 volume. 10 volume is about 3%. I is false. II: in CuSO4.5H2O four water molecules are coordinated to Cu2+ and the fifth is hydrogen bonded to sulphate and the coordinated waters, so not all five are H-bonded as stated; II is false. Neither (cf. K11 hydrogen peroxide: 30% as 100 volume; standard, pre-2023 NCERT).

42. NDA 2025 (II)

Which one of the following is considered to be the purest source of water?

(a) River water(b) Sea water(c) Well water(d) Rain water

Answer and solution

Answer: (d) Rain water

Method. Purity of natural waters

Rain water is formed by evaporation, i.e. natural distillation, so it carries the least dissolved matter; river, well and sea water contain dissolved salts. Standard result.

43. NDA 2024 (I)

Which one among the following statements with reference to the properties of water is not correct?

(a) The specific heat of water is abnormally high.(b) Latent heat of fusion of water is very low.(c) Density of water is higher than ice.(d) Pure water is a non-conductor of electricity.

Answer and solution

Answer: (b) Latent heat of fusion of water is very low.

Method. Properties of water

Specific heat of water is abnormally high (4.18 J/⁠g/⁠K) (a true); water is denser than ice (c true); pure water is a poor conductor (d true). The latent heat of fusion of water (about 334 J/⁠g) is high, not low; so (b) is not correct. Standard result.

44. NDA 2022 (I)

Which one of the following is the colour of hydrogen gas?

(a) Light yellow(b) Orange(c) Black(d) Colourless

Answer and solution

Answer: (d) Colourless

Method. Physical property of hydrogen

Hydrogen is a colourless, odourless gas. Standard fact.

45. NDA 2021 (I)

Which one of the following statements about dihydrogen (H₂) is not correct?

(a) H₂ is lighter than air and insoluble in water.(b) H₂ is inert at room temperature due to high H – H bond dissociation enthalpy.(c) H₂ reacts with alkali metals at high temperature to yield metal hydrides.(d) A mixture of NO₂ and H₂ is known as Syngas.

Answer and solution

Answer: (d) A mixture of NO₂ and H₂ is known as Syngas.

Method. Properties of dihydrogen

(a) is true; (b) true, H-H bond enthalpy 436 kJ/⁠mol; (c) true: 2Na + H2 -> 2NaH. (d) false: syngas (water gas) is CO + H2, not NO2 + H2. Standard (pre-2023 NCERT).

46. NDA 2019 (II)

A very large volume of hydrogen can be accommodated by making

(a) non-metallic hydrides.(b) hydrogen peroxide.(c) non-stoichiometric hydrides.(d) alkali metal hydrides.

Answer and solution

Answer: (c) non-stoichiometric hydrides.

Method. Hydrogen storage

Non-stoichiometric (interstitial) hydrides of d-block metals such as Pd absorb very large volumes of H2; they are the hydrogen-storage medium. Alkali-metal hydrides are stoichiometric ionic solids, non-metal hydrides are molecular. Standard result (pre-2023 NCERT).

11.5 Carbon, nitrogen and oxygen: the common gases and their oxides

47. NDA 2026 (II)

Consider the following statements regarding nitrogen:
I. Nitrogen contains a triple bond.
II. Nitrogen is a triatomic gas.
III. Forces acting between nitrogen molecules are weak van der Waals forces.
Which of the statements given above are correct?

(a) I and II only(b) II and III only(c) I and III only(d) I, II and III

Answer and solution

Answer: (c) I and III only

Method. Nitrogen molecule

I: N2 has the triple bond N≡N, correct. II: N2 is diatomic, so II is false. III: only weak van der Waals (London) forces act between N2 molecules (hence its very low boiling point), correct. So I and III. Standard result (pre-2023 NCERT).

48. NDA 2025 (I)

Which one of the following oxides is a neutral oxide?

(a) CO(b) CO₂(c) Na₂O(d) MgO

Answer and solution

Answer: (a) CO

Method. Neutral oxides

CO is neutral, neither acidic nor basic. CO2 is acidic; Na2O and MgO are basic (C3-A15, C3-A70).

49. NDA 2025 (I)

Which one of the following is also known as Dry ice in its solid form?

(a) SiO₂(b) CO₂(c) CaO(d) MgO

Answer and solution

Answer: (b) CO₂

Method. Dry ice

Dry ice is solid CO2. Standard fact.

50. NDA 2025 (I)

Which one of the following nitrogen oxides may dimerize?

(a) N₂O(b) NO₂(c) N₂O₃(d) N₂O₅

Answer and solution

Answer: (b) NO₂

Method. Dimerisation of oxides of nitrogen

NO2 (odd-electron, paramagnetic brown gas) dimerises to colourless N2O4 on cooling: 2NO2 <-> N2O4. N2O, N2O3 and N2O5 are not odd-electron molecules. Standard result (pre-2023 NCERT).

51. NDA 2023 (I)

Which among the following is the popular method for manufacture of ammonia?

(a) Ostwald’s process(b) Haber-Bosch process(c) Electric furnace process(d) Electrolysis process

Answer and solution

Answer: (b) Haber-Bosch process

Method. Industrial synthesis of ammonia

Haber-Bosch process: N2 + 3H2 <-> 2NH3 over iron catalyst. Ostwald's process makes HNO3 from NH3. Standard result (pre-2023 NCERT).

52. NDA 2019 (II)

Which one of the following is termed as ‘Dry ice’?

(a) Ice present in ice-cream(b) Solid water at Antarctica(c) Solid state of carbon dioxide(d) Solid water of ionosphere

Answer and solution

Answer: (c) Solid state of carbon dioxide

Method. Dry ice

Dry ice is solid carbon dioxide, which sublimes. Standard fact.

53. NDA 2019 (I)

Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but they do not react with each other to form oxides of nitrogen because

(a) the reaction requires initiation by a catalyst(b) oxides of nitrogen are unstable(c) the reaction is endothermic and requires very high temperature(d) the stoichiometry of N₂ and O₂ in air is not ideal for the reaction to take place

Answer and solution

Answer: (c) the reaction is endothermic and requires very high temperature

Method. Reaction of N2 with O2

N2 + O2 -> 2NO is endothermic (+180 kJ/⁠mol) and needs about 3000 K or an electric arc, because the N-N triple bond is very strong; hence air does not react at ordinary temperature. Standard result (pre-2023 NCERT).

54. NDA 2018 (II)

Which one of the following reactions will give NO (nitric oxide) gas as one of the products?

(a) 3Cu + 8HNO₃ (dilute) →(b) Cu + 4HNO₃ (conc.) →(c) 4Zn + 10HNO₃ (dilute) →(d) Zn + 4HNO₃ (conc.) →

Answer and solution

Answer: (a) 3Cu + 8HNO₃ (dilute) →

Method. Products of metal + nitric acid

3Cu + 8HNO3 (dilute) -> 3Cu(NO3)2 + 2NO + 4H2O gives NO. Conc. HNO3 with Cu or Zn gives NO2; dilute HNO3 with Zn gives N2O or NH4NO3. HNO3 is an oxidising acid and gives no H2 (C3-A29). Standard result.

55. NDA 2018 (II)

Which one of the following is called dry ice?

(a) Solid carbon dioxide(b) Liquid carbon dioxide(c) Liquid nitrogen(d) Liquid ammonia

Answer and solution

Answer: (a) Solid carbon dioxide

Method. Dry ice

Solid CO2 sublimes without leaving a liquid, hence 'dry ice'. Standard fact.

56. NDA 2018 (I)

The Poisonous nature of Carbon monoxide (CO) is due to its

(a) insolubility in water(b) ability to form a complex with haemoglobin(c) ability to reduce some metal oxides(d) property of having one sigma bond

Answer and solution

Answer: (b) ability to form a complex with haemoglobin

Method. Toxicity of carbon monoxide

CO binds to the iron of haemoglobin about 200 times more strongly than O2, forming carboxyhaemoglobin and blocking oxygen transport. Standard result (pre-2023 NCERT).

11.6 Halogens, noble gases, sulphur and phosphorus

57. NDA 2022 (II)

To help deep-sea divers breathe, they carry cylinders of oxygen mixed with

(a) chlorine(b) helium(c) nitrogen(d) ozone

Answer and solution

Answer: (b) helium

Method. Gas mixtures for divers

Nitrogen dissolves in blood under pressure and causes narcosis and bends; helium is much less soluble, so a He-O2 mixture is used. Chlorine and ozone are toxic. Standard result.

58. NDA 2021 (I)

Which one of the following is the chemical formula of Hypobromous acid?

(a) HBrO₄(b) HOBr(c) HBr(d) HBrO₃

Answer and solution

Answer: (b) HOBr

Method. Naming of oxoacids of bromine

HOBr (HBrO) is hypobromous acid. HBrO3 is bromic, HBrO4 perbromic, HBr hydrobromic. Standard nomenclature.

59. NDA 2020 (I)

Which one of the following statements regarding Bleaching powder and D.D.T. is correct?

(a) Both are inorganic compounds(b) Both are organic compounds(c) Both contain chlorine(d) Both contain calcium.

Answer and solution

Answer: (c) Both contain chlorine

Method. Constitution of bleaching powder and DDT

Bleaching powder is CaOCl2 (inorganic, contains Ca and Cl). DDT is C14H9Cl5 (organic, contains Cl, no Ca). The only common feature is chlorine. Standard result.

60. NDA 2017 (II)

The principal use of hydrofluoric acid is

(a) in etching glass(b) as a bleaching agent(c) as an extremely strong oxidizing agent(d) in the preparation of strong organic fluorine compounds

Answer and solution

Answer: (a) in etching glass

Method. Chemistry of hydrofluoric acid

HF attacks silica and silicates of glass (SiO2 + 4HF -> SiF4 + 2H2O), so its textbook use is etching glass. It is a weak acid, not a bleach or a strong oxidising agent. Standard reference (pre-2023 NCERT).

61. NDA 2017 (I)

Match List I with List II and select the correct answer using the code given below the Lists:
List I (Noble gas)
(A) Argon
(B) Neon
(C) Krypton
(D) Xenon
List II (Use)
(1) In lights for advertising display
(2) Airport landing lights and in light houses
(3) Light in photographer's flash gun
(4) In tungsten filament to last longer
Code:
A B C D

(a) 3 1 2 4(b) 3 2 1 4(c) 4 2 1 3(d) 4 1 2 3

Answer and solution

Answer: (d) 4 1 2 3

Method. Uses of noble gases

Argon: inert fill in tungsten filament bulbs so the filament lasts longer (4). Neon: advertising display tubes (1). Krypton: airport landing lights and lighthouses (2). Xenon: photographer's flash guns (3). So A-4, B-1, C-2, D-3. Standard noble-gas uses (pre-2023 NCERT, standard reference).

62. NDA 2017 (I)

Radon is

(a) an inert gas(b) an artificial fibre(c) an explosive(d) a metal

Answer and solution

Answer: (a) an inert gas

Method. Group 18 gases

Radon (Z = 86) is the heaviest noble (inert) gas, a radioactive product of the decay of radium. It is neither a fibre, an explosive nor a metal. Standard result (pre-2023 NCERT).

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