Chemical Reactions and Redox Reactions – NDA Chemistry PYQs

Solve NDA Chemistry previous-year questions on Chemical Reactions with detailed solutions and explanations.

Chapter-wise PYQs • Concept-based explanations • Exam insights

NDA 2026-II

Q. 1. 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

(d) I, II and III

Answer: (a) I and II only

Explanation: Statement I: Correct – O in peroxides (H₂O₂) is −1; in superoxides (KO₂) is −½. Statement II: Correct – fluorine is the most electronegative element and always has −1 in compounds. Statement III: Incorrect – in a reduction half-reaction, it is the OXIDANT that decreases in oxidation number (the substance being reduced). The reductant is the one being oxidised (oxidation number increases). So III is wrong. Concept Tested: Oxidation numbers – oxygen in peroxides/superoxides, fluorine

Q. 2. Consider the following pairs :

I. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) [Displacement reaction]

II. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) [Double displacement reaction]

III. Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g) [Redox reaction]

IV. 2KClO₃(s) → 2KCl(s) + 3O₂(g) [Thermal decomposition reaction]

How many of the pairs given above are correctly matched ?

(a) One

(b) Two

(d) All four

Answer: (d) All four

Explanation: I: Fe displaces Cu – displacement ✓. II: Ba²⁺ + SO₄²⁻ → BaSO₄ precipitate – double displacement ✓. III: Fe is reduced from +3 to 0, CO is oxidised to CO₂ – redox ✓. IV: KClO₃ breaks into KCl + O₂ when heated – thermal decomposition ✓. All four pairs are correctly matched. Concept Tested: Matching reaction types to equations – all four types

NDA 2026-I

Q. 3. 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

(d) I, II, III and IV

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

Explanation: All four are oxidation reactions. Rusting of iron: Fe is oxidised to Fe₂O₃. Burning of fuel: C/H compounds are oxidised. Rancidity of oils: unsaturated fats are oxidised by atmospheric O₂. Browning of fruits: enzymatic oxidation (polyphenols oxidised to quinones). All four involve oxidation. Concept Tested: Oxidation reactions – rusting, burning, rancidity, browning

Q. 4. Consider the following pairs of reactions and properties :

I. Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) [Fe is reductant]

II. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s) [Cu is being reduced]

III. Br₂(l) + 2I⁻(aq) → 2Br⁻(aq) + I₂(s) [Br₂ is reductant]

IV. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) [C atom in CH₄ is oxidized]

Which of the above pairs are correct?

(a) I and II only

(b) II, III and IV

(d) I, III and IV

Answer: (c) I, II and IV

Explanation: I: Fe goes 0→+2 (loses electrons) → Fe is oxidised = reductant ✓. II: Cu goes +2→0 (gains electrons) → Cu is being reduced ✓. III: Br₂ is reduced (Br goes 0→−1), so Br₂ is an oxidising agent, NOT a reductant – statement III is WRONG. IV: C in CH₄ goes −4→+4 (loses electrons) → oxidised ✓. So I, II and IV are correct. Concept Tested: Identifying correct redox roles in paired reaction statements

NDA 2025-II

Q. 5. Which one of the following is NOT a chemical change?

(a) Burning of coal

(b) Rusting of metal

(d) Reaction of acid with base

Answer: (c) Melting of ice

Explanation: Melting of ice (ice → water) is a physical change – only the state changes, not the chemical composition. Burning, rusting, and acid-base reactions all produce new chemical substances. Concept Tested: Physical change – melting of ice

Q. 6. Which one of the following statements is NOT correct?

(a) The interconversion of states of matter is a physical change

(b) During burning of a candle, both chemical and physical changes take place

(d) Rusting of iron is a chemical change

Answer: (c) During a physical change, the chemical composition of molecules changes

Explanation: By definition, a physical change does NOT change chemical composition. Statement (c) is the opposite of the correct definition – it is NOT correct. (a) is correct: changes of state are physical. (b) is correct: burning a candle involves both (wax melting = physical; wax combustion = chemical). (d) is correct: rusting = chemical change. Concept Tested: Physical change – definition and identification of false statement

Q. 7. In the following redox reaction, which compound is acting as a reducing agent? N₂H₄ + 2H₂O₂ → N₂ + 4H₂O

(a) H₂O₂

(b) N₂H₄

(d) H₂O

Answer: (b) N₂H₄

Explanation: In this reaction, N₂H₄ (hydrazine) is oxidized: N goes from −2 to 0 in N₂ (loses electrons). H₂O₂ is reduced: O goes from −1 to −2 in H₂O (gains electrons). The substance oxidized is the reducing agent: N₂H₄. Concept Tested: Identifying the reducing agent in a redox reaction – N₂H₄

Q. 8. Match List I with List II and select the answer using the code given below the Lists:

List I (Reaction)

A. CaO(s) + H₂O(l) → Ca(OH)₂(aq)

B. CaCO₃(s) → CaO(s) + CO₂(g)

C. Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)

D. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

List II (Type)

1. Single displacement

2. Double displacement

3. Combination

4. Decomposition

Code :

A B C D

(a) 2 1 4 3

(b) 2 4 1 3

(d) 3 1 4 2

(a) 2 1 4 3

(b) 2 4 1 3

(d) 3 1 4 2

Answer: (c) A-3, B-4, C-1, D-2

Explanation: A. CaO + H₂O → Ca(OH)₂: two substances combine → 3. Combination. B. CaCO₃ → CaO + CO₂: one compound breaks down → 4. Decomposition. C. Pb + CuCl₂ → PbCl₂ + Cu: Pb replaces Cu → 1. Single displacement. D. Na₂SO₄ + BaCl₂ → BaSO₄ + NaCl: ions exchange → 2. Double displacement. Code (c): A-3, B-4, C-1, D-2. Concept Tested: Matching reaction types – combination, decomposition, single/double displacement

Q. 9. Which one of the following is an endothermic reaction?

(a) CH₄(g) + 2O₂(g) → CO₂(g) + 2O₂(g)

(b) C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l)

(d) 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)

Answer: (d) 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)

Explanation: Thermal decomposition reactions generally require energy input (endothermic). The decomposition of lead nitrate on heating absorbs energy and is endothermic. Combustion of CH₄ and glucose are strongly exothermic. Formation of ammonia (Haber process) is exothermic. Note: option (a) appears to have a typo (product should be H₂O not O₂). The intended endothermic answer is (d). Concept Tested: Endothermic reaction – thermal decomposition of Pb(NO₃)₂

NDA 2025-I

Q. 10. 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

(d) If a substance gains oxygen during reaction, it is oxidized

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

Explanation: Classical definitions: Oxidation = loss of hydrogen OR gain of oxygen. Reduction = gain of hydrogen OR loss of oxygen. Option (a) says losing hydrogen = reduced – this is INCORRECT. Losing hydrogen means oxidation, not reduction. Options (b), (c), (d) are all correct. Concept Tested: Classical definitions of oxidation and reduction – hydrogen/oxygen

NDA 2024-II

Q. 11. The chemical reaction: 2AgCl (s) → 2Ag (s) + Cl₂ (g) takes place

(a) in dark

(b) in sunlight

(d) under high pressure

Answer: (b) in sunlight

Explanation: Silver chloride decomposes on exposure to light (photodecomposition). This is the basis of photography and explains why AgCl is stored in dark bottles. The reaction occurs in sunlight (or UV light), not on heating or in the dark. Concept Tested: Photodecomposition of AgCl – light-sensitive reaction

Q. 12. Which one among the following is an example of oxidation reaction?

(a) Freezing of water

(b) Dissolving sugar in water

(d) Boiling of petrol

Answer: (c) Rusting of iron

Explanation: Rusting of iron involves the oxidation of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (simplified). Iron gains oxygen and increases its oxidation state – this is oxidation. Freezing and boiling are physical changes. Dissolving sugar is a physical change (no new substance). Concept Tested: Oxidation reaction – rusting of iron as example

Q. 13. Which one among the following is an example of chemical change?

(a) Sublimation of camphor

(b) Melting of ice

(d) Mixing of NaOH and HCl

Answer: (d) Mixing of NaOH and HCl

Explanation: NaOH + HCl → NaCl + H₂O – a neutralisation reaction producing new substances (salt and water). This is a chemical change. Sublimation of camphor, melting of ice, and heating of an iron rod are physical changes (only state or temperature changes, no new substance formed). Concept Tested: Chemical change – neutralisation reaction as example

NDA 2024-I

Q. 14. What is the oxidation state of Vanadium in V₂O₅?

(a) +2

(b) +4

(d) +5

Answer: (d) +5

Explanation: In V₂O₅: O has oxidation state −2; total from 5 O = −10. For the compound to be neutral: 2V + (−10) = 0 → 2V = +10 → V = +5. Concept Tested: Oxidation state of vanadium in V₂O₅

NDA 2023-II

Q. 15. 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

(d) This reaction is applicable to joining of railway tracks

Answer: (b) It is highly endothermic reaction

Explanation: The thermite reaction (Fe₂O₃ + 2Al → 2Fe + Al₂O₃) is a highly EXOTHERMIC reaction that releases enormous heat – enough to melt iron. It is not endothermic. It is a displacement reaction (Al displaces Fe), Fe₂O₃ is reduced (acting as oxidizing agent), and thermite welding is used to join railway tracks. Concept Tested: Thermite reaction – exothermic, displacement, railway track welding

Q. 16. Match List I with List II and select the correct answer using the code given below the Lists :

List I (Chemical process)

A. Electrolysis of water

B. Burning of coal

C. Iron nail immersed in copper sulphate solution

D. Addition of barium chloride solution to aluminium sulphate solution

List II (Reaction)

1. Double displacement reaction

2. Combination reaction

3. Decomposition reaction

4. Displacement reaction

Code :

A B C D

(a) 3 4 2 1

(b) 4 2 1 3

(d) 4 1 2 3

(a) 3 4 2 1

(b) 4 2 1 3

(d) 4 1 2 3

Answer: (c) A-3, B-2, C-4, D-1

Explanation: A. Electrolysis of water → 2H₂O → 2H₂ + O₂ – one compound breaks into two = 3. Decomposition. B. Burning of coal: C + O₂ → CO₂ – two substances combine = 2. Combination. C. Iron nail + CuSO₄ → FeSO₄ + Cu – Fe replaces Cu = 4. Displacement. D. BaCl₂ + Al₂(SO₄)₃ → BaSO₄↓ + AlCl₃ – two compounds exchange ions = 1. Double displacement. Code (c): A-3, B-2, C-4, D-1. Concept Tested: Matching reaction types to chemical processes

Q. 17. Which one of the following processes involves chemical reaction ?

(a) Evaporation of petrol

(b) Dissolution of salt in water

(d) Burning of magnesium ribbon in air

Answer: (d) Burning of magnesium ribbon in air

Explanation: Burning of magnesium: 2Mg + O₂ → 2MgO – new substance (MgO) formed; chemical reaction. Evaporation, dissolution of salt, and storing gas are all physical processes – no new substances formed. Concept Tested: Chemical reaction – combustion of magnesium

Q. 18. Consider the following reaction : 2HgO → 2Hg + O₂ The respective state of HgO, Hg and O₂ in the above reaction is

(a) Liquid, solid, gas

(b) Solid, solid, gas

(d) Solid, liquid, gas

Answer: (d) Solid, liquid, gas

Explanation: HgO is a solid red/orange powder. On strong heating, it decomposes to produce mercury and oxygen. Mercury at the temperature of this reaction is a liquid (mercury’s boiling point is 357 °C; the reaction occurs near 400 °C). Oxygen is released as a gas. So: HgO = solid, Hg = liquid, O₂ = gas. Concept Tested: States of matter in thermal decomposition of HgO

NDA 2023-I

Q. 19. An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions ?

(a) Addition reaction

(b) Decomposition reaction

(d) Displacement reaction

Answer: (d) Displacement reaction

Explanation: Fe + CuSO₄ → FeSO₄ + Cu. Iron displaces copper from copper sulphate. Copper deposits on the iron nail, turning it brown (copper-coloured). This is a displacement reaction – a more reactive metal replaces a less reactive metal from its salt solution. Concept Tested: Displacement reaction – iron nail in CuSO₄

Q. 20. Airbags work on the principle of a chemical reaction triggered by the impact producing a gaseous product that causes a sudden volume change. Which one among the following chemical conversions is responsible for this ?

(a) Sodium azide into nitrogen gas

(b) Solid carbon dioxide into gaseous carbon dioxide

(d) Sudden conversion of gaseous carbon dioxide into carbon monoxide

Answer: (a) Sodium azide into nitrogen gas

Explanation: Airbags contain sodium azide (NaN₃). On impact, sodium azide rapidly decomposes to produce nitrogen gas: 2NaN₃ → 2Na + 3N₂. The sudden production of N₂ inflates the airbag. This is a chemical decomposition reaction. Concept Tested: Airbag chemistry – decomposition of sodium azide

NDA 2022-II

Q. 21. Which of the following compounds undergoes/undergo thermal decomposition?

(a) Zinc oxide

(b) Silver oxide and zinc oxide

(d) Magnesium oxide

Answer: (c) Silver oxide

Explanation: Silver oxide (Ag₂O) thermally decomposes on heating: 2Ag₂O → 4Ag + O₂. Zinc oxide (ZnO) does not decompose on normal heating. Magnesium oxide (MgO) is very stable and does not decompose easily. Only silver oxide undergoes thermal decomposition among these options. Concept Tested: Thermal decomposition – silver oxide

NDA 2022-I

Q. 22. Which one of the following is a chemical change?

(a) Dissolving sugar in water

(b) Melting of ice

(d) Milk turning sour

Answer: (d) Milk turning sour

Explanation: Milk turning sour is a chemical change – bacteria convert lactose to lactic acid, producing a new chemical substance. Dissolving sugar, melting of ice and crystallization are all physical changes – no new substances are formed. Concept Tested: Chemical change – milk souring due to bacterial action

Q. 23. Which one of the following is not an example of a redox reaction?

(a) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl

(b) 2NaH → 2Na + H₂

(d) CuSO₄ + Zn → Cu + ZnSO₄

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

Explanation: In (a) AlCl₃ + H₂O → Al(OH)₃ + HCl: this is a double decomposition/hydrolysis reaction. Check oxidation states: Al stays +3, Cl stays −1, H stays +1, O stays −2. No oxidation state changes – not a redox reaction. (b) NaH decomposition: H goes from −1 to 0 – redox. (c) Fe oxidation: Fe goes from 0 to +3 – redox. (d) Zn displaces Cu: Zn goes 0 to +2, Cu goes +2 to 0 – redox. Concept Tested: Identifying non-redox reactions – hydrolysis of AlCl₃

Q. 24. Which one of the following reactions is an example of decomposition reaction?

(a) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

(b) 2AgCl(s) → 2Ag(s) + Cl₂(g)

(d) Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Answer: (b) 2AgCl(s) → 2Ag(s) + Cl₂(g)

Explanation: A decomposition reaction is one where a single compound breaks down into two or more simpler substances. Option (b) AgCl → Ag + Cl₂ (photodecomposition in sunlight) is a decomposition reaction. Option (a) is combustion. Option (c) is a redox/displacement. Option (d) is a displacement reaction. Concept Tested: Decomposition reaction – photodecomposition of AgCl

NDA 2021-I

Q. 25. Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of

(a) Displacement reaction.

(b) Endothermic reaction.

(d) Exothermic reaction.

Answer: (d) Exothermic reaction.

Explanation: CaO + H₂O → Ca(OH)₂ + heat. This reaction releases heat and is exothermic (the vessel becomes warm). It is a combination/addition reaction (two substances combine), not decomposition or displacement. Concept Tested: Exothermic reaction – CaO with water

NDA 2019-II

Q. 26. Which one of the following compounds does not exhibit a different oxidation number of the same element?

(a) Pb₃O₄

(b) Fe₃O₄

(d) Mn₃O₄

Answer: (c) Fe₂O₃

Explanation: Pb₃O₄ contains Pb in +2 and +4 states. Fe₃O₄ contains Fe in +2 and +3 states. Mn₃O₄ contains Mn in +2 and +3 states. Fe₂O₃ contains only Fe³⁺ – all iron atoms have the same +3 oxidation state. So Fe₂O₃ does not exhibit different oxidation numbers for the same element. Concept Tested: Mixed oxidation state compounds – Fe₂O₃ as exception

NDA 2019-I

Q. 27. 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

(d) The reaction is an example of a redox reaction

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

Explanation: Fe displaces Cu from CuSO₄ because Fe is MORE reactive than Cu (higher in the reactivity series). So (c) is NOT correct – copper is LESS reactive than iron. (a) Fe loses electrons → reducing agent ✓. (b) FeSO₄ solution is green ✓. (d) Fe is oxidised, Cu²⁺ is reduced → redox ✓. Concept Tested: Reactivity series – Fe displacing Cu from CuSO₄

NDA 2018-II

Q. 28. Which one of the following is an oxidation-reduction reaction?

(a) NaOH + HCl → NaCl + H₂O

(b) CaO + H₂O → Ca(OH)₂

(d) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Answer: (c) 2Mg + O₂ → 2MgO

Explanation: In 2Mg + O₂ → 2MgO: Mg goes from 0 to +2 (oxidised); O goes from 0 to −2 (reduced). This is a redox reaction. Option (a) is an acid-base neutralization (no oxidation state change). Option (b) is a combination reaction with no redox. Option (d) is a double displacement/precipitation reaction with no redox. Concept Tested: Identifying redox reactions from a set of equations

NDA 2017-II

Q. 29. Which one of the following is a chemical change?

(a) Cutting of hair

(b) Graying of hair naturally

(d) Cutting of fruit

Answer: (b) Graying of hair naturally

Explanation: Graying of hair naturally involves a reduction in melanin production – a biochemical (chemical) change. Cutting of hair and cutting of fruit are physical changes (no new substance formed). Swelling of resin in water is a physical change (absorption). Concept Tested: Chemical vs physical change – graying of hair

Q. 30. 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

(d) 2 and 4 only

Answer: Editorial Review Required

Explanation: In CH₄ + 2O₂ → CO₂ + 2H₂O: Carbon goes from −4 (in CH₄) to +4 (in CO₂) – oxidised. Hydrogen goes from +1 (in CH₄) to +1 (in H₂O) – unchanged. Oxygen goes from 0 (O₂) to −2 – reduced. Only carbon is oxidised; hydrogen does not change oxidation state. Statement 1 alone is correct. However, option (b) 1 and 2 is listed as a common answer. This needs verification against the official NDA 2017-II key since statement 2 appears incorrect based on oxidation state analysis. Concept Tested: Oxidation state changes in combustion of methane

NDA 2016-II

Q. 31. Match List I with List II and select the correct answer using the code given below the Lists:

List I (Process)

A. Heating Camphor

B. Cooling of water vapour up to room temperature

C. Cooking an egg

D. Formation of water vapour at room temperature

List II (Type of change)

1. Chemical

2. Evaporation

3. Condensation

4. Sublimation

Code :

A B C D

(a) 4 3 1 2

(b) 4 1 3 2

(d) 2 3 1 4

(a) 4 3 1 2

(b) 4 1 3 2

(d) 2 3 1 4

Answer: (a) A-4, B-3, C-1, D-2

Explanation: A. Heating camphor → it sublimes (solid to gas directly) = 4. Sublimation. B. Cooling water vapour → it condenses = 3. Condensation. C. Cooking an egg → chemical change (protein denaturation) = 1. Chemical. D. Formation of water vapour at room temperature → evaporation = 2. Evaporation. Code (a): A-4, B-3, C-1, D-2. Concept Tested: Matching change processes to their types

Q. 32. Which one of the following species is not capable of showing disproportionation reaction?

(a) ClO⁻

(b) ClO₂⁻

(d) ClO₄⁻

Answer: (d) ClO₄⁻

Explanation: Disproportionation occurs when a species is simultaneously oxidised and reduced. ClO⁻, ClO₂⁻, and ClO₃⁻ can disproportionate because Cl can be oxidised to a higher state or reduced to a lower state. In ClO₄⁻, chlorine is in its highest oxidation state (+7) and cannot be further oxidised. Therefore ClO₄⁻ cannot disproportionate. Concept Tested: Disproportionation reaction – chlorine oxyanions

NDA 2016-I

Q. 33. Which one of the following is not a chemical change ?

(a) Ripening of fruits

(b) Curdling of milk

(d) Digestion of food

Answer: (c) Freezing of water

Explanation: Freezing of water (liquid → solid) is a physical change – only the state changes, not the chemical composition. Ripening of fruits, curdling of milk, and digestion of food all involve chemical reactions where new substances are produced. Concept Tested: Physical change – freezing of water

Q. 34. Which one of the following is a reduction reaction ?

(a) 2 Mg (s) + O₂ (g) → 2 MgO (s)

(b) S (s) + O₂ (g) → SO₂ (g)

(d) Mg (s) + S (s) → MgS (s)

Answer: (c) 2 HgO (s) → 2 Hg (l) + O₂ (g)

Explanation: In option (c), HgO loses oxygen to form Hg metal. Mercury goes from +2 (in HgO) to 0 (Hg) – it gains electrons – this is reduction. Options (a) and (b) are oxidation reactions (Mg and S gain oxygen). Option (d) is a combination reaction but not a reduction. Concept Tested: Reduction reaction – identification by oxygen loss

NDA 2015-II

Q. 35. In the reaction ZnO + C → Zn + CO, ‘C’ acts as

(a) an acid

(b) a base

(d) a reducing agent

Answer: (d) a reducing agent

Explanation: This is the same question as NDA 2015-I Q96. Carbon removes oxygen from ZnO, reducing it to zinc metal. Carbon is therefore the reducing agent. This repeated appearance in NDA confirms this is a high-frequency concept. Concept Tested: Carbon as a reducing agent – NDA 2015-I repeated in 2015-II

★ JOVIK Exam Insight: This exact question appeared in both NDA 2015-I and NDA 2015-II. Carbon as a reducing agent in ZnO + C → Zn + CO is a core NDA concept.

NDA 2015-I

Q. 36. Statement I : Oxygen gas is easily produced at a faster rate by heating a mixture of potassium chlorate and manganese dioxide than heating potassium chlorate alone. Statement II : Manganese dioxide acts as a negative catalyst.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (c) Statement I is true but Statement II is false

Explanation: Statement I is correct: MnO₂ speeds up the decomposition of KClO₃ to give O₂. Statement II is false: MnO₂ acts as a POSITIVE catalyst (it speeds up the reaction), not a negative catalyst (which would slow it down). Concept Tested: MnO₂ as a positive catalyst in KClO₃ decomposition

Q. 37. In the reaction ZnO + C → Zn + CO, ‘C’ acts as

(a) an acid

(b) a base

(d) a reducing agent

Answer: (d) a reducing agent

Explanation: In ZnO + C → Zn + CO: Zinc in ZnO has oxidation state +2; it is reduced to 0 in Zn. Carbon in C is 0; it is oxidised to +2 in CO. So carbon removes oxygen from ZnO (reducing ZnO), meaning carbon acts as a reducing agent. Concept Tested: Carbon as a reducing agent – reduction of metal oxides

NDA 2014-II

Q. 38. What is the oxidizing agent in the following equation? HAsO₂(aq) + Sn²⁺(aq) + H⁺(aq) → As(s) + Sn⁴⁺(aq) + H₂O(l)

(a) HAsO₂(aq)

(b) Sn²⁺(aq)

(d) Sn⁴⁺(aq)

Answer: (a) HAsO₂(aq)

Explanation: Sn²⁺ is oxidized to Sn⁴⁺ (loses electrons) – Sn²⁺ is the reducing agent. HAsO₂ provides As which goes from As³⁺ (in HAsO₂) to As⁰ (elemental As) – As is reduced (gains electrons). The species being reduced is the oxidizing agent: HAsO₂. Concept Tested: Identifying oxidizing and reducing agents from oxidation state changes

NDA 2013-I

Q. 39. Which one among the following is not a chemical change?

(a) Curdling of milk

(b) Ripening of fruit

(d) Burning of coal

Answer: (c) Evaporation of water

Explanation: Evaporation is a physical change – water changes state from liquid to vapour but remains chemically H₂O. Curdling of milk, ripening of fruit, and burning of coal all involve chemical reactions forming new substances. Concept Tested: Physical vs chemical change – evaporation as physical change

NDA 2012-II

Q. 40. In KMnO₄ molecule, the oxidation states of the elements Potassium (K), Manganese (Mn) and Oxygen (O) are respectively

(a) +1, +5, −2

(b) +1, +7, −2

(d) +1, +7, 0

Answer: (b) +1, +7, −2

Explanation: K has a fixed oxidation state of +1. O has a fixed oxidation state of −2. For KMnO₄ to be electrically neutral: +1 + Mn + 4(−2) = 0 → Mn = +8−1 = +7. So K = +1, Mn = +7, O = −2. Concept Tested: Oxidation state of Mn in KMnO₄

★ JOVIK Exam Insight: Mn in KMnO₄ = +7. This is one of the most common oxidation state questions in NDA. Always use: sum of all oxidation states = 0 for neutral compounds.

NDA 2012-I

Q. 41. Statement I: At high temperature, hydrogen can reduce PbO to elemental lead. Statement II: Hydrogen has great affinity to oxygen.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (a) Both the statements are individually true and Statement II is the correct explanation of Statement I

Explanation: PbO + H₂ → Pb + H₂O at high temperature. Hydrogen reduces PbO because H₂ has a strong affinity for oxygen – it reacts with oxygen to form water. This affinity for oxygen is exactly why hydrogen acts as a reducing agent here. Statement II directly explains Statement I. Concept Tested: Reduction of metal oxides by hydrogen – affinity for oxygen

Q. 42. Statement I: Conversion of blue copper sulphate to black cupric oxide on heating is a physical change. Statement II: A change in which chemical composition does not change is called physical change.

(a) Both the statements are individually true and Statement II is the correct explanation of Statement I

(b) Both the statements are individually true but Statement II is not the correct explanation of Statement I

(d) Statement I is false but Statement II is true

Answer: (d) Statement I is false but Statement II is true

Explanation: Statement II is correct: a physical change does not alter chemical composition. Statement I is false: converting CuSO₄ (copper sulphate) to CuO (cupric oxide) involves a change in chemical composition – it is a chemical change, not a physical change. Concept Tested: Physical vs chemical change – CuSO₄ to CuO on heating

Q. 43. Which of the following statements regarding oxidation and reduction are correct?

1. In oxidation, loss of electron takes place whereas in reduction, gain of electron takes place.

2. In oxidation, gain of electron takes place whereas in reduction, loss of electron takes place.

3. Oxidizing agent decreases the oxidation number but reducing agent increases the oxidation number.

4. Oxidizing agent increases the oxidation number but reducing agent reduces the oxidation number.

Select the correct answer using the code given below:

(a) 1 and 3

(b) 2 and 4

(d) 1 and 4

Answer: (d) 1 and 4

Explanation: Statement 1 is correct: oxidation = loss of electrons; reduction = gain of electrons. Statement 4 is correct: the oxidizing agent accepts electrons, so it causes the oxidation number of the other substance to increase; the reducing agent donates electrons. Statements 2 and 3 are incorrect reversals. Concept Tested: Oxidation and reduction – electron transfer and oxidation numbers

Q. 44. The following equation is an example of a redox reaction, in which Cl₂ is the oxidizing agent and FeBr₃ is the reducing agent: 2FeBr₃(aq) + 3Cl₂(g) = 2FeCl₃(aq) + 3Br₂(l) Which one among the following statements is incorrect for this redox reaction?

(a) Oxidizing agents are themselves reduced

(b) Reducing agents gain or appear to gain electrons

(d) Oxidizing agents oxidize other substances

Answer: (b) Reducing agents gain or appear to gain electrons

Explanation: In a redox reaction: oxidizing agents are reduced (they gain electrons) – (a) is correct. Reducing agents are oxidized (they lose electrons) – (c) is correct. Oxidizing agents oxidize other substances – (d) is correct. But (b) says reducing agents gain electrons – this is incorrect. Reducing agents LOSE electrons (they are oxidized). So (b) is the incorrect statement. Concept Tested: Roles of oxidizing and reducing agents in redox reactions

NDA 2011-II

Q. 45. In oxidation

1. Hydrogen is displaced from a substance.

2. an electropositive element is added to or proportion of electropositive element increases in a substance.

Select the correct answer using the code given below :

(a) 1 only

(b) 2 only

(d) Neither 1 nor 2

Answer: (a) 1 only

Explanation: In oxidation: hydrogen is removed (displaced) from a substance – statement 1 is correct. Statement 2 describes reduction, not oxidation: in reduction, an electropositive element (metal) is added or its proportion increases. Oxidation involves loss of electrons/hydrogen or gain of oxygen. Concept Tested: Classical definition of oxidation – hydrogen removal

Q. 46. When a copper rod is dipped in aqueous silver nitrate solution, the colour of the solution changes to blue. This is because

(a) Cu is more easily reduced than Ag

(b) Ag is more easily reduced than Cu

(d) Nitrate ion acts as a reducing agent

Answer: (b) Ag is more easily reduced than Cu

Explanation: Copper displaces silver from silver nitrate: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. This happens because silver (Ag⁺) is more easily reduced than copper (Cu²⁺) – Ag has higher reduction potential. Cu²⁺ ions in solution give the blue colour (copper nitrate solution is blue). Concept Tested: Displacement reaction – copper and silver nitrate

NDA 2011-I

Q. 47. Iron nails are dipped into blue copper sulphate solution. After some time iron nails are :

(a) dissolved and blue colour is discharged.

(b) dissolved but blue colour is not discharged.

(d) not dissolved but blue colour is discharged.

Answer: (a) dissolved and blue colour is discharged.

Explanation: Iron is more reactive than copper. Iron displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ + Cu. The iron nail dissolves (Fe goes into solution as Fe²⁺) and copper deposits on the nail. The blue colour of CuSO₄ fades as copper ions are replaced by iron ions (FeSO₄ is pale green). Concept Tested: Displacement reaction – iron and copper sulphate

★ JOVIK Exam Insight: Iron + CuSO₄ is a classic NDA displacement reaction. Iron is more reactive → displaces copper. Blue colour (Cu²⁺) is lost as Cu deposits. Iron nail appears copper-coated.

Q. 48. Statement I : Oxidation in our body cell releases dangerous free radicals Statement II : Our body itself produces anti-oxidants to neutralize harmful free radicals

(a) Both the statements are individually true and statement II is the correct explanation of statement I

(b) Both the statements are individually true but statement II is not the correct explanation of statement I

(d) Statement I is false but statement II is true

Answer: (a) Both the statements are individually true and statement II is the correct explanation of statement I

Explanation: Oxidation reactions in body cells produce free radicals (reactive molecules with unpaired electrons) that can damage cells. The body produces antioxidants (like superoxide dismutase, vitamins C and E) to neutralize these. Statement II directly explains the mechanism for handling what Statement I describes. Concept Tested: Oxidation and free radicals in biological systems

NDA 2010-II

Q. 49. An oxidizing agent is a substance which :

(a) increases the oxidation number of an element in a given substance

(b) decreases the oxidation number of an element in a given substance

(d) loses electrons in an oxidation-reduction reaction

Answer: (b) decreases the oxidation number of an element in a given substance

Explanation: An oxidizing agent oxidizes another substance – meaning it causes the oxidation number of the other substance to increase. In doing so, the oxidizing agent itself is reduced (its own oxidation number decreases). So an oxidizing agent decreases its own oxidation number. Option (b) correctly describes the oxidizing agent’s own change. Concept Tested: Oxidizing agent – definition and electron/oxidation-number change

Q. 50. Which one among the following is a chemical process?

(a) Distillation of sea (salty) water

(b) Crystallization of impure salt (NaCl)

(d) Sublimation of iodine (I₂)

Answer: (c) Production of iodine (I₂) from sea-weeds

Explanation: Extracting iodine from seaweed involves chemical reactions (oxidation of iodide ions to iodine). Distillation, crystallization and sublimation are all physical processes – no new chemical substances are formed. Concept Tested: Chemical vs physical processes – production of iodine

NDA 2010-I

Q. 51. Which of the following represent a chemical change?

1. Magnetization of iron

2. Condensation of liquid

3. Burning of fuel

4. Rusting of iron

Select the correct answer using the code given below:

(a) 1 and 2

(b) 2 and 3

(d) 1 and 4

Answer: (c) 3 and 4

Explanation: Burning of fuel (combustion) and rusting of iron are both chemical changes – new substances are formed (CO₂/H₂O from combustion; iron oxide from rusting). Magnetization of iron is a physical change (magnetic alignment, no new substance). Condensation is a physical change (liquid to gas reversal). Concept Tested: Chemical changes – burning and rusting as examples

NDA 2009-I

Q. 52. Which among the following is a chemical change?

(a) A wet towel dries in the sun

(b) Lemon juice added to tea causing its colour to change

(d) Coffee is brewed by passing steam through ground coffee

Answer: (b) Lemon juice added to tea causing its colour to change

Explanation: Lemon juice (acid) reacts with the compounds in tea, causing a chemical change in colour. The other options – drying (evaporation), hot air rising (convection), and brewing (extraction/dissolution) – are physical processes with no new chemical substances formed. Concept Tested: Chemical vs physical change – everyday examples