Redox, Oxidation Numbers, Displacement Reactions and Ionic Equations

1. In the reaction below, which species is oxidised?

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

A Zn
B Cu2+
C Zn2+
D Cu

2. In the reaction below, which species is reduced?

Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)

A Mg
B H+
C Mg2+
D H2

3. Which statement correctly describes oxidation in terms of electrons?

A gain of electrons
B loss of electrons
C gain of hydrogen
D loss of oxygen

4. Which statement correctly describes reduction in terms of oxidation number?

A oxidation number increases
B oxidation number decreases
C oxidation number stays the same
D oxidation number becomes zero only

5. What is the oxidation number of sulfur in H2SO4?

A –2
B +2
C +4
D +6

6. What is the oxidation number of manganese in MnO4–?

A +2
B +4
C +6
D +7

7. What is the oxidation number of chromium in Cr2O7 2–?

A +3
B +4
C +6
D +7

8. What is the oxidation number of nitrogen in NH4+?

A –3
B –1
C +3
D +5

9. Ammonium nitrate contains NH4+ and NO3– ions.

What are the oxidation numbers of nitrogen in NH4+ and NO3–?

	NH4+	NO3–
A	–3	+5
B	+3	–5
C	–3	–5
D	+5	–3

10. In which substance does chlorine have oxidation number +1?

A Cl2
B HCl
C NaClO
D NaCl

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. Which change is oxidation?

A Fe3+ to Fe2+
B Cu2+ to Cu
C I– to I2
D Cl2 to Cl–

12. Which change is reduction?

A Mg to Mg2+
B Fe2+ to Fe3+
C Br– to Br2
D Ag+ to Ag

13. In the reaction below, which species acts as the reducing agent?

Cl2(aq) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

A Cl2
B Br–
C Cl–
D Br2

14. In the reaction below, which species acts as the oxidising agent?

2Fe2+(aq) + Cl2(aq) → 2Fe3+(aq) + 2Cl–(aq)

A Fe2+
B Cl2
C Fe3+
D Cl–

15. Which equation shows oxidation only?

A Cu2+ + 2e– → Cu
B Fe2+ → Fe3+ + e–
C Cl2 + 2e– → 2Cl–
D O2 + 4e– → 2O2–

16. Which equation shows reduction only?

A Mg → Mg2+ + 2e–
B 2I– → I2 + 2e–
C Fe3+ + e– → Fe2+
D Zn → Zn2+ + 2e–

17. Which reaction is not a redox reaction?

A Zn + CuSO4 → ZnSO4 + Cu
B 2Mg + O2 → 2MgO
C HCl + NaOH → NaCl + H2O
D Cl2 + 2KBr → 2KCl + Br2

18. Which reaction involves both oxidation and reduction?

A AgNO3 + NaCl → AgCl + NaNO3
B H2SO4 + 2KOH → K2SO4 + 2H2O
C CaCO3 + 2HCl → CaCl2 + H2O + CO2
D 2Na + Cl2 → 2NaCl

19. In the reaction below, what happens to iron?

Fe2O3 + 3CO → 2Fe + 3CO2

A Iron is oxidised from +3 to 0.
B Iron is reduced from +3 to 0.
C Iron is oxidised from 0 to +3.
D Iron is reduced from 0 to +3.

20. In the same reaction, Fe2O3 + 3CO → 2Fe + 3CO2, what happens to carbon?

A Carbon is reduced from +2 to +4.
B Carbon is oxidised from +2 to +4.
C Carbon is reduced from +4 to +2.
D Carbon is oxidised from 0 to +4.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. Which ionic equation correctly represents the reaction between magnesium and dilute hydrochloric acid?

A Mg(s) + 2Cl–(aq) → MgCl2(aq)
B Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)
C Mg2+(aq) + 2H+(aq) → Mg(s) + H2(g)
D Mg(s) + HCl(aq) → MgCl(aq) + H2(g)

22. Which ionic equation correctly represents the reaction between copper and silver nitrate solution?

A Cu(s) + Ag+(aq) → Cu+(aq) + Ag(s)
B Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s)
C Cu2+(aq) + 2Ag(s) → Cu(s) + 2Ag+(aq)
D Cu(s) + 2NO3–(aq) → Cu(NO3)2(aq)

23. Which ionic equation correctly represents neutralisation?

A H+(aq) + OH–(aq) → H2O(l)
B Na+(aq) + Cl–(aq) → NaCl(aq)
C 2H+(aq) + O2–(aq) → H2O(l)
D H2(g) + O2(g) → H2O(l)

24. Which ionic equation correctly represents the reaction of carbonate ions with acid?

A CO3 2–(aq) + H+(aq) → CO2(g) + OH–(aq)
B CO3 2–(aq) + 2H+(aq) → CO2(g) + H2O(l)
C CO3 2–(aq) + 2OH–(aq) → CO2(g) + H2O(l)
D CO3 2–(aq) + H2O(l) → CO2(g) + 2OH–(aq)

25. Which ionic equation correctly represents the formation of silver chloride precipitate?

A Ag+(aq) + Cl–(aq) → AgCl(s)
B Ag(s) + Cl2(g) → AgCl(s)
C AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
D Ag+(aq) + NO3–(aq) → AgNO3(aq)

26. Which ions are spectator ions in this reaction?

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

A Ba2+ and SO4 2–
B Na+ and Cl–
C Ba2+ and Cl–
D Na+ and SO4 2–

27. Which ions are spectator ions in this reaction?

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

A Zn and Cu
B Zn2+ and Cu2+
C SO4 2– only
D Cu2+ only

28. Which ionic equation represents the precipitation of lead(II) iodide?

A Pb2+(aq) + 2I–(aq) → PbI2(s)
B Pb+(aq) + I–(aq) → PbI(s)
C Pb2+(aq) + I2(aq) → PbI2(s)
D Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

29. Which full equation has the ionic equation:

2H+(aq) + CO3 2–(aq) → CO2(g) + H2O(l)

A NaOH + HCl → NaCl + H2O
B Na2CO3 + 2HCl → 2NaCl + H2O + CO2
C AgNO3 + NaCl → AgCl + NaNO3
D Zn + 2HCl → ZnCl2 + H2

30. Which full equation has the ionic equation:

Fe(s) + Cu2+(aq) → Fe2+(aq) + Cu(s)

A Fe + CuSO4 → FeSO4 + Cu
B FeCl2 + Cu → Fe + CuCl2
C Fe2O3 + 3CO → 2Fe + 3CO2
D Fe + 2HCl → FeCl2 + H2

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A strip of zinc is placed in copper(II) sulfate solution.

Which observation is expected?

A blue solution becomes colourless and brown copper forms
B colourless solution becomes blue and copper dissolves
C green solution forms and hydrogen gas is produced
D no change occurs because zinc is less reactive than copper

32. A strip of copper is placed in zinc sulfate solution.

Which observation is expected?

A zinc deposits on copper
B blue solution forms
C no reaction
D copper dissolves and zinc ions are reduced

33. Which metal can displace iron from iron(II) sulfate solution?

A copper
B silver
C magnesium
D gold

34. Which solution can be used to show that chlorine is more reactive than iodine?

A chlorine water added to potassium iodide solution
B iodine solution added to potassium chloride solution
C potassium chloride solution added to potassium iodide solution
D potassium iodide solution added to sodium chloride solution

35. Chlorine water is added to potassium bromide solution.

Which statement is correct?

A Bromide ions are oxidised to bromine.
B Bromine is reduced to bromide ions.
C Chloride ions are oxidised to chlorine.
D Potassium ions are reduced to potassium.

36. Iodine is added to potassium chloride solution.

Which observation is expected?

A purple iodine disappears and chlorine forms
B brown solution forms because iodine displaces chlorine
C no displacement reaction occurs
D potassium metal is produced

37. Which halogen will displace iodine from potassium iodide solution?

A bromine only
B chlorine only
C chlorine and bromine
D iodine only

38. Which halide ion is the strongest reducing agent?

A F–
B Cl–
C Br–
D I–

39. Which halogen is the strongest oxidising agent?

A fluorine
B chlorine
C bromine
D iodine

40. Which displacement reaction is possible?

A Cu + MgSO4 → CuSO4 + Mg
B Ag + CuSO4 → Ag2SO4 + Cu
C Fe + CuSO4 → FeSO4 + Cu
D Au + ZnSO4 → AuSO4 + Zn

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. A metal X is placed in solutions of three metal nitrates. The observations are shown.

solution	observation
magnesium nitrate	no reaction
copper(II) nitrate	copper deposited
zinc nitrate	no reaction

Which order of reactivity is correct?

A Mg > Zn > X > Cu
B X > Mg > Zn > Cu
C Cu > X > Zn > Mg
D Mg > X > Zn > Cu

42. Metal P displaces metal Q from QSO4 solution. Metal R does not displace Q from QSO4 solution.

Which statement must be correct?

A P is more reactive than Q.
B R is more reactive than Q.
C Q is more reactive than P.
D P is less reactive than R.

43. A student adds iron filings to silver nitrate solution.

Which ionic equation is most likely?

A Fe(s) + Ag+(aq) → Fe+(aq) + Ag(s)
B Fe(s) + 2Ag+(aq) → Fe2+(aq) + 2Ag(s)
C Fe2+(aq) + 2Ag(s) → Fe(s) + 2Ag+(aq)
D Ag(s) + Fe2+(aq) → Ag+(aq) + Fe(s)

44. Which reaction shows hydrogen acting as a reducing agent?

A H2 + Cl2 → 2HCl
B CuO + H2 → Cu + H2O
C 2H2 + O2 → 2H2O
D H2O + CO2 → H2CO3

45. Which substance is oxidised in this reaction?

CuO + H2 → Cu + H2O

A CuO
B H2
C Cu
D H2O

46. Which substance is the oxidising agent in this reaction?

CuO + H2 → Cu + H2O

A CuO
B H2
C Cu
D H2O

47. In the reaction below, which statement is correct?

2FeCl2 + Cl2 → 2FeCl3

A Fe2+ is oxidised to Fe3+.
B Cl2 is oxidised to Cl–.
C Fe3+ is reduced to Fe2+.
D Chloride ions are reduced to chlorine.

48. Which change shows loss of oxygen?

A CuO → Cu
B Mg → MgO
C Fe2+ → Fe3+
D Cl– → Cl2

49. Which change shows gain of oxygen?

A ZnO → Zn
B CO → CO2
C Fe3+ → Fe2+
D Cu2+ → Cu

50. Which row is correct?

	oxidising agent	reducing agent
A	gains electrons	loses electrons
B	loses electrons	gains electrons
C	is oxidised	is reduced
D	decreases another species’ oxidation number	increases another species’ oxidation number

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

1. A

A right: Zn loses electrons to form Zn2+, so zinc is oxidised.
B wrong: Cu2+ gains electrons, so it is reduced.
C wrong: Zn2+ is the product of oxidation, not the species being oxidised.
D wrong: Cu is the reduced product.

2. B

A wrong: Mg loses electrons, so it is oxidised.
B right: H+ gains electrons to form H2, so it is reduced.
C wrong: Mg2+ is the oxidised product.
D wrong: H2 is the reduced product, not the species being reduced.

3. B

A wrong: Gain of electrons is reduction.
B right: Oxidation is loss of electrons.
C wrong: Gain of hydrogen is reduction.
D wrong: Loss of oxygen is reduction.

4. B

A wrong: Oxidation number increases during oxidation.
B right: Reduction means oxidation number decreases.
C wrong: No change means no redox for that species.
D wrong: Reduction does not always make oxidation number zero.

5. D

In H2SO4:
H = +1 each, total +2
O = –2 each, total –8
Overall molecule = 0

So sulfur must be +6.

A wrong: Too low.
B wrong: Too low.
C wrong: Sulfur is +4 in SO2, not H2SO4.
D right: +6.

6. D

In MnO4–:
O = –2 each, total –8
Overall ion = –1

Mn + (–8) = –1
Mn = +7

A wrong: Too low.
B wrong: Too low.
C wrong: Too low by 1.
D right: +7.

7. C

In Cr2O7 2–:
O = –2 each, total –14
Overall ion = –2

2Cr – 14 = –2
2Cr = +12
Cr = +6

A wrong: Too low.
B wrong: Too low.
C right: +6.
D wrong: Too high.

8. A

In NH4+:
H = +1 each, total +4
Overall ion = +1

N + 4 = +1
N = –3

A right: –3.
B wrong: Too high.
C wrong: Sign is wrong.
D wrong: N is not +5 in ammonium.

9. A

In NH4+: nitrogen = –3.
In NO3–: oxygen total = –6, overall charge = –1, so nitrogen = +5.

A right: –3 and +5.
B wrong: Both signs reversed.
C wrong: N in nitrate is not –5.
D wrong: Reversed.

10. C

A wrong: Cl2 has chlorine oxidation number 0.
B wrong: In HCl, chlorine is –1.
C right: In NaClO, Na is +1 and O is –2, so Cl is +1.
D wrong: In NaCl, chlorine is –1.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. C

A wrong: Fe3+ to Fe2+ is reduction because oxidation number decreases.
B wrong: Cu2+ to Cu is reduction.
C right: I– to I2 is oxidation because iodide loses electrons and oxidation number rises from –1 to 0.
D wrong: Cl2 to Cl– is reduction.

12. D

A wrong: Mg to Mg2+ is oxidation.
B wrong: Fe2+ to Fe3+ is oxidation.
C wrong: Br– to Br2 is oxidation.
D right: Ag+ to Ag is reduction because silver ions gain electrons.

13. B

A wrong: Cl2 is reduced, so it is the oxidising agent.
B right: Br– is oxidised to Br2, so Br– acts as the reducing agent.
C wrong: Cl– is a product.
D wrong: Br2 is the oxidised product.

14. B

A wrong: Fe2+ is oxidised, so it is the reducing agent.
B right: Cl2 gains electrons and is reduced to Cl–, so Cl2 is the oxidising agent.
C wrong: Fe3+ is the oxidised product.
D wrong: Cl– is the reduced product.

15. B

A wrong: Cu2+ gains electrons, so reduction.
B right: Fe2+ loses an electron to form Fe3+, so oxidation.
C wrong: Cl2 gains electrons, so reduction.
D wrong: O2 gains electrons, so reduction.

16. C

A wrong: Mg loses electrons, so oxidation.
B wrong: I– loses electrons, so oxidation.
C right: Fe3+ gains an electron to form Fe2+, so reduction.
D wrong: Zn loses electrons, so oxidation.

17. C

A wrong: Zinc is oxidised and copper ions are reduced.
B wrong: Magnesium is oxidised and oxygen is reduced.
C right: HCl + NaOH is neutralisation, not redox. Oxidation numbers do not change.
D wrong: Chlorine is reduced and bromide is oxidised.

18. D

A wrong: This is precipitation, not redox.
B wrong: This is neutralisation, not redox.
C wrong: Acid-carbonate reaction; no oxidation number change.
D right: Sodium is oxidised and chlorine is reduced.

19. B

In Fe2O3, iron is +3.
In Fe, iron is 0.
Oxidation number decreases from +3 to 0, so iron is reduced.

A wrong: Decrease is not oxidation.
B right: Iron is reduced from +3 to 0.
C wrong: Direction is reversed.
D wrong: Iron is not starting at 0.

20. B

In CO, oxygen is –2, so carbon is +2.
In CO2, oxygen total is –4, so carbon is +4.
Carbon oxidation number increases from +2 to +4, so carbon is oxidised.

A wrong: Increase is not reduction.
B right: Carbon is oxidised from +2 to +4.
C wrong: Direction is reversed.
D wrong: Carbon is not 0 in CO.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. B

A wrong: Chloride ions are spectator ions.
B right: Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).
C wrong: This shows magnesium ions becoming magnesium metal, the reverse reaction.
D wrong: HCl should be split into ions, and MgCl is not correct.

22. B

A wrong: Copper normally forms Cu2+ in this reaction, not Cu+.
B right: Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s).
C wrong: This is the reverse of the actual displacement.
D wrong: Nitrate ions are spectator ions.

23. A

A right: H+(aq) + OH–(aq) → H2O(l) is the ionic equation for neutralisation.
B wrong: Sodium chloride remains aqueous as ions.
C wrong: Oxide ions are not the usual neutralisation equation here.
D wrong: This is not neutralisation.

24. B

A wrong: Needs 2H+ and water forms, not OH–.
B right: CO3 2–(aq) + 2H+(aq) → CO2(g) + H2O(l).
C wrong: Acid, not hydroxide, reacts with carbonate.
D wrong: Carbonate does not decompose like this in water.

25. A

A right: Ag+(aq) + Cl–(aq) → AgCl(s).
B wrong: This is not the precipitation ionic equation from solution.
C wrong: This is a full equation, not ionic.
D wrong: Nitrate is spectator and silver nitrate remains soluble.

26. B

Ba2+ and SO4 2– form BaSO4(s).
Na+ and Cl– remain unchanged in solution.

A wrong: These form the precipitate.
B right: Na+ and Cl– are spectator ions.
C wrong: Ba2+ is not spectator.
D wrong: SO4 2– is not spectator.

27. C

Full ionic idea:
Zn(s) + Cu2+(aq) + SO4 2–(aq) → Zn2+(aq) + SO4 2–(aq) + Cu(s)

A wrong: Zn and Cu are not spectator ions; they change.
B wrong: Zn2+ is a product and Cu2+ reacts.
C right: SO4 2– is unchanged.
D wrong: Cu2+ is reduced, so it is not spectator.

28. A

A right: Pb2+(aq) + 2I–(aq) → PbI2(s).
B wrong: Lead(II) is Pb2+, and PbI is wrong.
C wrong: Iodide ions, not iodine molecules, form the precipitate.
D wrong: This is the full equation, not ionic.

29. B

A wrong: This gives H+ + OH– → H2O.
B right: Sodium carbonate and hydrochloric acid give carbonate + acid ionic equation.
C wrong: This gives Ag+ + Cl– → AgCl.
D wrong: This gives Zn + 2H+ → Zn2+ + H2.

30. A

A right: Fe + CuSO4 → FeSO4 + Cu gives Fe + Cu2+ → Fe2+ + Cu.
B wrong: This is the reverse.
C wrong: This is reduction of iron oxide by carbon monoxide.
D wrong: This is iron reacting with acid.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A

A right: Zinc displaces copper from copper(II) sulfate. Blue CuSO4 solution becomes colourless ZnSO4, and brown copper forms.
B wrong: This describes copper dissolving, not zinc displacing copper.
C wrong: Hydrogen is not produced here.
D wrong: Zinc is more reactive than copper.

32. C

A wrong: Copper is less reactive than zinc, so it cannot displace zinc.
B wrong: Zinc sulfate is colourless and copper does not dissolve.
C right: No reaction occurs.
D wrong: Zinc ions are not reduced by copper.

33. C

A wrong: Copper is less reactive than iron.
B wrong: Silver is less reactive than iron.
C right: Magnesium is more reactive than iron and can displace it.
D wrong: Gold is much less reactive.

34. A

A right: Chlorine displaces iodine from potassium iodide, showing chlorine is more reactive.
B wrong: Iodine cannot displace chlorine from potassium chloride.
C wrong: These are two halide salts, no halogen displacement.
D wrong: No displacement reaction occurs.

35. A

A right: Bromide ions lose electrons and are oxidised to bromine.
B wrong: Bromide is oxidised, not bromine reduced.
C wrong: Chlorine is reduced to chloride ions.
D wrong: Potassium ions are spectators.

36. C

A wrong: Iodine cannot displace chlorine from chloride solution.
B wrong: Iodine is less reactive than chlorine.
C right: No displacement reaction occurs.
D wrong: Potassium ions are spectators and are not reduced.

37. C

A wrong: Bromine can displace iodine, but chlorine can too.
B wrong: Chlorine can displace iodine, but bromine can too.
C right: Chlorine and bromine are both more reactive than iodine.
D wrong: Iodine cannot displace itself meaningfully.

38. D

A wrong: F– is the weakest reducing agent among halides.
B wrong: Cl– is weaker than Br– and I–.
C wrong: Br– is strong, but not the strongest here.
D right: I– is most easily oxidised, so it is the strongest reducing agent.

39. A

A right: Fluorine is the strongest oxidising agent among halogens.
B wrong: Chlorine is weaker than fluorine.
C wrong: Bromine is weaker than chlorine.
D wrong: Iodine is the weakest oxidising agent listed.

40. C

A wrong: Copper cannot displace magnesium.
B wrong: Silver cannot displace copper.
C right: Iron is more reactive than copper, so Fe displaces Cu from CuSO4.
D wrong: Gold cannot displace zinc.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. A

X does not displace Mg, so Mg is more reactive than X.
X displaces Cu, so X is more reactive than Cu.
X does not displace Zn, so Zn is more reactive than X.

Known order: Mg > Zn > X > Cu.

A right: Mg > Zn > X > Cu.
B wrong: X is not more reactive than Mg or Zn.
C wrong: Copper is not more reactive than X.
D wrong: Zinc must be above X.

42. A

A right: If P displaces Q, P is more reactive than Q.
B wrong: R does not displace Q, so R is less reactive than Q.
C wrong: Q is less reactive than P.
D wrong: We cannot compare P and R directly except both relative to Q; P is above Q and R below Q, so P is likely more reactive than R, not less.

43. B

A wrong: Iron usually forms Fe2+ in this displacement.
B right: Fe(s) + 2Ag+(aq) → Fe2+(aq) + 2Ag(s).
C wrong: Reverse reaction.
D wrong: Silver cannot displace iron.

44. B

A wrong: Hydrogen reacts with chlorine, but this does not show hydrogen reducing a metal oxide.
B right: Hydrogen removes oxygen from CuO, reducing CuO to Cu; hydrogen acts as the reducing agent.
C wrong: Hydrogen is oxidised in combustion, but not reducing a metal oxide here.
D wrong: No redox.

45. B

In CuO + H2 → Cu + H2O:
Hydrogen gains oxygen to form water, so hydrogen is oxidised.

A wrong: CuO is reduced.
B right: H2 is oxidised.
C wrong: Cu is the reduced product.
D wrong: H2O is the oxidised product.

46. A

A right: CuO oxidises hydrogen to water and is itself reduced, so CuO is the oxidising agent.
B wrong: H2 is the reducing agent.
C wrong: Cu is the reduced product.
D wrong: H2O is the oxidised product.

47. A

A right: Fe2+ becomes Fe3+, so iron(II) ions are oxidised.
B wrong: Cl2 is reduced to Cl–, not oxidised.
C wrong: Fe3+ is formed, not reduced.
D wrong: Chlorine is reduced to chloride ions, not the reverse.

48. A

A right: CuO → Cu shows loss of oxygen, so reduction.
B wrong: Mg → MgO shows gain of oxygen.
C wrong: This is electron loss/oxidation, not oxygen loss.
D wrong: This is oxidation of chloride ions, not oxygen loss.

49. B

A wrong: ZnO → Zn is loss of oxygen.
B right: CO → CO2 is gain of oxygen, so oxidation.
C wrong: Fe3+ → Fe2+ is reduction by electron gain.
D wrong: Cu2+ → Cu is reduction.

50. A

A right: An oxidising agent gains electrons and causes another species to be oxidised; a reducing agent loses electrons and causes another species to be reduced.
B wrong: Reversed.
C wrong: Oxidising agents are reduced; reducing agents are oxidised.
D wrong: Oxidising agents decrease another species’ electron count by taking electrons, but the clearer correct electron-transfer row is A; this row is awkward and not the standard definition.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.