CBSE 12th Chemistry Exam 2025 : Most Important Assertion Reason Questions with Answers for Last Minute Revision

CBSE 12th exams are underway and your CBSE 12th Chemistry exam is scheduled on 27th Feb, 2025. You have just a few hours left for CBSE 12th Chemistry exam.

We know how important it is to focus on the right topics and questions during revision, so this article provides important CBSE 12th Chemistry Most Important Assertion Reason Questions along with Answers for last minute revision.

Get most expected questions for CBSE Class 12 Chemistry. These questions are important and often appear in exams. Practice them well to prepare for the CBSE Class 12 Chemistry Exam 2025.

Most Important Assertion Reason Questions in CBSE Class 12 Chemistry Exam 2025

Following questions consist of two statements Assertion (A) and Reason (R).

Answer these questions selecting the appropriate option given below:

(a) Both A and R are true, and R is the correct explanation of A.

(b) Both A and R are true, and R is not the correct explanation of A.

(d) A is false but R is true.

1. Assertion: Molality is independent of temperature whereas molarity is a function of temperature.

Reason: Volume depends on temperature and mass does not depend on temperature.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

2. Assertion: An aqueous solution of NaCl freezes below 273 K.

Reason: Vapour pressure of the solution is less than that of the pure solvent.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

3. Assertion: The semipermeable membrane made of copper (II) ferrocyanide Cu₂[Fe(CN)₆] is not used for studying osmosis in a non-aqueous solution.

Reason: Copper (II) ferrocyanide is soluble in non-aqueous medium and insoluble in water.

Answer ⇒ (a) Both A and R are true, and R is the correctexplanation of A.

Because cupric ferrocyanide is soluble innon-aqueous solvents.

4. Assertion: An ideal solution obeys Henry's law.

Reason: In an ideal solution, solute-solute as well as solvent-solvent interactions are similar to solute-solvent interaction. (2020 HMJ/5)

Answer ⇒ (d) A is false but R is true.

5. Assertion: Relative lowering in vapourpressure is a colligative property.

Reason: Relative lowering in vapour pressure depends upon mole fraction of pure solvent. (2021)

Answer ⇒ (c) A is true but R is false.

Relative lowering in vapour pressure depends only on the number of solute particles and not on its nature. It depends on the mole fraction of solute.

6. Assertion: The boiling point of an azeotropicmixture of water and ethanol is less than thatof water and ethanol.

Reason: Azeotropic mixture of water and ethanol show positive deviation from Raoult's law.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

7. Assertion: An isotonic solution exerts same osmotic pressure under similar conditions.

Reason: Solute-solvent dipolar interactions exist in the pair of isotonic solution.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Because isotonic solution has the same concentration on both sides of the semipermeable membrane.

8. Assertion: Working of Galvanic cell is spontaneous.

Reason: Free energy, i.e., ΔG of galvanic cell increases.

Answer ⇒ (c) A is true but R is false.

The reaction in galvanic cell produces voltage and current flow in an external circuit, is spontaneous as it occurs with no intervention and has a negative Gibb’s free energy.

9. Assertion: ZnCl₂ does not combine with NH₃ released in electrode process of dry cell.

Reason: The dry cell may damage due to extra pressure of NH₃ gas.

Answer ⇒ (d) A is false but R is true.

ZnCl₂ combines with NH₃ produced to form the complex salt [Zn(NH₃)₂]²⁺; otherwise, the pressure developed due to NH₃ would crack the seal of the cell.

10. Assertion: Current stops flowing when $E_{cell} = 0$ .

Reason: Equilibrium of the cell reaction is attained.

Answer ⇒ (a) Both A and R are true, and R is the explanation of A.

11. Assertion (A): The units of rate constant of a zero order reaction and rate of reaction are the same.

Reason (R): In zero order reaction, the rate of reaction is independent of the concentration of reactants. (CBSE 2024)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

12. Assertion: The rate of reaction increases generally by 2 to 3 times for every 10° rise in temperature.

Reason: Increase in temperature increases the collision frequency. (Competency)

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Because the rate of reaction is dependent on temperature which is expressed in terms of temperature coefficient which is a ratio of 2 rate constants differing by a temperature of 10°C.

13. Assertion: The molecularity of the reaction

$H_{2} + B r_{2} \to 2 H B r$ is 2.

Reason: The order of this reaction is 3/2.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Because 2 molecules of the reactants are involved in given elementary reactions.

14. Assertion: For a zero-order reaction, the unit of rate constant and rate of reaction are same.

Reason (R): Rate of reaction for zero-order reaction is independent of concentration of reactant. (2023 Series: HFG1E/5)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

15. Assertion: The half-life of a reaction is the time in which the concentration of the reactant is reduced to one half of its initial concentration.

Reason: In first-order kinetics, when the concentration of reactant is doubled, its half-life is doubled. (2023 Series: HFG1E/5)

Answer ⇒ (c) A is true but R is false.

When initial concentration of a reactant in first order kinetic is doubled, its half-life is not affected as t₁/₂ is independent of the initial concentration of reactant.

∴ t₁/₂ = 0.693 / K

16. Assertion: Hydrolysis of an ester follows first-order kinetics.

Reason: Concentration of water remains nearly constant during the course of the reaction. (2020 HMI/4)

Answer ⇒ (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

17. Assertion: Actinoids form relatively less stable complexes as compared to lanthanoids.

Reason: Actinoids can utilise their 5f-orbitals along with 6d-orbitals in bonding but lanthanoids do not use their 4f-orbitals for bonding. (Competency)

Answer ⇒ (d) A is false but R is true.

Actinoids form relatively more stable complexes than lanthanoids due to the utilization of 5f and 6d orbitals, which lanthanoids cannot do.

18. Assertion: Ln (III) Compounds are generally colourless.

Reason: The ionic size of Ln(III) ions decreases with increase in atomic number.

Answer ⇒ (d) A is false but R is true.

Ln (III) Compounds are generally coloured due to f–f transition.

19. Assertion: Actinides show greater range of oxidation states.

Reason: 5f, 6d and 7s levels have comparable energies.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

20. Assertion: Transition metals have high enthalpy of atomisation.

Reason: Greater number of unpaired electrons in transition metals results in weak metallic bonding. (2023 Series: HFG1E/2)

Answer ⇒ (c) A is true but R is false.

High effective nuclear charge and a large number of valence electrons in transition metals cause very strong metallic bonding, which results in high enthalpies of atomisation.

21. Assertion: Transition metals show their highest oxidation state with oxygen.

Reason: The ability of oxygen to form multiple bonds to metals. (2023 Series: HFG1E/2)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

22. Assertion: Transition metals have low melting points.

Reason: The involvement of greater number of (n - 1) d and ns electrons in the interatomic metallic bonding. (2020 HMJ/4)

Answer ⇒ (d) A is false but R is true.

23. Assertion: Transition metals have high melting points.

Reason: Transition metals have completely filled d-orbitals. (2020 HMJ/4)

Answer ⇒ (c) A is true but R is false.

24. Assertion: An aqueous pink solution of cobalt (II) chloride changes to deep blue on addition of excess of HCl.

Reason: [Co(H₂O)₆]²⁺ is transformed into [CoCl₄]^2- and tetrahedral complexes have smaller crystal field splitting than octahedral complexes. (Competency)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

25. Assertion: [Cd(CN)₄]^2- complex is more stable than [Cd(NH₃)₄]²⁺ complex.

Reason: Stronger is the ligand, greater is the stability.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

26. Assertion: [Ni(NH₃)₆]²⁺ does not form inner orbital octahedral complex.

Reason: Ni²⁺ ion has 3d⁸ 4s⁰ configuration, hence, only sp³d² hybridization is possible.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

27. Assertion: Tetrahedral complexes show cis- and trans- isomerism.

Reason: The four corners of a tetrahedron are not present in the same plane.

Answer ⇒ (d) A is false but R is true.

Tetrahedral complexes has no cis- or trans- isomerism because the atoms are not bonded to a central atom cannot be placed either next to or across from each other.

28. Assertion: Boiling point of alkyl halides increases with increase in molecular weight.

Reason: Boiling point of alkyl halides is in the order RI > RBr > RCl > RF.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Greater the molecular mass, stronger the van der Waals' forces of attraction and hence higher is the melting point/boiling point.

29. Assertion: Boiling points of alcohols are lower than hydrocarbons.

Reason: Among isomeric alcohols, boiling point decreases in the order: 1° > 2° > 3°.

Answer ⇒ (d) A is false but R is true.

Alcohols undergo intermolecular H-bonding and hence, their boiling points are higher than those of hydrocarbons. Among isomeric alcohols, boiling point decreases in the order: 1° > 2° > 3°.

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30. Assertion: tert-Butyl alcohol undergoes acid-catalysed dehydration readily than propanol.

Reason: 3° Alcohols do not give Victor-Meyer’s test.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Alcohols which form the more stable carbocation undergo dehydration more readily. Since tert-butyl alcohol forms a more stable tert-butyl cation, therefore, it undergoes dehydration more readily than propanol.

31. Assertion: With Br₂-H₂O, phenol gives 2,4,6-tribromophenol but with Br₂-CS₂, it gives 4-bromophenol as the major product.

Reason: In water, ionisation of phenol is enhanced but in CS₂, it is greatly suppressed.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

32. Assertion: Phenol decomposes NaHCHO₃ solution to evolve CO₂ gas.

Reason: Picric acid is 2,4,6-trinitrophenol.

Answer ⇒ (d) A is false but R is true.

Phenol is a weaker acid than carbonic acid and hence, does not decompose NaHCO₃ solution to evolve CO₂ gas. Picric acid is 2,4,6-trinitrophenol.

33. Assertion: Reimer-Tiemann reaction of phenol with CHCl₃ in NaOH at 340 K gives salicylic acid as the major product.

Reason: The reaction occurs through intermediate formation of ⁺CHCl₂.

Answer ⇒ (c) A is true but R is false.

Intermediate formed is dichlorocarbene.

34. Assertion (A): p-nitrophenol is less acidic than phenol.

Reason (R): Nitro group is electron withdrawing and helps in the stabilisation of p-nitrophenoxide ion. (CBSE 2024)

Answer ⇒ (d) A is false but R is true.

p-Nitrophenol is more acidic than phenol because of EWG (Electron Withdrawing Groups) i.e., -NO₂.

35. Assertion: Primary and secondary alcohols can be distinguished by Victor-Meyer’s test.

Reason: Primary alcohols form nitrolic acid which dissolves NaOH to form blood red colouration, but secondary alcohols form pseudonitrots which give blue colouration with NaOH.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

36. Assertion: Benzaldehyde is more reactivethan ethanol towards nucleophilic attack.

Reason: All the carbon atoms of benzaldehydeare sp² hybridised.

Answer ⇒ (d) A is false but R is true.

Benzaldehyde is less reactive than ethanol towards nucleophilic attack due to electron donating effect of the benzene ring. The carbonyl carbon of benzaldehy de becomes less electrophilc.

37. Assertion: Benzoic acid does not undergoFriedal-Crafts reaction. (2020 Series: HMJ/4)

Reason: The carboxyl group is activating and undergo electrophilic substitution reaction.

Answer ⇒ (c) A is true but R is false.

38. Assertion: Benzaldehyde is less reactive than ethanol towards nucleophilic addition reactions.

Reason: Ethanal is more sterically hindered. (2020 Series: HMJ/5)

Answer ⇒ (c) A is true but R is false.

39. Assertion (A): Benzoic acid does not undergo Friedel-Crafts reaction.

Reason (R): Carboxyl group is deactivating and the catalyst aluminium chloride gets bonded to the carboxyl group. (CBSE 2024)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

40. Assertion: Amine have lower boiling point than of corresponding alcohols or carboxylic acids.

Reason: The electronegativity of oxygen is lower than that of nitrogen.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

41. Assertion: Primary amines have higher boiling point than tertiary amines.

Reason: Tertiary amines show extensive intermolecular H-bonding. (Competency)

Answer ⇒ (c) A is true but R is false.

Tertiary amines do not home hydrogen atoms so cannot form H-bonds. Therefore, have low boiling point than Primary amines.

42. Assertion: In Sandmeyer reaction, diazonium salt can be heated with Cu powder.

Reason: Sandmeyer reaction occurs by a free radical mechanism.

Answer ⇒ (d) A is false but R is true.

43. Assertion: Ortho and para-nitrophenols can be separated by steam distillation. (2020 Series: HMJ/5)

Reason: Ortho isomer associates through intermolecular hydrogen bonding, while para isomer associates through intramolecular hydrogen bonding.

Answer ⇒ (c) A is true but R is false.

44. Assertion: Glucose and fructose are reducing sugars.

Reason: Glucose and fructose contain a free aldehydic and ketonic group adjacent to a > CHOH group respectively. (Competency)

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

Reducing sugars contain a free aldehydic or ketonic group adjacent to α > CHOH group and reduce Tollen’s reagent, Schiff’s reagent or Benedict’s solution.

45. Assertion: Fructose reduces Fehling’s solution and Tollens’ reagent.

Reason: Fructose does not contain any aldehyde group.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

Fructose on warming with dilute alkali, gives rise to an equilibrium mixture of glucose, fructose and mannose. The ability of fructose to reduce Fehling solution and Tollens’ reagent is probably due to the isomerisation of fructose to glucose and mannose (i.e., called Lobry de Bruyn and Elkenstein rearrangement).

46. Assertion: DNA undergoes replication.

Reason: DNA contains cytosine and thymine as pyrimidine bases.

Answer ⇒ (b) Both A and R are true, and R is not the correct explanation of A.

The genetic information of the cell is contained in the sequence of base А, T, G and C in DNA molecule when a cell divides, DNA molecules replicate and make exact copies of themselves so
that each daughter cell will have DNA identical to that of the parent cell.

47. Assertion: Haemoglobin is a globular protein.

Reason: Globular proteins are insoluble in water.

Answer ⇒ (c) A is true but R is false.

Globular proteins have weak intermolecular forces of attraction and hence they are soluble in water.

48. Assertion: Alpha (α)–amino acids exist as internal salt in solution as they have amino and carboxylic acid groups in near vicinity.

Reason: H⁺ ion given by carboxylic group (-COOH) is captured by amino group (-NH₂) having lone pair of electrons.

Answer ⇒ (a) Both A and R are true, and R is the correct explanation of A.

NH₂—CH₂—COOH is a typical α-amino acid. In solution, it exists as an internal salt or zwitter ion, (NH₃₊—CH₂—COO^-) because the proton (H⁺) of —COOH group is captured by the NH₂ group, as NH₂ has a lone pair of electrons on the N atom.

49. Assertion: Uracil occurs in DNA.

Reason: DNA undergoes replication.

Answer ⇒ (d) A is false but R is true.

Uracil occurs in RNA.

50. Assertion: The backbone of DNA and RNA molecules is a chain consisting of heterocyclic base, pentose sugar and phosphate group.

Reason: Nucleotides and nucleosides mainly differ from each other in presence of phosphate group. (2023 Series: HFGI/5)

Answer ⇒ (b) Both A and R are true, and R is not thecorrect explanation of A.

CBSE 12th Chemistry Exam 2025 : Most Important Assertion Reason Questions with Answers for Last Minute Revision

Most Important Assertion Reason Questions in CBSE Class 12 Chemistry Exam 2025

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