In short: Assertion-Reason questions appear in Section A of the CBSE Class 12 Chemistry paper (70 marks theory). Typically 2 out of 16 MCQs are assertion-reason format, carrying 1 mark each. The format gives two statements — Assertion (A) and Reason (R) — and you must identify the relationship. Combined with competency MCQs and case-based questions, approximately 50% of the paper tests competency skills.

How Assertion-Reason Works in Chemistry

Every assertion-reason question follows a fixed four-option format. Memorise these options — they are the same in every paper:

Option	Meaning
(a)	Both A and R are true, and R is the correct explanation of A
(b)	Both A and R are true, but R is NOT the correct explanation of A
(c)	A is true but R is false
(d)	A is false but R is true

The key challenge is distinguishing (a) from (b). Both have "A true, R true" — but only in (a) does the reason causally explain the assertion. In (b), both statements are factually correct but talk about different aspects or the reason does not logically lead to the assertion.

The Chemistry chapter-wise marks weightage shows that Physical Chemistry, Organic Chemistry, and Inorganic Chemistry each carry roughly equal weightage. A-R questions appear from all three sections.

Solutions (Chapter 2)

Q1. Assertion: The boiling point of a solution is higher than that of the pure solvent. Reason: The vapour pressure of a solution is lower than that of the pure solvent (Raoult's Law).

Answer: (a) — The lowering of vapour pressure directly causes the solution to require a higher temperature to boil. The reason correctly explains the assertion.

Q2. Assertion: When NaCl is dissolved in water, the elevation in boiling point is more than expected for a non-electrolyte of the same molality. Reason: NaCl dissociates into Na⁺ and Cl⁻ ions, producing more particles in solution.

Answer: (a) — The van 't Hoff factor i > 1 for NaCl because of dissociation, which increases colligative effects.

Q3. Assertion: Osmotic pressure is a colligative property. Reason: Osmotic pressure depends on the nature of the solute particles.

Answer: (c) — A is true (osmotic pressure depends on the number of particles, not their nature). R is false (colligative properties by definition do NOT depend on the nature of the solute).

Q4. Assertion: A 0.1 M solution of glucose and a 0.1 M solution of urea have the same osmotic pressure at the same temperature. Reason: Both are non-electrolytes and produce the same number of particles per formula unit.

Answer: (a) — For non-electrolytes, osmotic pressure depends only on molarity and temperature (π = iMRT, with i = 1 for both).

Electrochemistry (Chapter 3)

Q5. Assertion: The standard electrode potential of Cu²⁺/Cu is positive (+0.34 V). Reason: Copper is more easily oxidised than hydrogen.

Answer: (c) — A is true (E° for Cu²⁺/Cu is indeed +0.34 V). R is false (a positive reduction potential means Cu²⁺ is more easily reduced than H⁺, not that Cu is more easily oxidised).

Q6. Assertion: In a galvanic cell, the cathode is the positive terminal. Reason: Reduction occurs at the cathode.

Answer: (a) — Reduction at the cathode consumes electrons, making it electron-poor (positive). The reason correctly explains the assertion.

Q7. Assertion: The molar conductivity of a strong electrolyte increases slightly with dilution. Reason: Strong electrolytes are completely dissociated at all concentrations.

Answer: (a) — Since strong electrolytes are fully dissociated, dilution only reduces interionic attractions, causing a slight increase in molar conductivity.

Q8. Assertion: Rusting of iron is an electrochemical process. Reason: During rusting, iron is oxidised and oxygen is reduced in the presence of water.

Answer: (a) — The reason describes the two half-reactions that constitute the electrochemical corrosion process.

Chemical Kinetics (Chapter 4)

Q9. Assertion: The order of a reaction can be zero. Reason: The order of a reaction is determined experimentally.

Answer: (b) — Both are true. Zero-order reactions exist, and order is indeed determined experimentally. However, the fact that order is experimental does not explain why it can be zero — it simply states how order is found.

Q10. Assertion: The molecularity of a reaction cannot be zero or fractional. Reason: Molecularity is the number of reacting molecules in the rate-determining step.

Answer: (a) — Since molecularity counts actual molecules, it must be a positive integer. The definition in R explains why it cannot be zero or fractional.

Q11. Assertion: For a first-order reaction, the half-life is independent of the initial concentration. Reason: The half-life of a first-order reaction is t₁/₂ = 0.693/k.

Answer: (a) — The formula in the reason shows no concentration term, directly explaining the assertion.

Q12. Assertion: A catalyst increases the rate of a reaction. Reason: A catalyst provides an alternative pathway with lower activation energy.

Answer: (a) — The lower activation energy path directly explains the increased rate.

d-Block and f-Block Elements (Chapter 8)

Q13. Assertion: Transition metals exhibit variable oxidation states. Reason: The energies of (n−1)d and ns electrons are comparable.

Answer: (a) — Because d and s orbital energies are close, varying numbers of electrons can participate in bonding, leading to multiple oxidation states.

Q14. Assertion: Most transition metal compounds are coloured. Reason: Transition metal ions have completely filled d-orbitals.

Answer: (c) — A is true (d-d transitions cause colour). R is false (colour arises from partially filled d-orbitals, not completely filled ones. Zn²⁺ with d¹⁰ is colourless).

Q15. Assertion: Zinc is not considered a typical transition element. Reason: Zinc has a completely filled d-orbital (3d¹⁰) in its ground state and common ion Zn²⁺.

Answer: (a) — A transition element should have partially filled d-orbitals in at least one stable oxidation state. Zn²⁺ has 3d¹⁰, which is completely filled.

Q16. Assertion: The +2 oxidation state is the most common among transition metals. Reason: The two 4s electrons are lost first during ionisation.

Answer: (a) — The ns electrons are lost before (n−1)d electrons, making +2 the baseline oxidation state.

Q17. Assertion: KMnO₄ acts as a strong oxidising agent. Reason: In KMnO₄, manganese is in the +7 oxidation state, the highest for Mn.

Answer: (a) — The highest oxidation state (+7) means Mn can easily gain electrons (be reduced), making KMnO₄ a strong oxidiser.

Q18. Assertion: Lanthanoid contraction is the gradual decrease in the size of lanthanoid ions from La³⁺ to Lu³⁺. Reason: The poor shielding effect of 4f electrons causes a greater effective nuclear charge.

Answer: (a) — 4f electrons shield poorly, so each additional proton is less screened, pulling electrons closer and shrinking the ion.

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Coordination Compounds (Chapter 9)

Q19. Assertion: [Fe(CN)₆]⁴⁻ is diamagnetic. Reason: CN⁻ is a strong field ligand that causes pairing of d-electrons.

Answer: (a) — In the presence of CN⁻ (strong field), Fe²⁺ (d⁶) undergoes complete pairing, leaving zero unpaired electrons → diamagnetic.

Q20. Assertion: [NiCl₄]²⁻ is paramagnetic while [Ni(CN)₄]²⁻ is diamagnetic. Reason: Cl⁻ is a weak field ligand and CN⁻ is a strong field ligand.

Answer: (a) — Weak field Cl⁻ does not cause pairing (tetrahedral, sp³, 2 unpaired electrons). Strong field CN⁻ causes pairing (square planar, dsp², 0 unpaired electrons).

Haloalkanes and Haloarenes (Chapter 10)

Q21. Assertion: SN2 reactions occur with inversion of configuration. Reason: In SN2, the nucleophile attacks from the side opposite to the leaving group.

Answer: (a) — The backside attack directly causes inversion (Walden inversion). The mechanism explains the observation.

Q22. Assertion: Tertiary halides prefer SN1 mechanism over SN2. Reason: Tertiary carbocations are highly stable due to hyperconjugation and inductive effects.

Answer: (a) — SN1 proceeds through a carbocation intermediate. The stability of tertiary carbocations makes SN1 favourable.

Q23. Assertion: Vinyl chloride (CH₂=CHCl) is less reactive towards nucleophilic substitution than ethyl chloride. Reason: The C−Cl bond in vinyl chloride has partial double bond character due to resonance.

Answer: (a) — Resonance between the lone pair on Cl and the C=C π bond strengthens the C−Cl bond, making it harder to break.

Alcohols, Phenols, and Ethers (Chapter 11)

Q24. Assertion: Phenol is more acidic than ethanol. Reason: The phenoxide ion formed after losing H⁺ is stabilised by resonance.

Answer: (a) — Resonance delocalises the negative charge over the benzene ring, making phenoxide more stable than ethoxide. Greater stability of conjugate base = greater acidity.

Q25. Assertion: Lucas test can distinguish between primary, secondary, and tertiary alcohols. Reason: Tertiary alcohols react immediately with Lucas reagent (HCl + ZnCl₂) to form a turbid solution.

Answer: (a) — The rate of reaction with Lucas reagent depends on carbocation stability: tertiary (immediate) > secondary (5 min) > primary (requires heating).

Aldehydes, Ketones, and Carboxylic Acids (Chapter 12)

Q26. Assertion: Aldehydes are more reactive than ketones towards nucleophilic addition. Reason: Aldehydes have less steric hindrance and weaker +I effect compared to ketones.

Answer: (a) — Both steric and electronic factors favour nucleophilic attack on aldehydes over ketones.

Q27. Assertion: Formic acid (HCOOH) can reduce Tollens' reagent, but acetic acid (CH₃COOH) cannot. Reason: Formic acid has an aldehyde-like −CHO group.

Answer: (a) — The H−C(=O)−OH structure of formic acid contains a −CHO functionality that can be oxidised, giving it reducing properties. For more on organic reactions, see the named reactions and mechanisms guide.

Amines (Chapter 13)

Q28. Assertion: Aniline does not undergo Friedel-Crafts reaction. Reason: The −NH₂ group in aniline reacts with the Lewis acid catalyst AlCl₃ to form a salt.

Answer: (a) — The lone pair on nitrogen coordinates with AlCl₃, deactivating the amine and preventing the Friedel-Crafts mechanism from proceeding.

Q29. Assertion: Aliphatic amines are stronger bases than aromatic amines. Reason: In aromatic amines, the lone pair on nitrogen is delocalised into the benzene ring.

Answer: (a) — Delocalisation reduces the availability of the lone pair for protonation, making aromatic amines weaker bases.

Biomolecules (Chapter 14)

Q30. Assertion: Sucrose is a non-reducing sugar. Reason: In sucrose, the glycosidic bond is formed between the reducing groups of both glucose and fructose.

Answer: (a) — Since both anomeric carbons (reducing ends) are involved in the glycosidic linkage, neither is free to open and reduce a reagent.

How to Approach Chemistry Assertion-Reason

The Three-Step Method

Judge A independently. Is the assertion a correct chemical statement? If false, your answer is (d) — regardless of R.
Judge R independently. Is the reason a correct chemical statement? If A is true but R is false, answer is (c).
Check the causal link. If both are true, ask: "Does R explain why A is true?" If yes → (a). If no → (b).

When to Choose (b)

Option (b) is the trickiest. Here are common patterns where both are true but unrelated:

A states a property. R states a different property of the same substance.
A states an observation. R states the definition of a different concept.
A and R are facts from the same chapter but different topics.

The smart study techniques guide explains how to build analytical skills needed for these questions.

Common Mistakes in Chemistry A-R Questions

Choosing (a) whenever both statements look correct. This is the most frequent error. Always verify the causal connection.
Not checking if R is factually true. Students focus so much on the assertion that they accept the reason without verification. R can contain subtle errors — like saying "completely filled" instead of "partially filled."
Confusing oxidation with reduction. In electrochemistry A-R questions, mixing up which electrode gets oxidised vs reduced leads to wrong answers.
Ignoring exceptions. Statements with "always," "all," or "never" in chemistry are often false. Zinc being a "transition metal" is a common trap.
Spending too much time on one question. Each A-R question is worth 1 mark. If you cannot decide between (a) and (b) within 1 minute, make your best guess and move on.

For full paper strategy, see the board exam time management guide.

FAQs

How many assertion-reason questions are in CBSE Class 12 Chemistry 2027?

Typically 2 out of 16 MCQs in Section A are assertion-reason format, carrying 1 mark each (2 marks total). Some years have had up to 4 A-R questions.

Which chapters are most common for Chemistry A-R questions?

Solutions, Electrochemistry, d-Block Elements, and Organic Chemistry (Haloalkanes, Amines) are the most frequent sources. These chapters have many concept pairs that lend themselves to the A-R format.

Is option (b) rare in actual papers?

No. In fact, CBSE examiners deliberately include (b) to test whether students can distinguish correlation from causation. In recent papers, approximately 30-40% of A-R questions have (b) as the correct answer.

How do I practise A-R questions effectively?

Start with the 30 questions in this post. Then solve A-R questions from CBSE sample papers and previous years. For each question, write down WHY you chose your answer — this builds the analytical habit.

Can A-R questions come from Biomolecules or Polymers?

Yes, though less frequently. Biomolecules (reducing/non-reducing sugars, protein structure) and Polymers (addition vs condensation) occasionally appear in A-R format. The CBSE Class 12 sample paper practice sets include these.

Are the A-R options always the same four?

Yes. CBSE uses the same four-option format for assertion-reason across all subjects: (a) both true + correct explanation, (b) both true + not correct explanation, (c) A true R false, (d) A false R true.

CBSE Class 12 Chemistry Assertion-Reason Questions Chapter-Wise Practice 2027