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Assertion-reason questions look like ordinary one-mark questions, but students drop marks on them even when they know the chapter. The trouble is usually method, not knowledge: they judge the two statements together, or assume the reason must explain the assertion because it sounds related. This post gives a four-step method, works it through real questions from CBSE's 2026-27 sample papers, and points out a trap in the option lists.
Where do assertion-reason questions appear?
In CBSE Class 12 science, they sit in Section A. The Physics sample paper for 2026-27 says Section A has sixteen one-mark questions, "twelve MCQ and four Assertion-Reasoning based". The Biology and Chemistry sample papers also use questions 13 to 16 for this format. Each paper carries a note that there is no change in the question paper design for 2026-27. Check the sample papers on cbseacademic.nic.in for your own year and subject.
Each question gives two statements, an Assertion (A) and a Reason (R), and four options built from three questions: Is A true? Is R true? Does R explain A?
What is the four-step method?
Judge A on its own. Cover R with your hand. Is the assertion true as written, every word of it?
Judge R on its own. Now cover A. Is the reason true as a general statement?
Test the link, only if both are true. Read them as one sentence: "A, because R." Does R give the actual cause or principle behind A, or just a true fact on the same topic?
Match to the options printed in this paper. Do not answer from memory of what "D" usually means.
Step 3 is where most marks go. Two true statements about the same chapter are not automatically cause and effect.
Worked example 1: a numerical assertion (physics)
From the physics sample paper, question 15. In Young's double slit experiment, the slits are 0.25 mm apart, the screen is 1.0 m away, and the light has wavelength 500 nm.
A: The fringe width observed on the screen will be 2.0 mm.
R: The fringe width in YDSE is directly proportional to the slit separation and inversely proportional to the wavelength of light.
Step 1. Fringe width β = λD/d = (500 × 10⁻⁹ m)(1.0 m) ÷ (0.25 × 10⁻³ m) = 2.0 × 10⁻³ m = 2.0 mm. A is true.
Step 2. The formula β = λD/d shows β is proportional to wavelength and inversely proportional to slit separation. R states the reverse, so R is false.
Step 3. Not needed. Answer: C, A is true but R is false. The physics marking scheme gives C and explains that fringe width is inversely proportional to slit separation and directly proportional to wavelength.
Notice that the numbers alone settle A. Many students who see a correct number assume the whole question is "A".
Worked example 2: two true statements that are not linked (physics)
Question 13 of the same paper.
A: In Rutherford's scattering experiment, most of the alpha particles passed through the gold foil undeflected.
R: The electrons in the atom are very light and do not affect the path of the alpha particles significantly.
Both are true. But read "A, because R": most alpha particles pass undeflected because most of the atom is empty space and the nucleus is tiny. The lightness of electrons is true and relevant to scattering, but it is not the reason most particles go straight through. Answer: B. The marking scheme says exactly this.
Worked example 3: the option trap (biology)
Question 14 of the biology sample paper.
A: There is no variation in the percentage recombination due to loosely or tightly linked genes.
R: Linked genes result in occurrence of higher proportion of parental gene combinations.
Step 1. Tightly linked genes show low recombination and loosely linked genes show higher recombination, so A is false. Step 2. R is a correct statement about linkage. Step 4. In the biology option list, D is "A is False but R is true". Answer: D, as the biology marking scheme confirms. A student who learned "D means both false" from the physics paper would look for a different option and get confused.
Worked example 4: when R does explain A (chemistry)
The chemistry sample paper's question 15 asserts that ethers mix with water like alcohols of similar molecular mass, with the reason that the oxygen of an ether can form hydrogen bonds with water. Both are true, and the hydrogen bonding is the cause of the miscibility. The chemistry marking scheme gives A.
Which mistakes cost the most?
Judging both statements at once. One false word in A makes A false, however true R is.
Assuming a related reason is the explanation. Ask whether R is the cause, or just a true fact nearby.
Answering from habit. Option letters change between papers. Read them each time.
Skipping the arithmetic. If A contains a number, compute it. It takes thirty seconds.
A practice routine for the next two weeks
Collect 40 items from the current sample papers for your subjects and your textbook exercises.
Do steps 1 and 2 only for ten items a day, writing T or F for each statement with a one-line reason.
Add step 3 from day four: for every pair of true statements, write the real cause of A in your own words. A large review by Dunlosky and colleagues (2013) rated this kind of "why" questioning, called elaborative interrogation, as moderate utility, with practice testing rated high.
Write your own assertion-reason pair from each chapter you revise, including one where both are true but unlinked.
For the other question formats in CBSE papers, see our guide to competency-based questions.
If you often cannot tell whether a reason is the real cause, the gap is usually in the concept itself. The Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can check the principle before you judge the link.
Tonight, open your subject's 2026-27 sample paper, solve questions 13 to 16 with the four steps, and check them against the marking scheme.
