Assertion-Reason is the question type where students lose marks not because they don't know the chapter, but because they don't know the format. You get two statements — an Assertion (A) and a Reason (R) — and you have to judge each one separately, then judge whether they're actually connected. Here's how to get it right every time.
Every assertion-reason question in a CBSE paper offers the same four choices:
Most marks are lost between (A) and (B) — students correctly see that both statements are true and stop there, without checking whether the reason actually causes the assertion or is just another true fact sitting next to it.
Assertion: A balanced chemical equation must have the same number of atoms of each element on both sides.
Reason: This follows from the law of conservation of mass — mass can't be created or destroyed in a chemical reaction.
Answer: (A). Both statements are true, and the reason is exactly why we balance equations — the number of atoms has to stay constant because mass does.
Assertion: Burning magnesium in air is a chemical reaction.
Reason: The magnesium changes from a silvery metal to white magnesium oxide.
Answer: (B). Both statements are factually true — but the colour change is something you observe, not the reason it's a chemical reaction. The real reason is that a new substance with different properties is formed; colour change is just a clue, not the cause.
Assertion: HCF(12, 20) = 4.
Reason: HCF is found by multiplying the greatest power of each common prime factor.
Answer: (C). The assertion is correct — but the reason describes how to find the LCM, not the HCF. For HCF you take the smallest common power of each shared prime factor. This is the classic HCF/LCM mix-up examiners test for.
Assertion: The chemical formula of a compound can be changed while balancing a chemical equation, to make the atoms equal on both sides.
Reason: Changing a formula would alter the identity of the substance being represented.
Answer: (D). The assertion is false — you only ever adjust coefficients when balancing, never the formula itself. The reason is true on its own, which is exactly why the assertion is wrong.
Do it in that order. Reading them together first is what causes students to assume a connection that isn't there.
Assertion-Reason practice for every Class 10 chapter
Science, Social Science, Maths, English, Hindi — all NCERT-grounded
Open Class 10 Science →More on the same chapters: Class 10 Maths — Real Numbers, Class 10 Social Science — Resources and Development, or browse every Class 10 chapter. If you want the wider study method these questions fit into, read how to study any NCERT chapter in 30 minutes.