Science — CBSE Class 10 board question

(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

Decomposition reactions generally require energy input, making them endothermic. Decomposition of organic matter into compost is indeed exothermic, but this does not explain why decomposition reactions are endothermic.

• A is true: Most decomposition reactions (thermal, electrolytic, photolytic) require energy — they are endothermic.
• R is true: Composting (biological decomposition) releases heat — it is exothermic.
• R does NOT explain A: In fact, composting is an exception to the general rule stated in A. A reason that contradicts or is unrelated to the assertion cannot be its correct explanation, so option (B) is correct, not (A).
• Key examiner tip: Always check whether R logically supports and explains A, not merely whether both statements are independently true.