Milk turns into curd, and curd does not turn back into milk

Why

Gaurav's conclusion actually lands correctly — curd-forming genuinely IS a chemical change, so full marks there. But look closely at the rule he used: 'physical changes can always be reversed.' That rule itself isn't quite right, even though it happened to point him the correct way this time. Some physical changes are easy to reverse, like melting wax, but others are physical changes that are very hard or messy to reverse in practice, like shredding paper into confetti — still the same paper chemically, but nobody's un-shredding it by hand. Curd-forming IS chemical, and for its own real reason: bacteria in the milk change milk's proteins and sugars into new substances (lactic acid, among others) that behave differently — thicker, sourer — and that transformation of the milk's actual chemistry, not merely its reversibility, is what makes it chemical. Test: try to identify what NEW substance appeared (curd's sourness, from lactic acid) rather than just asking whether you can undo it. Gaurav reached the right answer, but a rule that only sometimes works isn't the real reason — chemical change means a new substance formed, and irreversibility is a common clue, not the actual definition.