Which of the following is NOT a physical change?

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Accepted Answer

Step-by-step analysis:

1. Definition: A physical change alters the physical state or appearance of a substance without forming new substances; a chemical change produces new substances with different chemical composition.

2. Evaluate each option:
   - (1) Heating an iron rod to red hot: only temperature and color change; composition of iron remains the same ⇒ physical change.
   - (2) Curdling of milk: proteins (casein) denature and coagulate producing curd and whey — new substances and irreversible chemical processes (often enzymatic or acidic) ⇒ chemical change.
   - (3) Evaporation of diesel: liquid to vapor phase change with no chemical change ⇒ physical change.
   - (4) Sublimation of $NH_4Cl$: solid sublimes to gas and can re-deposit as solid; no new substance formed ⇒ physical change.

Conclusion: Only option (2) is NOT a physical change — it is a chemical change.

FINAL_ANSWER: b

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