Which of the following is NOT a physical change?

Correct — A. Curdling is bacterial fermentation: lactose is converted to lactic acid, the milk protein casein coagulates, and no amount of cooling or stirring gives milk back from curd.

A new substance with new properties has formed and the change is irreversible, which makes curdling a chemical change.

The other three alter only state or appearance, with the composition untouched, so all three are physical. The stem wants the one that is NOT physical: option (a).

Why the others are wrong

• (b)
  Heating of iron rod to red hot — Heating an iron rod to red hot changes its temperature and its colour, not its composition — it is still iron, and it returns to grey as it cools. That is a physical change.
• (c)
  Evaporation of diesel — Evaporation of diesel is a liquid-to-vapour phase change. The molecules are unaltered, and cooling the vapour condenses the diesel back.
• (d)
  Sublimation of NH₄Cl — Sublimation is a solid passing straight to vapour without melting, and school chemistry classes it as a physical change. That leaves curdling as the one chemical change on offer.

The test is not reversibility on its own. It is whether a new substance with new properties has been formed.

A physical change alters state, shape, size or appearance while the chemical composition stays put, and it can usually be undone.

A chemical change rearranges bonds and produces something new, often with heat, gas, a smell or a permanent colour change alongside, and it does not undo by simple physical means.

Curdling, rusting, burning, cooking and digestion are the standard chemical examples. Melting, boiling, sublimation, dissolving sugar in water and cutting are the standard physical ones.

Key facts

• A chemical change produces a new substance and cannot be reversed by simple physical means.
• Curdling of milk is a fermentation in which lactose is converted to lactic acid and casein coagulates.
• Sublimation is the direct solid-to-vapour transition, and it is a physical change.
• Heating a metal until it glows changes its temperature and colour, not its chemical identity.

Common traps

• Passing over curdling as physical because it merely looks like thickening. The chemistry and the taste both change, and it cannot be undone.
• Calling red-hot iron a chemical change because it looks so transformed.
• Forgetting that a NOT question needs three correct classifications before the odd one can be picked.

No directly related past PYQ was found.