Which is not a reversible change?

A physical change alters only the state or form of a substance while its chemical composition remains the same, so it can usually be reversed simply by reversing the temperature or pressure condition (cooling reverses melting or vaporisation, and heating reverses condensation or freezing). A chemical change (a chemical reaction) produces new substances with a different composition, so the original substance generally cannot be recovered just by reversing the physical conditions -- this irreversibility is what marks it as a chemical change.

• Water changing to water vapour is evaporation, a change of state of the same substance (water); cooling reverses it back to liquid water through condensation, so this change is reversible.

• Melting of iron is a change of state of the same substance (solid iron to molten iron); cooling reverses it back to solid iron through solidification, so this change is reversible.

• Burning of kerosene is combustion, a chemical reaction that converts kerosene into carbon dioxide and water vapour, substances with a different composition; these products cannot be recombined back into kerosene, so this change is irreversible.

• Condensation of water vapour is a change of state of the same substance (water); heating reverses it back to vapour through evaporation, so this change is reversible.

Cross-check: three of the four processes are only a change of state of one substance (water/water vapour, and solid/molten iron), each freely reversible by adjusting temperature. Only burning kerosene changes the substance's chemical identity into carbon dioxide and water vapour, which cannot be converted back into kerosene by any physical means -- that irreversibility identifies burning of kerosene as the change that is not reversible.