Which one of the following is an example of a physical change?

A physical change is a process where the physical properties of a substance (such as its state, shape, or size) change, but its chemical identity remains the same. No new substances are formed, and the change is often reversible.

• Melting of ice: This is a physical change because it is merely a change of state from solid H₂O to liquid H₂O. The chemical composition remains unchanged, and it can be reversed by freezing.
• Ripening of banana: This is a chemical change involving enzymatic reactions that alter the chemical composition (converting starch to sugar).
• Souring of milk: This is a chemical change where bacteria convert lactose into lactic acid, creating a new substance.
• Passing CO₂ gas into water: This is a chemical change because CO₂ reacts with water to form a new chemical compound, carbonic acid (H₂CO₃).