An example of a physical change is

Analyze the question: We need to pick the option that is a physical change, meaning no new substance is formed. Only the state or appearance changes, and the change is usually reversible.

Concept:

• Physical change: Changes of state (melting, freezing, boiling, condensation), dissolving sugar or salt in water, and so on. No new substance forms.
• Chemical change: A new substance with different properties forms. Signs include gas evolution, colour change, heat release, and formation of a precipitate.

Step-by-step analysis:

• A. Slaking of lime: CaO + H₂O → Ca(OH)₂. Calcium hydroxide is a new substance, so this is a chemical change.
• B. Liquefaction of liquids: The wording is odd. Strictly, liquids are already liquid, so "liquefaction of gases" would be the usual phrase. Still, liquefaction is a change of state (gas to liquid by cooling or pressure), and no new substance forms. This is a physical change.
• C. Dissolution of limestone in acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Gas is given off and new substances form, so this is a chemical change.
• D. Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. Rust is a new compound, so this is a chemical change.

Answer: Options A, C and D are all chemical changes. Option B is the only physical change, so the answer is B.

Tip: Ask yourself, "Is a new substance formed?" If yes, it is chemical. If it is just a change of state or a mixing or dissolving, it is physical. Words like slaking, rusting, burning, fermenting, and reacting with acid usually signal chemical changes, while melting, freezing, boiling, condensing, liquefaction, and dissolving usually signal physical ones.