Question

which of the following is an irreversible physical change?
a melting a piece of lead
b sharpening a pencil
c dissolving sugar into water
d boiling water

Explanation:

• Option a: Melting lead is reversible (can solidify again).
• Option b: Sharpening a pencil removes wood, and the removed wood can't be restored to the original pencil, so it's irreversible physical change.
• Option c: Dissolving sugar in water is reversible (sugar can be retrieved by evaporation).
• Option d: Boiling water is reversible (steam can condense back to water).

Answer:

b. sharpening a pencil