Concept:A physical change affects the physical state or form of a substance without forming a new chemical substance.
Explanation:Liquefaction involves a change of state, such as a gas turning into a liquid, so it is a physical change.
During liquefaction, the molecules remain unchanged; only their spacing and kinetic energy change.
For example, water vapour and liquid water are both
H2O.
No new compound is formed, so no chemical reaction occurs.
Slaking of lime is chemical because
CaO+H2O→Ca(OH)2.
Dissolving limestone in acid gives new products:
CaCO3+2HCl→CaCl2+H2O+CO2.
Rusting of iron also forms a new substance, hydrated iron(III) oxide,
Fe2O3⋅xH2O.
Therefore, among the given options, only liquefaction is a physical change.
Answer:B. liquefaction of liquids