Phase changes solid -> liquid -> gas involve what signs for enthalpy and entropy changes?

Phase changes solid -> liquid -> gas involve what signs for enthalpy and entropy changes?

• A. +ΔH,+ΔS
• B. -ΔH,-ΔS
• C. +ΔH,-ΔS
• D. -ΔH,+ΔS

Answer: (A)

When a phase change happens from solid to liquid or from liquid to gas, energy must be put in to overcome the forces holding the molecules in place. That means the enthalpy change is positive because heat is absorbed during melting and vaporization. At the same time, each step increases disorder: a solid has a highly ordered structure, a liquid is less ordered, and a gas is highly disordered with many accessible configurations. This rise in randomness means the entropy increases, so the entropy change is positive for both melting and vaporization. So, both enthalpy and entropy increase for the solid → liquid → gas sequence.

When a phase change happens from solid to liquid or from liquid to gas, energy must be put in to overcome the forces holding the molecules in place. That means the enthalpy change is positive because heat is absorbed during melting and vaporization.

At the same time, each step increases disorder: a solid has a highly ordered structure, a liquid is less ordered, and a gas is highly disordered with many accessible configurations. This rise in randomness means the entropy increases, so the entropy change is positive for both melting and vaporization.

So, both enthalpy and entropy increase for the solid → liquid → gas sequence.