Phase Transition at Boiling Point

Steps:

• At the boiling point, heat added overcomes intermolecular forces without changing temperature.
• Temperature remains constant, so average kinetic energy of molecules stays the same.
• Molecules transition from close-packed liquid state to dispersed gas state.
• This dispersion results in greater distance between molecules.

Why C is correct:

• Molecular spacing increases because gases have lower density than liquids, as defined by the ideal gas law (PV = nRT), where volume expands for the same number of molecules at constant T and P.

Why the others are wrong:

• A: Average kinetic energy depends on temperature, which is constant during boiling.
• B: Average size of molecules is fixed by their chemical structure and does not change.
• D: Total number of molecules is conserved in a closed system during phase change.

Final answer: C