Final answer:
To demonstrate free energy of transfer between phases, the change in enthalpy during a phase change is observed since it involves the heat of fusion or heat of vaporization required to occur at constant temperature.
Step-by-step explanation:
To demonstrate free energy of transfer between phases, the key aspect to consider is a change in enthalpy (ΔH), which can be observed during a phase change. When a substance changes phase, for instance from solid to liquid (melting) or from liquid to gas (vaporization), the heat of fusion or the heat of vaporization is the amount of energy required per unit mass to complete the transition without a change in temperature.
During these phase changes, the temperature of the system does not change even though energy is being transferred. This energy transfer is required to break or form the bonds between molecules during a phase change. Therefore, option d) Change in enthalpy is correct because it relates to the heat required to cause a phase change at constant temperature. It is explained by the formula Q = m·ΔH, where Q is the heat absorbed or released, m is the mass of the sample, and ΔH is the heat of fusion or vaporization, depending on the phase change occurring.