Which term describes the additional thermal energy needed to convert a solid to a liquid at its melting point?

The term that describes the additional thermal energy needed to convert a solid to a liquid at its melting point is the heat of fusion. This process involves the absorption of energy by the solid as it changes its phase to a liquid. At the melting point, the substance absorbs this extra energy without a rise in temperature, allowing the molecules to overcome the attractive forces holding them in the solid structure.

Understanding the specific phase changes is vital in thermodynamics, where heat of fusion specifically refers to the transition from solid to liquid. Other concepts, such as heat of vaporization, pertain to the transition from liquid to gas, and thus do not apply to the melting process. Similarly, thermal expansion is a different phenomenon that describes the increase in volume of a substance due to heating, while specific heat refers to the amount of energy required to raise the temperature of a unit mass of a substance by one degree Celsius. Hence, the definition and context of heat of fusion make it the appropriate answer.