In molar heat of fusion, which state of matter is not involved?

The concept of molar heat of fusion specifically pertains to the phase transition between solid and liquid states. Molar heat of fusion is the amount of energy required to change one mole of a solid into a liquid at its melting point, without changing its temperature.

In this process, the solid gains energy, which allows its particles to overcome the intermolecular forces keeping them in a fixed position. This leads to a state change from solid to liquid. Since this transformation involves only solid and liquid states, the gaseous state does not come into play during this particular phase change. The presence of gas is irrelevant in the context of heat of fusion, as it concerns only the melting of solids into liquids.

Plasma, being another distinct state of matter characterized by ionized particles, is also not involved in the process of fusion. However, the focus here is particularly on the liquid and solid states. Thus, the correct identification of the absence of gas in the process of heat of fusion succinctly captures the nature of this thermodynamic concept.