What is the melting temperature?

The melting temperature refers to the specific temperature at which a solid phase transitions into a liquid phase under equilibrium conditions. At this temperature, both the solid and liquid forms of a substance coexist, and the process can occur in either direction, allowing for the solid to melt into a liquid or the liquid to solidify back into a solid. This equilibrium state is essential, as it signifies that the two phases are stable at this particular temperature, reflecting the unique properties of the substance involved.

Understanding this concept is vital for comprehending phase changes in thermochemistry. The other options describe different physical changes that are not relevant to the definition of melting temperature. For example, liquefaction of a gas is indicative of the condensation process rather than melting, boiling relates specifically to the phase transition from liquid to gas, and the transition to plasma involves a significant increase in energy, leading to ionization, which is not related to melting temperature.