According to the melting process, when does a solid transition to a liquid?

The transition of a solid to a liquid, known as melting, occurs when the vapor pressure of the solid equals that of the liquid. At this point, the two phases are in equilibrium, meaning that the molecules of the solid have sufficient energy to overcome the intermolecular forces holding them in the solid state. When this equilibrium is achieved, melting can occur.

The concept of vapor pressure is crucial here; it reflects the tendency of particles to escape from the solid phase into the gas phase. As the temperature increases, the kinetic energy of the molecules also increases, making it possible for the solid to transition into a liquid. Therefore, when the solid's vapor pressure reaches a critical point where it is equal to that of its liquid phase, the solid begins to melt.

This understanding of phase changes is fundamental in thermodynamics and directly relates to concepts such as heating curves and phase diagrams, which describe how substances behave under varying conditions of temperature and pressure.