Melting, the process by which a solid transforms into a liquid, is a fundamental phase transition that requires an input of energy, typically supplied as heat. This energy must facilitate a complete physical change in the material’s structure. The incoming thermal energy is absorbed in two distinct ways: first, to raise the substance’s temperature, and then solely to restructure the material at the molecular level.
Energy Storage in the Solid State
Before a solid begins to melt, the added heat energy is stored in the material as increased molecular motion. In a solid, molecules or atoms vibrate rapidly around fixed positions within a highly ordered structure. These vibrations represent the internal kinetic energy of the particles.
As a solid is heated, the absorbed thermal energy increases the speed and amplitude of molecular oscillations. The resulting higher internal kinetic energy is registered externally as a measurable rise in the material’s temperature. This form of heat absorption, where energy leads to a temperature increase, is known as sensible heat.
The temperature of the solid continues to climb in proportion to the heat added until the material reaches its specific melting point. At this thermal threshold, the kinetic energy of the vibrating molecules becomes high enough to challenge the forces holding the rigid structure together. The temperature will not increase further until the structural change is complete, signaling the start of the phase transition.
Overcoming Molecular Forces
The transition from a solid to a liquid demands a physical restructuring of the material’s internal arrangement. Solids are held together by various attractive intermolecular forces, such as van der Waals forces, dipole-dipole interactions, or hydrogen bonds. These forces hold the particles in a fixed, repeating pattern, like a crystal lattice.
Once the solid reaches its melting point, further energy input is diverted away from increasing the kinetic energy of the molecules. Instead, the incoming heat works to overcome the attractive forces holding the structure rigid. This energy separates the molecules enough to allow them to slip past one another, breaking the fixed positions of the solid state.
The molecules gain potential energy as they move into the less-ordered liquid state. In the liquid phase, particles remain close together and experience attraction, but they are no longer locked into place and can move and flow freely. This structural breakdown is the physical mechanism of melting, which occurs uniformly throughout the material.
The Energy Cost of Melting
The energy required to convert a solid entirely into a liquid without changing its temperature is quantified by the Latent Heat of Fusion. This value represents the specific amount of heat energy needed to change a unit mass of a substance from solid to liquid at its melting temperature. For example, melting one kilogram of ice at 0°C requires the absorption of approximately 334 kilojoules of energy.
The phenomenon of the temperature remaining constant during the phase change is known as the temperature plateau. During this time, the solid and liquid phases coexist in equilibrium. The heat being continuously supplied is used exclusively to break the remaining intermolecular bonds to complete the structural transition.
This heat input increases the material’s potential energy, not its kinetic energy. Only once the material is completely liquid does the temperature begin to rise again. At that point, the liquid begins to absorb sensible heat, and the added energy once again increases the kinetic energy and temperature of the particles.