Melting is a fundamental physical process where matter changes its physical form. This transition occurs when the internal energy of a substance is increased, typically by applying heat, causing a change in its state. Understanding this change is key to grasping how matter behaves when subjected to energy input.
Defining the Transformation: Solid to Liquid
Melting is the phase transition from a solid state to a liquid state. This change involves a substance moving from a rigid, fixed structure to a fluid one. The process is sometimes referred to as fusion.
A familiar example is the transformation of ice into water when it reaches its melting point. Other common examples include solid butter turning into liquid when heated, or metals being melted down for casting. The change is always physical, meaning the substance remains chemically the same, only its state has been altered.
The Role of Heat and Melting Point
The transition from a solid to a liquid requires an input of thermal energy. As a solid is heated, its temperature rises until it reaches the melting point. This is the specific temperature at which the solid and liquid phases can exist in equilibrium, and it is a characteristic property for any pure substance.
Once this temperature is reached, the continued addition of heat energy does not increase the temperature further. This energy is entirely used to facilitate the change of state, known as the latent heat of fusion. Latent heat is the energy absorbed to convert a specific quantity from a solid to a liquid at a constant temperature. For instance, one kilogram of ice at 0°C must absorb 333.55 kilojoules to melt completely into liquid water at the same temperature. This absorption without a temperature increase makes melting an isothermal process until the transition is complete.
What Happens at the Molecular Level
The heat supplied increases the kinetic energy of the particles within the solid. In a solid state, the atoms or molecules are packed tightly in an orderly, fixed arrangement, only able to vibrate in place. As the temperature approaches the melting point, the particles vibrate more vigorously.
When the kinetic energy becomes high enough to overcome the attractive intermolecular forces holding the structure together, the rigid order breaks down. The particles gain enough freedom to move past one another, remaining in close contact. This ability to slide and flow is the defining feature of the liquid state. The latent heat of fusion provides the energy needed to break these forces and transition the substance from its highly ordered solid structure to the less ordered, fluid arrangement of a liquid.