Matter exists most commonly as solids, liquids, and gases. A phase change describes the physical process where a substance transitions from one state to another without altering its chemical composition. These transitions are governed by the addition or removal of thermal energy, which influences the motion and arrangement of the particles. Fusion specifically involves the transition from the highly ordered solid state to the more fluid liquid state.
The Physical Process of Fusion
Fusion refers exclusively to the process of melting, where a solid substance becomes a liquid. This physical change is initiated by supplying thermal energy to the solid material. As energy is absorbed, the constituent particles gain kinetic energy, causing them to vibrate more rapidly within their fixed positions in the crystal lattice.
The increase in kinetic energy allows the particles to overcome the attractive forces, such as intermolecular forces, that held them rigidly in place. Once these forces are disrupted, the organized structure of the solid collapses into a less ordered state. The particles are then free to move and slide past one another, resulting in the characteristic flow properties of a liquid.
This process must be distinguished from nuclear fusion, which involves the joining of two light atomic nuclei to form a heavier nucleus, releasing immense amounts of energy, as occurs in the sun. Chemical fusion (melting) is a simple physical change affecting only the spacing and movement of molecules, requiring a small input of energy to break intermolecular bonds.
Energy Dynamics: The Latent Heat of Fusion
Fusion is an endothermic process, meaning the substance must continuously absorb energy from its surroundings to complete the phase change. The specific amount of energy required to convert a set quantity of a solid into a liquid at its melting point is termed the Latent Heat of Fusion, also known as the Enthalpy of Fusion. This value is a unique physical property for every pure substance.
While heat is continuously added, the temperature of the substance remains constant during the entire transition from solid to liquid. This occurs because the absorbed thermal energy is not used to increase the translational kinetic energy of the particles, which would register as a temperature rise. Instead, the input energy is directed toward breaking the intermolecular forces that maintain the solid structure.
This energy is “latent” or hidden because it does not cause a temperature change, overcoming the potential energy barrier of the solid state. Once all the solid has converted to liquid, further addition of thermal energy will begin to increase the liquid’s temperature.
Defining the Melting Point
The melting point is defined as the specific temperature at which the solid and liquid phases of a substance can coexist indefinitely in thermodynamic equilibrium. This temperature acts as a characteristic figure for a pure compound, serving as one of its defining physical properties.
At the melting point, the rate at which solid particles transition to liquid is exactly balanced by the rate at which liquid particles return to the solid state. The presence of impurities disrupts this equilibrium by interfering with the crystal lattice formation. This causes the observed melting point to be lowered and the transition to occur over a broader temperature range.
Pressure can also slightly influence the melting point. However, this effect is minor for solid-liquid transitions compared to the liquid-gas transition, making the melting point a reliable marker for material identification.