Melting

Melting, also known as fusion, is a fundamental physical process. It is a phase transition where a substance changes from a solid to a liquid. This transition is vital for understanding how matter behaves under different conditions. Melting occurs when the internal energy of a solid increases. This increase usually happens through the application of heat or pressure. As energy rises, the substance reaches its specific melting point.

To understand the mechanism, we must look at the molecular level. In a solid, ions or molecules are arranged in an ordered structure. When the melting point is reached, this internal ordering breaks down. The particles move into a much less ordered state. This shift is classified as a first-order phase transition. From a thermodynamic view, the Gibbs free energy of the liquid becomes lower than the solid. At this exact point, the change in Gibbs free energy is zero. However, there are non-zero changes in enthalpy and entropy. Enthalpy of fusion and entropy of fusion describe these energy changes.

Scientists use different theoretical criteria to analyze how melting occurs. The Lindemann criterion suggests that melting is caused by vibrational instability. In a crystal, atoms vibrate in place. When the average amplitude of these thermal vibrations becomes too high, the structure fails. This happens when the displacement of atoms reaches a certain ratio compared to interatomic distances. The Lindemann parameter is typically between 0.20 and 0.25. Another theory is the Born criterion. This focuses on a rigidity catastrophe. It occurs when the elastic shear modulus vanishes. At this point, the crystal lacks the rigidity to withstand a mechanical load and becomes liquid.

Most substances follow predictable patterns during this process. As temperature increases, molten substances generally show reduced viscosity. Viscosity is a measure of how much a fluid resists flowing. Most liquids become thinner and flow more easily as they get hotter. However, elemental sulfur is a notable exception. Between 130 °C and 190 °C, its viscosity actually increases due to polymerization. Some organic compounds also undergo melting through mesophases. These are unique states that exist with partial order between a solid and a liquid.

There are also rare exceptions to the standard rules of thermodynamics. Low-temperature helium behaves very differently than other elements. Helium-3 and Helium-4 are the only known exceptions to general rules. Below 0.3 K, Helium-3 has a negative enthalpy of fusion. Below 0.8 K, Helium-4 also shows a slightly negative enthalpy of fusion. This means that at certain constant pressures, you must actually remove heat to melt these substances. This is the opposite of how most materials behave when they undergo fusion.

Under specific conditions, substances can bypass their expected melting points. This phenomenon is known as supercooling. A liquid can remain in a metastable state even below its freezing point. For example, pure water on very clean glass can supercool by several degrees. Fine emulsions of pure water have been cooled to −38 °C without forming ice. This happens because nucleation, the start of crystal formation, requires a trigger. Physical vibrations or fluctuations in the material can trigger this change. Without such a trigger, the liquid stays liquid until it suddenly crystallizes.

Melting also relates to the formation of different types of matter. Glasses are amorphous solids rather than crystalline ones. They are often made when molten material cools very rapidly. This rapid cooling happens below the glass transition temperature. The material does not have enough time to form a regular crystal lattice. We can also observe quasi-liquid films on crystalline surfaces even below the melting point. The thickness of these films depends on the temperature. This pre-melting effect influences how snowflakes grow and how glaciers move.

Finally, melting can occur through non-traditional methods. In ultrashort pulse physics, a process called nonthermal melting can happen. This does not happen because of increased atomic kinetic energy. Instead, it occurs because of changes in the interatomic potential. This is caused by the excitation of electrons using a femtosecond laser. Since electrons act like a glue holding atoms together, changing them can break atomic bonds. In the field of genetics, melting refers to something different. It describes the process of separating double-stranded DNA into single strands using heat or chemicals.