Melting

A solid can turn into a liquid.

This is called melting. It happens when things get warm. Heat can also make things melt. This change can happen from pressure too.

When things melt, they lose their shape. The tiny parts inside stop staying in order. They move around more freely. This makes the solid flow like a liquid.

Some things melt in a special way. They might stay part solid and part liquid. This is a state in between.

Melting is a very cool thing to see.

Melting is a way that a solid turns into a liquid. This is also called fusion.

Melting happens when the power inside a solid goes up. This usually happens when you add heat. It can also happen if you add pressure. When a solid reaches its melting point, its tiny parts change. In a solid, these parts stay in a neat order. During melting, that order breaks down. The parts move into a less ordered state. This makes the solid flow like a liquid.

Most liquids flow more easily as they get hotter. This is called reduced viscosity. Viscosity is how thick a liquid is. One exception is sulfur. Sulfur actually gets thicker as it gets hotter.

Sometimes, things do not melt at the right time. A liquid can stay liquid even below its freezing point. This is called supercooling. It happens if the liquid stays very still. There is no shake or vibration to start the change. Even solids can have a thin liquid film on their surface. This can help things like snowflakes grow.

Melting is a very important thing that happens in our world. Scientists call this process fusion. It is when a solid turns into a liquid.

This change happens because the internal energy of the solid goes up. Usually, this happens when you add heat. You can also cause melting by adding pressure. Every material has its own melting point. This is the specific temperature where the change starts. When this happens, the solid becomes a liquid.

How does it work on the inside? In a solid, the tiny molecules or ions stay in a neat order. As the temperature rises, this order begins to break down. The parts move into a less ordered state. This change is called a first-order phase transition. This means the energy changes in a specific way. Scientists measure this using enthalpy and entropy. At the melting point, the Gibbs free energy of the liquid becomes lower than the solid. This makes the liquid the more stable state for that material.

There are different ways to look at why solids break apart. One idea is called the Lindemann criterion. It says melting happens because of vibrations. The atoms in a crystal shake more and more. Eventually, the shaking becomes too much to stay in place. Another idea is the Born criterion. This says a crystal melts when it loses its rigidity. It can no longer hold its shape against a load. This is like a building becoming too weak to stand.

Some things act in very strange ways. Most liquids get thinner and flow easier as they get hotter. This is called reduced viscosity. However, elemental sulfur is different. Between 130 °C and 190 °C, sulfur actually gets thicker. There are also special cases with helium. Helium-3 and Helium-4 are very unusual. Below certain tiny temperatures, you must actually remove heat to melt them. This is the opposite of how most things work.

Sometimes, the melting point does not happen right away. A liquid can stay liquid even below its freezing point. This is called supercooling. Pure water on clean glass can do this. Even water in fine emulsions can stay liquid down to −38 °C. This happens if the liquid stays very still. Without a shake or vibration, the change might not start. You might also see a thin liquid film on a solid surface. This can help snowflakes grow or help glaciers move.

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.