The definitive answer is that a change in state, such as melting, is categorized as a physical change. Science divides all transformations into two main categories: physical changes, which alter form but not identity, and chemical changes, which result in the formation of entirely new substances. Melting, where a solid transitions into a liquid, leaves the substance’s molecular structure completely unaltered.
What Defines a Physical Change
A physical change involves an alteration to a substance’s physical properties, such as its shape, size, density, or state of matter, without changing its chemical composition. The core identity of the material remains the same throughout the transformation. For instance, when water freezes into ice, it is still composed of $\text{H}_2\text{O}$ molecules, only arranged differently. These changes are often easily reversible through physical means, a concept known as a phase transition. Common examples of physical changes are changes in state, which include melting (solid to liquid), boiling (liquid to gas), and freezing (liquid to solid).
The Molecular Mechanics of Melting
The process of melting occurs when a solid absorbs thermal energy, which translates into increased kinetic energy for its constituent particles. In a solid state, molecules or atoms are locked into fixed positions within a crystalline lattice structure. As the temperature reaches the specific melting point, the increased kinetic energy causes the particles to overcome the attractive forces holding them in their rigid positions. Once the particles gain enough energy to break free from the lattice, the solid transforms into a liquid, where the molecules are now able to slide past one another. This entire process only disrupts the weak intermolecular forces that hold separate molecules together, such as hydrogen bonds or van der Waals forces.
What Defines a Chemical Change
In sharp contrast to a physical change, a chemical change, also known as a chemical reaction, results in the formation of one or more entirely new substances with different chemical properties. This transformation involves the rearrangement of atoms as existing chemical bonds are broken and new bonds are formed. The starting materials, or reactants, are consumed to produce chemically distinct products. A chemical change is frequently accompanied by observable indicators, such as a permanent color change, the production of gas bubbles not caused by boiling, or the release of heat and light. For example, when wood burns, the cellulose and lignin (reactants) combine with oxygen to form ash, carbon dioxide, and water vapor (products). Similarly, the rusting of iron is a chemical reaction where iron metal reacts with oxygen and water to form iron oxide.