Which of the following describes the process of melting?
Melting describes a physical change in which a substance transitions from the solid state to the liquid state under a specified pressure (commonly \(1\ \text{atm}\)).
Core description
Melting is an endothermic phase transition: heat flows into the substance, allowing particles in the solid to gain enough energy to move past one another and form a liquid.
For a pure substance at constant pressure, the temperature remains essentially constant at the melting point while the phase change proceeds.
Multiple-choice form
- A solid absorbs heat and changes into a liquid as particle motion increases and the rigid structure breaks down.
- A liquid releases heat and changes into a solid as particles lock into a fixed arrangement.
- A liquid absorbs heat and changes into a gas as particles separate widely and fill the container.
- A gas releases heat and changes into a liquid as particles slow and pack closer together.
Option A matches melting: a solid-to-liquid change driven by heat absorption.
Energy and temperature during melting
Heat added to a solid can produce two different effects:
- Temperature change within a single phase (solid warming or liquid warming), commonly summarized by \(q = m \cdot c \cdot \Delta T\).
- Phase change at the melting point (solid and liquid coexisting), summarized by \(q = m \cdot \Delta H_{\text{fus}}\), where \(\Delta H_{\text{fus}}\) is the enthalpy (latent heat) of fusion.
During the melting interval for a pure substance at constant pressure, added energy goes primarily into weakening and rearranging intermolecular attractions (or, for ionic solids, disrupting the lattice enough to permit flow) rather than raising temperature.
Particle-level interpretation
Solids maintain shape because particles vibrate about fixed positions within an ordered or partially ordered structure. In a liquid, particles remain close but move past one another, enabling flow and allowing the liquid to adopt the container’s shape.
Melting corresponds to crossing an energetic threshold where the solid’s structure can no longer remain rigid under the thermal motion present at the melting point.
Common pitfalls
- Melting vs dissolving: melting forms a liquid of the same substance; dissolving forms a solution containing a solute and solvent.
- Temperature plateau: for a pure substance, temperature does not rise during the main melting interval while heat continues to enter.
- Pressure and purity: melting point shifts with pressure and impurities; the standard melting point assumes a stated pressure and a reasonably pure sample.
Phase-change comparison
| Phase change | Direction | Heat flow at constant pressure | Particle-level summary |
|---|---|---|---|
| Melting (fusion) | Solid → liquid | Absorbed (endothermic) | Particles gain mobility; rigid structure becomes able to flow. |
| Freezing | Liquid → solid | Released (exothermic) | Particles lose mobility; structure becomes rigid. |
| Vaporization | Liquid → gas | Absorbed (endothermic) | Particles separate widely; gas expands to fill volume. |
| Condensation | Gas → liquid | Released (exothermic) | Particles cluster; liquid forms with close particle spacing. |
Heat-flow statements assume constant pressure and the usual sign convention where heat absorbed by the system is positive.
Visualization: heating curve segment highlighting melting
Direct answer statement
Melting is the endothermic transition where a solid absorbs heat at its melting point and becomes a liquid as particles gain sufficient energy to partially overcome the forces that hold the solid structure together.