Why does melting require energy even though temperature stays constant?
Melting requires energy because the process involves breaking the intermolecular forces that hold particles in a solid structure — and overcoming these forces demands energy input. Even though temperature does not rise during melting, energy is still being absorbed continuously. Instead of increasing particle speed (which would raise temperature), the energy goes into loosening the orderly arrangement of particles so they can move freely as a liquid.
In a solid, particles are locked into a fixed lattice due to strong intermolecular attractions. When heat is added, the particles vibrate more vigorously. Once they reach the melting point, the added energy no longer increases kinetic energy but instead begins to disrupt the structure. The energy needed to separate particles without increasing their speed is known as the latent heat of fusion.
During melting, the temperature stays constant because all the absorbed energy is used to break forces, not to speed up particles. Temperature is a measure of average kinetic energy. Since kinetic energy does not increase during the phase change, temperature remains steady until the entire solid has become liquid. Only after the solid fully melts does additional energy resume raising the temperature of the liquid.
This energy requirement explains why melting is an endothermic process. The system must absorb heat to weaken intermolecular forces and increase particle spacing. Substances with stronger intermolecular forces require more energy to melt, leading to higher melting points and larger latent heats.
The constant-temperature plateau seen during melting demonstrates that phase changes involve structural transformations, not kinetic ones. It’s only after the change is complete that particles can again convert added energy into motion, raising temperature.
In short, melting requires energy because it breaks the forces tying particles in a rigid structure, and while this happens, temperature remains constant because the energy is used for structural change, not particle motion.
Frequently Asked Questions
Why doesn’t temperature rise during melting?
Because all added energy goes into breaking intermolecular forces, not increasing particle motion.
Do all solids require the same amount of energy to melt?
No. Substances with stronger forces—like ionic solids—require much more energy than molecular solids.
What happens if heating stops during melting?
Melting pauses. The substance stays at the melting point until enough energy is absorbed to complete the phase change.
RevisionDojo Call to Action
Want phase changes and energy concepts to finally make sense? RevisionDojo breaks down chemistry at the particle level so you can understand every idea with clarity and confidence.
