Melting

Melting is an endothermic phase change where a solid turns into a liquid at a constant temperature called the melting point ¹.

Kinetic Particle Mechanism

Under IGCSE Kinetic Particle Theory ¹:

• Solid: Particles vibrate about fixed positions in a regular lattice.
• Heating: Thermal energy increases particle kinetic energy, making particles vibrate faster.
• Melting Point: Energy overcomes lattice forces, allowing particles to slide past each other as a liquid.

[Solid Lattice]      Heat Energy      [Liquid Flow]
 (o)(o)(o)(o)     ---------------->    (o)  (o) (o)
 (o)(o)(o)(o)       Melting Point        (o)  (o)

Heating Curve & Latent Heat

Temperature remains constant at the melting plateau until all solid melts ²:

Temp ^
     |            / Liquid
M.P. +===========+ (Melting Plateau)
     |          /
     +---------> Heat Added

Absorbed energy supplies the specific latent heat of fusion ($\Delta E = m \cdot l_f$) to overcome intermolecular forces without raising temperature ($\Delta T = 0$) ².

Purity Criterion

Pure substances melt sharply at a precise temperature (e.g. ice at $0\text{ }^\circ\text{C}$). Impurities depress the melting point and cause melting over a temperature range ¹.

Related

• Endothermic Change - Process absorbing thermal energy from the surroundings.
• Intermolecular Force - Attractive forces overcome by heat absorption.
• Boiling - Bulk liquid-to-gas transition at the boiling point.
• Evaporation - Surface liquid-to-gas transition below the boiling point.
• Freezing - Reverse exothermic transition.
• Lattice - Solid crystal structure broken down during melting.
• Pure Substance - Uniform chemical species displaying sharp melting plateaus.
• State of Matter - Classical states of matter.
• Chemistry - Core science area.