מעברי פאזות: התכה והקפאה

When heat is added to a solid, particles gain thermal energy and vibrate faster. They partially overcome intermolecular forces, move about, and rearrange. Eventually, the lattice collapses and the solid transitions to liquid. This process is called melting or fusion.

Melting requires only partial overcoming of intermolecular forces while molecules remain in close contact. Vaporization requires complete separation of molecules by breaking nearly all intermolecular forces. For example, melting ice requires 6.02 kilojoules per mole, while vaporizing water requires 40.8 kilojoules per mole.

The molar heat of fusion is the enthalpy change required to completely melt one mole of a solid at its melting point. Melting is endothermic, requiring energy input with a positive enthalpy value. For ice, the molar heat of fusion is 6.0 kilojoules per mole at 0 degrees Celsius.

Freezing is the reverse of melting, where liquid-phase molecules lose thermal energy and pack closely to re-establish intermolecular forces. Freezing is exothermic with negative enthalpy. Although the enthalpy value is negative, its magnitude equals the enthalpy of fusion for the same substance.

The melting point depends on the strength of attractive forces between particles in the crystal. Molecules with weak intermolecular forces form crystals with low melting points. Crystals with stronger attractive forces between particles melt at higher temperatures. Stronger intermolecular interactions result in higher melting points.

At the melting or freezing point, solid and liquid phases coexist in equilibrium. Melting and freezing occur at equal rates, so quantities of solid and liquid remain constant. In an insulated container, this equilibrium can persist for extended periods with no net phase change.

During melting, added heat energy overcomes intermolecular forces rather than increasing molecular motion. Temperature remains constant at the melting point until all solid has melted. Only after complete melting does continued heating increase the liquid's temperature. This plateau occurs because energy breaks bonds rather than accelerating particles.