What happens to particles when a solid melts?

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.

Why does melting require less energy than vaporization?

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.

What is the molar heat of fusion?

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.

How does freezing differ from melting in terms of energy?

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.

What determines the melting point of a crystalline solid?

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.

What happens when a solid-liquid mixture is in equilibrium?

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.

Why does temperature remain constant during melting despite continuous heat input?

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.

11.10

Phase Transitions: Melting and Freezing

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The particles of a solid tightly pack together through attractive forces and vibrate at fixed positions without disrupting the lattice.

The addition of heat causes the particles’ thermal energy to rise, and they vibrate faster. The particles move about and rearrange by partially overcoming the intermolecular forces. Subsequently, the lattice collapses, and the solid melts.

This transition from solid to liquid is called melting or fusion, and the temperature at which it occurs is called the melting point or the fusion point.

The change in enthalpy that is required to completely melt 1 mole of a solid at its melting point is called its molar heat of fusion or its molar enthalpy of fusion. Since melting nearly always requires energy, it is an endothermic process with a positive enthalpy value — with a few exceptions.

For example, when a mole of ice absorbs 6.02 kilojoules of heat energy from its surroundings, its temperature increases. When the temperature hits 0 °C, it begins to melt.

For any substance, the heat of fusion is lower than the heat of vaporization. For instance, while melting a mole of ice requires merely 6.02 kilojoules of energy, vaporizing a mole of water requires 40.8 kilojoules of energy.

This is because vaporization involves the complete separation of molecules by breaking free from nearly all intermolecular forces. In comparison, melting involves only partially overcoming the attractive forces while the molecules continue to stay in close contact.

The reverse of fusion, that is, the transition from liquid to solid, is called freezing or solidification. When liquid-phase molecules lose energy, their thermal motion decreases and the molecules pack close enough to re-establish the intermolecular forces.

Eventually, the liquid converts into its solid form. Freezing is an exothermic process, and its enthalpy value is negative — with a few exceptions.

Substances typically freeze at about the same temperature at which they melt. Although the enthalpy of freezing is negative, its magnitude is the same as the enthalpy of fusion.

When a substance is held at its melting point or freezing point, the solid and liquid phases coexist.

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Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy…