7.3 Phase Change and Specific Latent Heat

During melting or boiling, energy transfer can continue while temperature remains constant. This is not a contradiction. Temperature tracks average random kinetic energy, but during phase change much of the incoming energy is redirected into rearranging intermolecular structure, increasing potential-energy contribution rather than kinetic contribution.

At the melting point, added energy weakens and breaks enough solid-structure constraints to allow liquid motion. At the boiling point, added energy overcomes remaining intermolecular attraction strongly enough for particles to separate into vapor phase. In both plateaus, internal energy rises while temperature can stay fixed.

Specific latent heat of fusion $L_f$ applies to solid-liquid transitions. Specific latent heat of vaporization $L_v$ applies to liquid-gas transitions. For many substances $L_v$ is much larger than $L_f$, reflecting the greater intermolecular-separation work needed to reach vapor state.

In multi-step thermal problems, split the process into explicit segments: sensible heating within one phase ($Q = mc\Delta T$), then plateau segment(s) ($Q = mL$), then post-transition heating if needed. Summing these segments is safer than trying to force one equation through the entire path.

A heating curve is best read as an energy-allocation diagram. Sloped sections mean input energy is mostly increasing kinetic contribution and temperature rises. Flat sections mean energy is being diverted into phase reconfiguration at nearly constant temperature. Both represent real internal-energy increases, but through different microscopic channels.

This interpretation also explains why adding steam to cooler water can be so effective in heating: condensation releases a large latent-energy package before additional cooling of condensed water is even considered. Latent terms can dominate total thermal balance.

Interpretation guidance: move the energy slider slowly across each boundary marker. Notice that equal slider movement produces very different temperature responses depending on stage. This is exactly the practical meaning of latent heat: large energy transfer can occur with nearly zero temperature change.