Thermal physics

Melting, vaporization, condensation

During a phase change, a substance absorbs or releases latent heat; a pure substance's temperature usually stays constant during the transition at fixed pressure.

Ice can melt into liquid water; liquid water vaporizes into gas; vapor may condense back to liquid. The transferred energy changes molecular arrangement and spacing, not necessarily temperature.

Heating curve

When ice is heated near 1 atm, its temperature rises to 0 °C and stays nearly constant while melting. Liquid water then warms to its boiling point; during vaporization heat continues to enter while temperature remains nearly constant.

Quick check

While ice melts at ordinary pressure, the mixture stays near 0 °C even as energy continues to enter. That energy rearranges molecules and loosens intermolecular attractions; only after all the ice has melted does additional energy raise the liquid water's temperature. Similarly, during boiling, latent heat of vaporization enables molecules to leave the liquid phase. For mass $m$, the energy for melting or vaporization is $Q=mL$, where $L$ depends on the material and process. Condensation releases the corresponding latent heat to the surroundings. On a temperature-versus-time graph, a plateau can mark a phase change for a pure substance at constant pressure. Do not apply $Q=mc\Delta T$ to the phase-change interval itself. For a process with several stages, calculate sensible heating and latent heat separately, then add the energies with consistent signs.

For 0.10 kg of ice at 0 °C with $L_f=334$ kJ/kg, the heat needed to melt it completely is $Q=mL_f=33.4$ kJ. If heating continues afterward, calculate the energy to raise the water's temperature separately. In real situations, pressure can change the boiling point, while impurities can alter the melting point.