Phase Changes

Phase transitions at constant temperature through latent heat. Theory, formulas, worked examples and interactive graphs.

Conceptual context

A phase change is a transition between phases of a substance: melting, solidification, vaporization, condensation, sublimation, or deposition. Its central feature is that exchanged energy does not change temperature while two phases coexist. That energy is the total latent heat $Q_{lat}$.

The central physical question is: how much energy is needed to change the state of a substance, and why does temperature not vary during the transition? The answer depends on three quantities: the mass $m$ that changes state, the specific latent heat $L$ of the material, and the transition temperature $T_{trans}$. This leaf is not only about warming or cooling; it is about deciding when heat changes the structure of matter.

🟢 Essential level

If ice at 0 °C is heated, its temperature does not immediately rise. The incoming heat is used to break the structure of the solid phase and convert ice mass into liquid water. As long as ice and water coexist, the system remains at $T_{trans}$.

The larger the mass $m$ changing state, the larger $Q_{lat}$ is. If the material has a large $L$, each kilogram requires much energy. This is why vaporizing water requires much more energy than melting the same mass of ice: vaporization separates molecules almost completely.

The operating idea is simple: first check whether two phases are present; then ask how much mass is changing state; finally decide whether energy remains to raise or lower temperature. On a phase-change plateau, more heat means more transformed mass, not more degrees Celsius.

An everyday example fixes the idea: a pot of boiling water can keep receiving fire for minutes without the thermometer moving from 100 °C, while the water level slowly drops from vaporization. All the extra energy goes into converting liquid water into steam, not into heating the water that remains.

🔵 Formal level

Basic formulas

The general relation of this leaf is:

Latent heat of a complete transition

The thermal condition during the plateau is:

Constant temperature during the transition

For melting or solidification, the material fusion value is used:

Latent heat of melting or solidification

For vaporization or condensation, the vaporization value is used:

Latent heat of vaporization or condensation

Real student questions

Why does temperature not change if I keep heating? Because energy is used to change phase, not to increase thermal agitation.

Can I always use the same $L$? No. Use the melting, vaporization, or sublimation value that matches the material and pressure of the problem.

What if there is not enough energy? The transition is partial and the system remains at $T_{trans}$ with two phases. In that case the natural unknown is often transformed mass, not final temperature.

Final synthesis

A phase change exchanges $Q_{lat}$ at constant temperature $T_{trans}$. The operating rule is simple: mass changing state $m$ times specific latent heat $L$. The real difficulty is deciding whether the transition is complete, selecting the correct value of $L$, and respecting that temperature remains fixed while phases coexist.