Understanding the Constant Temperature During Phase Changes: A Comprehensive Guide

Why Does the Temperature Remain Constant During a Phase Change?

The temperature remains constant during a phase change because the energy added or removed from the substance is utilized to change its state rather than increasing its temperature. This phenomenon is known as latent heat. In this article, we will delve deeper into energy transfer, latent heat, molecular behavior, and thermodynamic equilibrium to explain this concept.

Energy Transfer

When a substance undergoes a phase change such as melting, freezing, boiling, or condensing, energy is transferred to or from the substance. For instance, when ice melts to water, heat energy is added. This transfer of energy is crucial in understanding why the temperature remains constant during a phase change.

Latent Heat

Latent Heat is the energy absorbed or released during a phase change. The latent heat of fusion is the energy required to change the state of a substance from a solid to a liquid at its melting point, while the latent heat of vaporization is the energy required to change a substance from a liquid to a gas at its boiling point. These energies break or form intermolecular bonds, not increasing the kinetic energy of the molecules, which is what temperature measures.

Molecular Behavior

During a phase change, the temperature remains constant because the energy is used to reorganize the molecular structure. For example, when ice melts, energy is used to overcome the forces holding the solid structure together, allowing the molecules to move freely as a liquid. This reorganization is a key reason why the temperature does not change during a phase change.

Equilibrium

A system remains in thermal equilibrium during the phase transition. This means that while energy is being added or removed, the temperature does not increase or decrease until the phase change is complete. The system's energy is being used to change its state rather than increasing its temperature.

In summary, the constancy of temperature during a phase change is due to the energy being utilized for changing the state of the substance instead of increasing its thermal energy.

Regions with Multiple Densities at the Same Temperature and Pressure

There are regions in a substance's PVT phase diagram where phases of more than one density exist at the same temperature and pressure. In these regions, adding or subtracting heat while holding either temperature (T) or pressure (P) constant can change the mass fractions in the two phases, thus changing the volume. However, both temperature and pressure remain constant until a boundary is reached, and only one phase remains.

The phase transition is achieved through thermodynamic equilibrium. Molecules receive new kinetic energy during this transition, but this added energy is balanced by the thermodynamic equilibrium effect. Once the old phase has completely disappeared, the thermodynamic equilibrium will be lost.