How Microscopic Energy Changes Explain Heating and Phase Changes

Temperature, heat, and internal energy describe related but different ideas. A particle model helps separate them.

Internal energy has kinetic and potential contributions

For a thermodynamic system, internal energy includes microscopic kinetic energy from particle translation, rotation, and vibration, plus potential energy associated with interactions among particles. It does not include the kinetic energy of the whole container moving across a room or the gravitational potential energy of the entire sample as a single object.

Heating within one phase raises average kinetic energy

Temperature is linked to the average kinetic energy of particles. When a solid, liquid, or gas absorbs heat without reacting or changing phase, particle motion becomes more energetic and the temperature usually rises.

For a temperature interval that stays within one phase, a common macroscopic relation is

$$q=mc\Delta T,$$

where $m$ is mass and $c$ is specific heat capacity. This equation tracks the heat needed for the temperature change; it does not say that every joule of internal energy is translational kinetic energy.

A phase change can absorb energy at constant temperature

During melting or boiling at constant pressure, added energy weakens or overcomes intermolecular attractions and changes the arrangement of particles. The temperature can remain constant while the system's internal energy increases because the potential-energy contribution changes.

For example, ice and liquid water can coexist at the melting point. Heat added during melting converts solid water to liquid without raising the equilibrium temperature until the phase change is complete. After all the ice has melted, additional heat raises the liquid water's temperature.

This gives a useful reading rule for a heating curve:

• sloped segment: temperature and average kinetic energy change;
• flat phase-transition segment: temperature stays constant while intermolecular arrangement and potential energy change.

Heat is transfer, not stored material

Heat is energy transferred because of a temperature difference. Once transferred, that energy contributes to the system's internal energy or is used in work. A sample contains internal energy; it does not contain heat as a separate substance.