What is the process of heat transfer from molecule to molecule within a substance called?

The process of heat transfer from molecule to molecule within a substance is called conduction. This mode of heat transfer occurs when kinetic energy is transferred between neighboring molecules as they collide or interact. In solids, where the molecules are closely packed together, conduction is particularly effective because the energy can easily pass through the material by direct contact of particles. The temperature difference within the material drives the energy transfer from the warmer molecules to the cooler ones until thermal equilibrium is reached.

For instance, if one end of a metal rod is heated, the molecules at that end gain energy and vibrate more vigorously. They then collide with adjacent molecules, transferring some of that energy along the length of the rod. This rapid transfer results in the entire rod eventually becoming warmer, showcasing the principle of conduction in action.

In contrast, convection refers to the transfer of heat through fluids (liquids and gases) by the movement of the fluid itself, and radiation is the transfer of heat in the form of electromagnetic waves. Thermal expansion, while related to temperature changes, pertains to the increase in volume of a substance as it is heated, rather than a mechanism of heat transfer. Understanding these distinctions helps clarify why conduction is the correct answer to this question.