Final answer:
The thermal energy within a substance can be described using units such as kilocalorie (kcal) and joule (J). Thermal energy is the average kinetic energy of the particles in a substance, and it increases with the velocity of the particles.
Step-by-step explanation:
The units that can be used to describe the thermal energy within a substance are kilocalorie (kcal) and joule (J).
Thermal energy is the average kinetic energy of the particles (molecules or atoms) in a substance. It increases with the velocity of the particles. The total internal energy of a system is the sum of the kinetic and potential energies of its atoms and molecules.