The thermal energy resulting from the motion of atoms, molecules, or ions is called:

• A. Enthalpy (H)
• B. Internal energy (U)
• C. Work (w)
• D. Heat (q)

Answer: (D)

Heat is the energy that flows between objects due to a difference in temperature, and it is intimately linked to the microscopic motion of particles. The random motion of atoms, molecules, or ions contributes to this thermal energy, and when that energy moves from one object to another, we call it heat. Enthalpy describes the heat content of a system at constant pressure, a state property, not the energy in transit. Internal energy is the total microscopic energy stored within the system, including kinetic and potential energy of all particles, not the energy transferred. Work is energy transfer due to macroscopic motion against a force. So the energy described here—the energy that moves as a result of particle motion and temperature differences—is best identified as heat.