What happens to the molecules inside a solid when heat energy is added?

• A. They dissolve into separate molecules.
• B. The molecules move faster.
• C. They stop moving.
• D. They change color.

Answer: (B)

Adding heat energy raises the average kinetic energy of the molecules, so in a solid they vibrate more vigorously and, as temperature rises, move a bit faster within the lattice. This increased motion is the main effect of heating a solid. They don’t stop moving, they don’t dissolve into separate molecules, and color changes aren’t a general result of heating—those would require different processes.