When you heat a piece of iron from 200 degree Kelvin to 400
degree Kelvin, then the kinetic energy of the molecules increases and so they
start travelling faster at a random motion. Whenever the temperature of any
object increases the atoms and molecules present within the object start moving
faster as they are boosted by additional kinetic energy. Under normal
circumstances the particles of the iron are held together in an orderly fashion
and with increased heat, this orderliness starts dissipating as the particles
gain in energy and start moving faster randomly.
