Question

what do u mean by " all collisions between atoms and molecules are perfectly elastic." ?

Answer 1

All collisions between atoms and molecules are perfectly elastic, this means that there is no energy is gained or lost during the collision between the atoms and molecules.

As an elastic collision occurs between two bodies in which the total kinetic energy of the two bodies after the encounter is equal to their total kinetic energy before the encounter. Elastic collisions occur only if there is no net conversion of kinetic energy into other forms.