Answer:
Molecules speed up when they get energy from other molecules and slow down when they give energy to other molecules.
Explanation:
We know from the first law of thermodynamics that energy can not be created nor destroyed.
However, according to the kinetic molecular theory, molecules collide frequently with each other. All molecules in a substance do not possess equal energy hence some molecules are more energetic than others.
When a more energetic molecule collides with a less energetic molecule, the more energetic molecule transfers energy to the less energetic molecule. The less energetic molecule speeds up while the molecule that collided with it slows down due to energy transfer from one molecule to another.
