Question

What happens to the motion of molecules in a substance is cooled?

Answer 1

The motion becomes smaller. Lets tale the example of water. At gaseous state, the particles moves rapidly in all directions. As it is cooled, it becomes liquid and the motion becomes smaller because the particles are now closer to each other as the attraction forces becomes stronger. As it is cooled further, it becomes solid. At solid state, the particles now vibrate in fixed position. Apply this concept to motion of molecules in a substance.