Why does heating leads to expansion?

As it is the atoms and molecules of all substances are in a state of vibration all the time.

When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms and molecules increases. As the spacing between atoms and molecules increases, there is an expansion in the substances and the matter that is being heated. The reverse is also true; substances and matter contract on cooling.

The motion and spacing of the particles determine the state of matter of the substance. Solids, liquids, and gases all expand when the heat is added and contract on cooling, that is, when the heat is removed.