Question

why and how do the molecules in substances like solid,liquid and gases,etc attract each other?

Answer 1

First, all molecules exert some kind of force on all other molecules. The net sum of these forces betwen two molecules is the van der waals force. It is such that each molecule is slightly attracted to each other from a long distance, very attracted to each other in the short distance, and repulsed if they are too close. There is, in short, a happy medium where the molecules will experience no net force between them.

In a gas, the molecules are so fast moving that they act like billiard balls. They bounce off each other and shoot off far away. If there is a self-attraction, it is very slight. This would be like people rushing about in a busy city. No one is slowing down enough to have a conversation let alone hold hands.

In a liquid, the molecules have slowed down enough that they are at that happy range, more or less. Two molecules act like weights on a spring, bouncing back and forth. However, the molecules have not slowed down enough that they begin to build a regular crystal between other molecules. In short, it is like a group dance where dance partners are constantly being exchanged.

In a solid, the molecules are slow enough that the join with several other molecules at once. This is like a dance where everyone joins hands with their partner and several others. It is very rare that partners are changed at all. In fact, the dancers can barely leave their spot for the crush of the crowd.

So really, the molecular attraction between a solid and a liquid are roughly the same. The difference is that the molecules in a liquid are moving too quickly to settle down.