Intermolecular Force is the force of attraction between the atoms or molecules of a matter.
Matter is not continuous it is rather discrete i.e. like space filled with balls. It is aggregation of molecules in such a way that there exists certain inter scapes between the molecules. This is why when we apply pressure to a body or it’s temperature is decreased the volume of the body diminish , Due to the applied pressure of decrease in temperature the intermolecular space decreases and the volume of the body decreases.
For example: When pressure is applied to certain volume of steam it’s volume decreases accordingly and when both pressure and temperature are lowered the intermolecular space decreases considerably and ultimately it changes into liquid water. Similarly when solids like sugar are dissolved in water , their is no corresponding increase in the volume of solution because water molecules are adjusted in the intermolecular space of sugar molecules.
Intermolecular Force and Motion:
Molecules are regarded to be in continuous rapid vibratory motion within their inter molecular space. Thus there is always a tendency of the molecules to fly away from one another . This type of motion is termed as intermolecular motion. This flying away tendency of molecules is countered by a force of attraction between the molecules , which keeps the molecules of a body together. This force of attraction which keeps the body of a matter together is termed as Intermolecular force.