Every material is made up of molecules. Molecules exert forces on each other which is called inter-molecular force.
There are two kinds of molecular forces:
1. Adhesive Forces: The force of attraction between the molecules of different substances is called adhesive force. It is different for different pair of substances. For e.g., gum is more adhesive than water.
2. Cohesive Forces: The force of attraction between the molecules of same substances is called cohesive force. It varies as r-7, where r is the intermolecular separation. So, force of cohesion is more when distance is small and it is negligible when distance is large.
Examples of cohesive and adhesive forces:
1. Water wets the glass but mercury does not: Cohesive force acts between the molecules of a liquid and adhesive force acts between the molecule of a solid and a liquid. The adhesive force between the molecules of water-glass is more the cohesive force between the molecules of water. So the water molecules stick with the molecules of glass and the surface of glass becomes wet. The adhesive force between the molecules of mercury and glass is lesser than the cohesive force between the molecules of mercury. So, the molecules of mercury does not wet the glass.
2. Due to the adhesive forces between the molecules of ink and paper, the ink sticks on the paper. While writing on the block board, due to the adhesive forces between the molecules of chalk and black board,the letters are written on the board.
