Cohesion:- The attraction between molecules that holds particles of a substance together. Cohesion differs from adhesion in being the force of attraction between adjacent particles within the same body; adhesion is the interaction between the surfaces of different bodies. The force of cohesion in gases can be observed in the liquefaction of a gas, which is the result of a number of molecules being pressed together to produce forces of attraction high enough to give a liquid structure.
Cohesion in liquids is reflected in the surface tension caused by the unbalanced inward pull on the surface molecules, and also in the transformation of a liquid into a solid state when the molecules are brought sufficiently close together. Cohesion in solids depends on the pattern of distribution of atoms, molecules, and ions, which in turn depends on the state of equilibrium (or lack of it) of the atomic particles. Many organic compounds form molecular crystals, for example, in which the atoms are bound strongly into molecules but the molecules are bound weakly to each other.

Adhesion, attraction between the surfaces of two bodies. The two adjacent surfaces may be of different chemical compositions. Adhesion is distinguished from cohesion which is the attraction between portions of a single body. For example, if a sheet of glass is lowered into water and withdrawn, some water will cling to the glass (adhesion) but the rest will be pulled back into the main body of water (cohesion).
The force of attraction involved is due to electromagnetic interactions produced by variations in the distribution of electrons in the molecules of the facing surfaces. The distance between the molecules of the facing surfaces is a determining factor in the amount of force exerted. A surface that may appear smooth to the naked eye may actually be so rough that its molecules are not close enough to a facing surface to produce an electromagnetic bond. Gauge blocks, pieces of metal used for taking accurate measurements, are so smooth that their facing surfaces can be made to stick to each other simply by pressing them together hard.
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