Phenomenon of intermolecular forces holding 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 (condensation) 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. In many organic compounds, which form molecular crystals, for example, the atoms are bound strongly into molecules, but the molecules are bound weakly to each other.
Condition existing at the free surface of a liquid, resembling the properties of an elastic skin under tension. The tension is the result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel. More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other. The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap,...
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