Cohesion Definition in Chemistry

Water droplet in a glass of water
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The word cohesion comes from the Latin word cohaerere, which means "to stick together or stay together." In chemistry, cohesion is a measure of how well molecules stick to each other or group together. It is caused by the cohesive attractive force between like molecules. Cohesion is an intrinsic property of a molecule, determined by its shape, structure, and electric charge distribution. When cohesive molecules approach each other, the electrical attraction between portions of each molecule holds them together.

Cohesive forces are responsible for surface tension, the resistance of a surface to rupture when under stress or tension.

Examples

A common example of cohesion is the behavior of water molecules. Each water molecule can form four hydrogen bonds with neighbor molecules. The strong Coulomb attraction between the molecules draws them together or makes them "sticky." Because the water molecules are more strongly attracted to each other than to other molecules, they form droplets on surfaces (e.g., dew drops) and form a dome when filling a container before spilling over the sides. The surface tension produced by cohesion makes it possible for light objects to float on water without sinking (e.g., water striders walking on water).

Another cohesive substance is mercury. Mercury atoms are strongly attracted to each other; they bead together on surfaces. Mercury sticks to itself when it flows.

Cohesion vs. Adhesion

Cohesion and adhesion are commonly confused terms. While cohesion refers to the attraction between molecules of the same type, adhesion refers to the attraction between two different types of molecules.

A combination of cohesion and adhesion is responsible for capillary action, which is what happens when water climbs up the interior of a thin glass tube or the stem of a plant. Cohesion holds the water molecules together, while adhesion helps the water molecules stick to glass or plant tissue. The smaller the diameter of the tube, the higher water can travel up it.

Cohesion and adhesion are also responsible for the meniscus of liquids in glasses. The meniscus of water in a glass is highest where the water is in contact with the glass, forming a curve with its low point in the middle. The adhesion between the water and glass molecules is stronger than the cohesion between the water molecules. Mercury, on the other hand, forms a convex meniscus. The curve formed by the liquid is lowest where the metal touches the glass and highest in the middle. That's because mercury atoms are more attracted to each other by cohesion than they are to glass by adhesion. Because the shape of the meniscus depends partly on adhesion, it will not have the same curvature if the material is changed. The meniscus of water in a glass tube is more curved than it is in a plastic tube.

Some types of glass are treated with a wetting agent or surfactant to reduce the amount of adhesion so that capillary action is reduced and also so that a container delivers more water when it is poured out. Wettability or wetting, the capacity for a liquid to spread out on a surface, is another property affected by cohesion and adhesion.