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The cohesive forces are responsible for surface tension, that is, the resistance that a surface shows to break when subjected to some type of tension.
examples
A common example of cohesion is the behavior of water molecules. Each water molecule can form four hydrogen bonds with adjacent molecules. This strong electrostatic attraction between the water molecules keeps them together, giving rise to the drops that we can observe on different surfaces and the kind of dome that can form in a glass filled with water over its edge without spilling. The surface tension generated by the cohesion of water molecules also allows objects that are not too heavy to float on its surface.
Another example of cohesion is that of mercury, a liquid metal at room temperature. Mercury atoms are held together by generating high surface tension, which gives rise to the droplets that can be seen when poured onto a surface.
Cohesion and adhesion
These two terms are often confused; cohesion refers to the attraction between molecules of the same type, within the same material, while adhesion refers to the attraction between different types of molecules, for example between the surfaces of different materials.
The combination of these two forces is responsible for capillarity , which causes water to rise up the inside of a small-diameter glass tube or up the stem of a plant. Cohesion holds water molecules together by rising above the hydrostatic level, while adhesion helps water molecules stick to glass or plant tissue. The smaller the diameter of the tube, the higher the water can rise.
Both forces are also responsible for the meniscusformed by liquids on the contact surfaces of the containers that contain them. The concave meniscus that forms the water in a glass beaker causes its level to be higher where the water is in contact with the glass, forming a downward curve between the glass of the glass and the surface of the water away from the glass. The adhesion between the water molecules and the glass is stronger than the cohesion between the water molecules. Mercury is the opposite example, forming a convex meniscus, with an upward curve between the glass surface and the liquid level. This is because mercury atoms are more attracted to each other by cohesion than to glass by adhesion. The adhesion depends on the materials that are in contact, and this can be observed in the change of curvature of the water meniscus if we change the material of the container.
Some types of glass are treated with a surfactant chemical to reduce stickiness and thus reduce wicking. The ability of a liquid to spread on a surface or form drops is another property associated with cohesion and adhesion.
