Cohesion (
n. lat.
cohaerere "stick or stay together") or
cohesive attraction or
cohesive force is a
physical propertyA physical property is any aspect of an object or substance that can be measured or perceived without changing its identity. Physical properties can be intensive or extensive. An intensive property does not depend on the size or amount of matter in the object, while an extensive property...
of a substance, caused by the intermolecular attraction between like-molecules within a body or substance that acts to unite them.
WaterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
, for example, is strongly cohesive as each molecule may make four hydrogen bonds to other water molecules in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules.
Van der Waals gasesThe van der Waals equation is an equation of state for a fluid composed of particles that have a non-zero size and a pairwise attractive inter-particle force It was derived by Johannes Diderik van der Waals in 1873, based on a modification of the ideal gas law, who received the Nobel prize in 1910...
such as
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
, however, have weak cohesion due only to Van der Waals forces that operate by induced polarity in non-polar molecules.
Cohesion, along with
adhesionAdhesion is the tendency of certain dissimilar molecules to cling together due to attractive forces.-Mechanisms of adhesion:Five mechanisms have been proposed to explain why one material sticks to another:-Mechanical adhesion:...
(attraction between unlike molecules), helps explain phenomena such as
meniscusMeniscus, plural: menisci/meniscuses, from the Greek for "crescent", is a curve in the surface of a molecular substance and is produced in response to the surface of the container or another object. It can be either concave or convex. A convex meniscus occurs when the molecules have a stronger...
,
surface tensionSurface tension is a property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid .Applying Newtonian physics to the forces that arise due to surface tension accurately predicts many liquid behaviors...
and
capillary actionCapillary action, capillarity, capillary motion, or wicking refers to two phenomena:1 The movement of liquids in thin tubes.
2 The flow of liquids through porous media, such as the flow of water through soil....
.
MercuryMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
in a glass flask is a good example of the effects of the ratio between cohesive and adhesive forces. Because of its high cohesion and low
adhesionAdhesion is the tendency of certain dissimilar molecules to cling together due to attractive forces.-Mechanisms of adhesion:Five mechanisms have been proposed to explain why one material sticks to another:-Mechanical adhesion:...
to the glass, mercury does not spread out to cover the bottom of the flask, and if enough is placed in the flask to cover the bottom, it exhibits a strongly convex
meniscusMeniscus, plural: menisci/meniscuses, from the Greek for "crescent", is a curve in the surface of a molecular substance and is produced in response to the surface of the container or another object. It can be either concave or convex. A convex meniscus occurs when the molecules have a stronger...
, where the meniscus of water is concave. It will not
wetWetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
the glass, unlike water and many other liquids, and if the glass is tipped, it will 'roll' around inside.
External links
- The Bubble Wall (Audio slideshow from the National High Magnetic Field Laboratory explaining cohesion, surface tension and hydrogen bonds)