View source for Cohesion (chemistry)

{{short description|Property of substances whose particles stick together}}
{{More citations needed|date=March 2010}}
[[File:Clayton Anderson zero g.jpg|thumb|right|The cohesive character of water is clearly seen]]
{{Continuum mechanics}}

In [[chemistry]] and [[physics]], '''cohesion''' ({{etymology|la|{{wikt-lang|la|cohaesiō}}|cohesion, unity}}), also called '''cohesive attraction''' or '''cohesive force''', is the action or [[chemical property|property]] of like [[molecule]]s sticking together, being mutually attractive. It is an [[intrinsic property]] of a [[Chemical substance|substance]] that is caused by the shape and structure of its molecules, which makes the distribution of surrounding [[electron]]s irregular when molecules get close to one another, creating [[Intermolecular force|electrical attraction]] that can maintain a microscopic structure such as a [[drop (liquid)|water drop]]. Cohesion allows for [[surface tension]], creating a "solid-like" state upon which light-weight or low-density materials can be placed.

[[File:Hg_Mercury.jpg|thumb|left| [[Mercury (element)|Mercury]] exhibits more cohesion than adhesion with glass]]
[[File:RainDrops1.jpg|thumb|Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, [[surface tension]], [[Van der Waals force]], [[Plateau–Rayleigh instability]]]]

[[Water]], 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. In simple terms, the polarity (a state in which a molecule is oppositely charged on its poles) of water molecules allows them to be attracted to each other. The polarity is due to the electronegativity of the atom of oxygen: oxygen is more electronegative than the atoms of hydrogen, so the electrons they share through the covalent bonds are more often close to oxygen rather than hydrogen. These are called polar covalent bonds, covalent bonds between atoms that thus become oppositely charged.<ref>Neil Campbell, Biology, 9th edition, p.92</ref> In the case of a water molecule, the hydrogen atoms carry positive charges while the oxygen atom has a negative charge. This charge polarization within the molecule allows it to align with adjacent molecules through strong intermolecular hydrogen bonding, rendering the bulk liquid cohesive. [[Van der Waals equation|Van der Waals gases]] such as [[methane]], however, have weak cohesion due only to [[van der Waals forces]] that operate by induced [[Chemical polarity|polarity]]  in non-polar molecules.

Cohesion, along with [[adhesion]] (attraction between unlike molecules), helps explain phenomena such as [[Meniscus (liquid)|meniscus]], surface tension and [[capillary action]].

[[Mercury (element)|Mercury]] 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 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 [[wikt:convex|convex]] meniscus, whereas the meniscus of water is [[wikt:concave|concave]]. Mercury will not [[Wetting|wet]] the glass, unlike water and many other [[liquid]]s,<ref>[https://books.google.com/books?id=MFQAAAAAYAAJ&dq=mercury+cohesion&pg=PA47 ''Common science'' by Carleton Wolsey Washburne]</ref> and if the glass is tipped, it will 'roll' around inside.

==See also==
* [[Adhesion]] – the attraction of molecules or compounds for other molecules of a different kind
* [[Specific heat capacity]] – the amount of heat needed to raise the temperature of one gram of a substance by one degree [[Celsius]]
* [[Heat of vaporization]] – the amount of energy needed to change one gram of a liquid substance to a gas at constant temperature
* [[Zwitterion]] – a molecule composed of individual functional groups which are [[ions]], of which the most prominent examples are the [[amino acids]]
* [[Chemical polarity]] – a neutral, or uncharged molecule or its chemical groups having an [[Electric dipole moment|electric]] [[dipole]] [[Electric dipole moment|moment]], with a negatively charged end and a positively charged end

== References ==
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==External links==
{{Commons category|Cohesion}}
* [https://nationalmaglab.org/education/magnet-academy/watch-play/demos/bubble-wall The Bubble Wall]{{Dead link|date=December 2023}} (audio slideshow from the National High Magnetic Field Laboratory explaining cohesion, surface tension and hydrogen bonds)
* [http://water.usgs.gov/edu/adhesion.html "Adhesion and Cohesion of Water" – US Geological Survey]

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[[Category:Molecular physics]]
[[Category:Intermolecular forces]]
[[Category:Physical quantities]]