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Cohesion (Chemistry) – Definition & Explanation

Cohesion in chemistry refers to the attractive forces between identical molecules of the same substance. It is responsible for the structural integrity of liquids and solids.

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Cohesion in chemistry refers to the attractive forces between identical molecules of the same substance. It is responsible for the structural integrity of liquids and solids.

What is Cohesion?

Cohesion in chemistry and physics refers to the attractive intermolecular forces that act between identical molecules or atoms of the same substance. This phenomenon is fundamentally responsible for keeping liquids and solids together as unified matter rather than dispersing. Cohesion is clearly distinct from adhesion, which describes the attractive forces between molecules of different substances.

Causes and Molecular Basis

Cohesive forces arise from various intermolecular interactions, which differ in strength depending on the substance:

Van der Waals forces: Weak, short-range attractive forces that occur between all molecules and are based on temporary dipole moments.
Hydrogen bonds: Particularly strong intermolecular forces that act, for example, between water molecules, and are responsible for the exceptional properties of water.
Dipole-dipole interactions: Forces between the permanent dipoles of polar molecules.
Metallic bonds: In metals, delocalized electrons create very strong cohesion among metal atoms.
Ionic and covalent bonds: In ionic crystals and covalent network solids, these strong chemical bonds ensure structural cohesion.

Cohesion in Water

The most well-known example of cohesion is water. Water molecules (H₂O) form strong hydrogen bonds with one another because the oxygen atom is highly electronegative, making the O-H bonds strongly polar. This pronounced cohesion is responsible for several unique properties of water:

Surface tension: At the surface of water, molecules experience a net inward force because they have no equivalent neighbors above them. This creates measurable surface tension, enabling insects like water striders to walk on the surface.
Capillary action: The interplay of cohesion and adhesion allows water to rise in narrow tubes, which is critical in biology for water transport in plants.
High enthalpy of vaporization: A significant amount of energy is required to vaporize water, as the hydrogen bonds must be overcome.

Cohesion in Solids

In solids, cohesion is typically very strong and determines physical properties such as hardness, melting point, and mechanical stability. Key categories include:

Molecular crystals (e.g., ice, naphthalene): Cohesion via Van der Waals forces and hydrogen bonds; relatively low melting points.
Ionic crystals (e.g., sodium chloride): Strong electrostatic cohesive forces between positively and negatively charged ions; high melting points.
Covalent network solids (e.g., diamond, quartz): Extremely strong covalent bonds act as cohesive forces; very high melting points and hardness.
Metals: Delocalized electrons form a so-called electron sea that holds the positive metal ions together.

Cohesive Energy

Cohesive energy (also referred to as lattice energy or binding energy) is a quantitative measure of the strength of cohesive forces within a substance. It indicates how much energy must be expended to completely separate a substance into its individual atoms, molecules, or ions. The higher the cohesive energy, the more stable the substance, and generally, the higher its melting and boiling points.

Distinction: Cohesion vs. Adhesion

While cohesion describes the attraction between identical particles, adhesion refers to the attraction between particles of different substances. Both forces often act simultaneously and together determine phenomena such as wettability, capillarity, and the behavior of liquids at interfaces. If cohesion exceeds adhesion, a liquid in narrow tubes will curve downward (as with mercury in glass); if adhesion exceeds cohesion, the liquid will rise upward (as with water in glass).

Relevance in Medicine and Biology

Cohesive forces also play an important role in biological and medical contexts:

The water transport in plants is explained by the cohesion-tension theory, which states that water columns in plant vessels are held together by cohesive forces.
The surface tension of bodily fluids such as blood influences their flow properties and distribution in tissues.
In pharmaceutics, cohesive forces are relevant for tablet compression and the stability of solid dosage forms.
The pulmonary surfactant system reduces the surface tension (cohesion) of alveolar fluid to prevent the collapse of the small air sacs in the lungs.

References

Atkins, P. W.; de Paula, J.: Physical Chemistry. 10th Edition, Oxford University Press, Oxford, 2014.
Mortimer, C. E.; Müller, U.: Chemistry: The Central Science. 12th Edition, Thieme, Stuttgart, 2015.
IUPAC: Compendium of Chemical Terminology (Gold Book). International Union of Pure and Applied Chemistry, 2nd Edition, 1997. Available at: https://goldbook.iupac.org

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