cohesion
Summary
Cohesion is the attraction between molecules of the same substance, such as hydrogen bonding among water molecules. It produces surface tension. The testable point is cohesion versus adhesion: when adhesion to the container wall exceeds cohesion, the meniscus is concave and the liquid rises in a narrow tube.
Detail
Meaning: Cohesion is the intermolecular attraction between like molecules within a liquid. A molecule at the surface has neighbors on fewer sides, so it is pulled inward and the surface behaves like a stretched skin. This gives surface tension, round droplets, and lets a small object such as a water strider rest on water.
Equation: γ = F/L, where γ is surface tension (N/m), F is the force acting parallel to the surface (N), and L is the length of surface edge the force acts along (m). A soap film has two surfaces, so L is counted twice.
On the MCAT: Passages compare cohesion with adhesion, the attraction between a liquid and a different material such as glass. If adhesion exceeds cohesion (water in glass), the meniscus is concave and the liquid climbs in a narrow tube by capillary action. If cohesion exceeds adhesion (mercury in glass), the meniscus is convex and the liquid is depressed. Stronger cohesion means higher surface tension. Raising temperature usually lowers surface tension, and surfactants such as soap lower it.
Common confusion: Cohesion is like-to-like attraction; adhesion is unlike-to-unlike. Students swap them when predicting meniscus shape. Settle it by asking which attraction is stronger: the liquid for the wall (adhesion) or the liquid for itself (cohesion).
Sources
- Physics (Giancoli)
- Physics (Cutnell and Johnson)
- College Physics (Serway and Vuille)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.