cohesive forces
Summary
Cohesive forces are attractions between molecules of the same substance, such as hydrogen bonds between water molecules. They produce surface tension. Testable point: if adhesion to the wall exceeds cohesion the meniscus is concave; if cohesion exceeds adhesion it is convex.
Detail
Meaning: Cohesive forces are attractions between molecules of the same substance, such as hydrogen bonds between water molecules. A molecule in the bulk of a liquid is pulled equally in all directions, but a surface molecule is pulled inward, so the surface acts like a stretched skin. This is surface tension. Adhesive forces are attractions between the liquid and a different material, such as glass.
Equation: γ = F/L, where γ is surface tension (N/m), F is the force along the surface (N), and L is the length of surface edge it acts along (m). A film with two surfaces gives F = 2γL.
On the MCAT: Passages ask about meniscus shape and capillary action. If adhesion exceeds cohesion (water in glass), the meniscus is concave and the liquid rises in a narrow tube. If cohesion exceeds adhesion (mercury in glass), the meniscus is convex and the level is depressed. Also expect why droplets are round, and why raising the temperature or adding a surfactant lowers surface tension.
Common confusion: Students swap cohesion and adhesion. Cohesion is liquid to liquid (like to like); adhesion is liquid to the container wall (unlike materials).
Sources
- Physics
- Physics for Scientists and Engineers
- Fundamentals of Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.