adhesive forces

Meaning: Adhesive forces are intermolecular attractions (hydrogen bonding, dipole forces, dispersion forces) between molecules of two different substances, such as water and glass. Cohesive forces act between like molecules, such as water to water, and give rise to surface tension. The balance of the two sets how a liquid behaves at a surface. When adhesion is stronger than cohesion (water in glass), the liquid wets the wall, climbs it, and forms a concave meniscus. When cohesion is stronger (mercury in glass), the liquid pulls together, forms a convex meniscus, and is pushed down in a narrow tube.

Equation: h = 2γcosθ / (ρgr), where h is capillary rise (m), γ is surface tension (N/m), θ is the contact angle between liquid and wall (degrees or radians), ρ is liquid density (kg/m³), g is gravitational acceleration (m/s²), and r is tube radius (m). A contact angle below 90° gives a rise; above 90° gives a depression.

On the MCAT: Passages describe capillary action, water climbing in a thin tube or in plant xylem, or the meniscus shape of different liquids. You predict that a narrower tube gives a larger rise, or decide from meniscus shape which force is stronger.

Common confusion: Students mix up adhesion and cohesion. Adhesion is between different substances; cohesion is within one substance. Surface tension comes from cohesion, not adhesion. A concave meniscus signals adhesion greater than cohesion.