What causes adhesion and cohesion?

• A. Hydrogen bonding
• B. Ionic exchange capacity
• C. Osmotic pressure
• D. Gravitational forces

Answer: (A)

Hydrogen bonding is the key. Water molecules are polar, with a partial negative charge on oxygen and partial positive charges on hydrogens, so they readily form hydrogen bonds with each other. That bonding between water molecules is cohesion, helping water stick to itself and form a continuous column. It also allows water to bond to other surfaces that have compatible polar or charged groups, which is adhesion. In soils and plants, this enables water to cling to soil particles and move through tiny pore spaces, supporting capillary rise and moisture retention.

Ionic exchange capacity, osmotic pressure, and gravitational forces describe different phenomena. Ionic exchange capacity relates to soil’s ability to exchange cations with the solution, osmotic pressure comes from solute concentration differences across membranes, and gravity acts on mass at larger scales, not on the molecular attractions that produce cohesion and adhesion.