IB Biology A1.1.3 Cohesion of water molecules
Hydrogen bonds make water molecules cohere, producing continuous columns under tension and surface tension that living organisms exploit for transport and support.
- Syllabus
- First assessment 2025
- Objective
- ib_null_biology_001_001_003
- Level
- All levels
Quick summary
Cohesion is the attraction of water molecules to other water molecules. It results from hydrogen bonding between their partial charges. Across a water surface, cohesive forces create surface tension because molecules are pulled towards neighbouring molecules rather than towards the air. This can support small organisms such as pond skaters. Cohesion is also essential in plants: water molecules remain connected as a continuous column in xylem vessels. Transpiration can place this column under tension and pull it upwards without the column breaking. Strong exam answers begin with polarity or hydrogen bonding, name cohesion, and then link it to a specific biological outcome such as xylem transport or surface tension. Students should distinguish cohesion from adhesion, which is attraction between water and a different material and contributes to capillary action.
Concept visual
One diagram for the relationship students need to remember.
Common mistakes
Defining cohesion as attraction between water and another surface.
Attributing xylem continuity to covalent bonds between water molecules.
Naming surface tension without linking it to cohesive intermolecular attraction.
Explaining upward movement without stating that the water column withstands tension.
Key takeaways
- Cohesion is attraction between water molecules.
- Hydrogen bonding produces cohesive forces.
- Cohesion maintains continuous water columns in xylem.
- Surface tension is a consequence of cohesion at an interface.
