Water has cohesion because hydrogen bonds attract water molecules to one another. It has adhesion because polar water molecules are also attracted to other polar or charged materials.
The Mechanism
A water molecule is polar because oxygen attracts the shared electrons in its covalent bonds more strongly than hydrogen. This gives the oxygen atom a partial negative charge and the hydrogen atoms partial positive charges.
The partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another. This intermolecular attraction is a hydrogen bond. Although each hydrogen bond is relatively weak, large numbers of them collectively produce important properties.
| Property | Molecular cause | Biological consequence |
|---|---|---|
| Cohesion | Hydrogen bonds form between water molecules | Water forms a continuous column in xylem and produces surface tension |
| Adhesion | Water forms attractions with polar or charged surfaces | Water adheres to plant cell walls and soil particles, contributing to capillary action |
In plants, transpiration removes water from moist cell walls in leaves. This generates tension in the xylem. Because water molecules cohere, they pull one another upward as a continuous column; adhesion to the polar components of xylem walls also supports movement through narrow spaces.
Cohesion also produces surface tension. Molecules at the surface are pulled inward by hydrogen bonding, creating a surface strong enough to provide a habitat for organisms such as pond skaters.
A common misconception is that cohesion and adhesion are the same. Cohesion is attraction between water molecules, whereas adhesion is attraction between water and a different polar or charged material.
Exam Technique
This content is assessed at both SL and HL in subtopic A1.1 Water. For an “explain” question, link the full sequence: unequal electron sharing → water polarity → hydrogen bonding → cohesion or adhesion → a named biological consequence. Do not merely state that water is “sticky”; identify the molecules or surfaces being attracted.