Water striders can walk on water because surface tension resists their legs breaking through the water’s surface. Their hydrophobic, widely spread legs distribute their weight and allow the surface to support them.
This is part of IB Biology A1.1 Water and is required at both SL and HL.
The Mechanism
Water molecules are polar because oxygen attracts the shared electrons in each covalent bond more strongly than hydrogen. The partially negative oxygen atom of one molecule is attracted to a partially positive hydrogen atom of another, forming a hydrogen bond.
These intermolecular hydrogen bonds produce cohesion, meaning attraction between water molecules. Within the water, each molecule is attracted to molecules in all directions. At the surface, however, molecules have fewer neighbours above them, so the cohesive forces produce a net inward attraction. This causes the surface to resist being stretched or broken, an effect called surface tension.
Several features allow a water strider to exploit this property:
Factor | Contribution to walking on water |
|---|---|
Cohesion between water molecules | Produces surface tension that resists penetration of the surface |
Long, widely spread legs | Distribute the insect’s weight over a larger area, reducing pressure at each contact point |
Hydrophobic leg surface | Prevents wetting and helps the legs remain above the surface |
Microscopic hairs on the legs | Trap air and increase water repellence |
The legs create dimples in the surface but normally do not break through it. To move, the water strider pushes backward against the water’s surface, and the resulting reaction force propels it forward.
A common misconception is that water has a literal elastic “skin.” It does not: surface tension is an emergent effect of cohesive forces between water molecules at the water/air boundary.
Exam Technique
For an explain question, link the ideas in sequence: water polarity → hydrogen bonding → cohesion → surface tension → support of the insect. Do not state only that the insect is “light”; its hydrophobic legs and weight distribution are also important.
