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Answer: Surface tension is the property of a liquid's surface that lets it resist an outside force, because its molecules cohere. Water has high surface tension: molecules at the surface have no water above them, so they hold more tightly to neighbors beside and below, forming a film strong enough to float a steel paper clip.

Cohesion and adhesion

• Cohesion: water attracted to water. It makes drops bead up and creates surface tension.
• Adhesion: water attracted to other substances, like a droplet clinging to a pine needle.

Together they are the stickiness of water, and gravity tugs against both.

Why it matters

Surface tension lets some insects walk on water and helps water form drops. Combined with adhesion it drives capillary action, which moves water through soil and up into plants.

Try it

Gently lay a paper clip flat on still water. It floats even though steel is far denser than water; a drop of soap lowers surface tension and it sinks.

Claims

• id: C1 claim: Surface tension lets a liquid surface resist an external force. source: S1 evidence: "The property of the surface of a liquid that allows it to resist an external"
• id: C2 claim: Cohesive forces between molecules cause surface tension. source: S1 evidence: "The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension."
• id: C3 claim: High surface tension lets a steel paper clip float. source: S1 evidence: "The high surface tension helps the paper clip - with much higher density - float"
• id: C4 claim: Cohesion means water is attracted to water; adhesion, to other substances. source: S2 evidence: "Cohesion: Water is attracted to water, and Adhesion: Water is attracted to other substances."

Sources

• [S1] Surface Tension and Water. USGS. https://www.usgs.gov/special-topics/water-science-school/science/surface-tension-and-water . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Adhesion and Cohesion of Water. USGS. https://www.usgs.gov/special-topics/water-science-school/science/adhesion-and-cohesion-water . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_m8bvb754,
  author = {{iinventedlight}},
  title = {{Surface tension of water}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/surface-tension-water/},
  note = {Permanent ID iil:note:m8bvb754. License: CC-BY-4.0.},
}

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