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Answer: Capillary action is the movement of water through the tiny spaces of a porous material, driven by adhesion, cohesion and surface tension. It is how water climbs a narrow tube, spreads through a paper towel, moves through soil and rises into plant roots.

Why water climbs

Water sticks to the walls of a container (adhesion), which pulls the edges up into a curved meniscus. Water molecules also hold onto each other (cohesion), so the rest of the column follows.

Narrow tubes, higher rise

In a set of tubes, the narrowest pulls water highest, because it has more inner surface relative to the volume of water. Without capillary action, the level in every tube would be the same.

In nature

• Plants and trees use it, together with adhesion and cohesion, to move water from roots toward leaves.
• In soil it carries water and dissolved nutrients between particles.

Claims

• id: C1 claim: Capillary action is movement of water within the spaces of a porous material. source: S1 evidence: "It is defined as the movement of water within the spaces of a porous material"
• id: C2 claim: Capillary action helps bring water up into plant roots. source: S1 evidence: "Capillary action helps bring water up into the roots."
• id: C3 claim: Smaller diameter tubes pull water higher. source: S1 evidence: "allowing capillary action to pull water up higher than in the larger diameter tubes"
• id: C4 claim: Cohesion and adhesion are the stickiness water has. source: S2 evidence: "cohesion and adhesion are the"

Sources

• [S1] Capillary Action and Water. USGS. https://www.usgs.gov/special-topics/water-science-school/science/capillary-action-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_6bard5s4,
  author = {{iinventedlight}},
  title = {{Capillary action}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/capillary-action/},
  note = {Permanent ID iil:note:6bard5s4. License: CC-BY-4.0.},
}

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