Capillary action (or capillarity) is the ability of a liquid to rise or fall in a narrow tube (capillary) without any external force like pumping.
It happens because of two forces:
- Cohesion → Attraction between liquid molecules themselves.
- Adhesion → Attraction between liquid molecules and the walls of the tube.
- If adhesion > cohesion, the liquid rises in the tube (e.g., water in glass).
- If cohesion > adhesion, the liquid level falls in the tube (e.g., mercury in glass).
Examples of Capillary Action:
- Water rising in thin plant stems and roots (helps transport water in plants).
- Ink moving in blotting paper.
- Oil rising in the wick of a lamp.
- Paper towels absorbing water.
Key point:
Capillary rise or fall depends on the liquid, the tube material, and the diameter of the tube (smaller tubes show stronger effect).
In short: Capillary action is the rising or falling of a liquid in a thin tube due to the balance of cohesive and adhesive forces.