Capillary Action
Definition and meaning of Capillary Action in chemistry.
Capillary action is the spontaneous movement of a liquid through a narrow tube or porous material, even against gravity, driven by intermolecular forces between the liquid and the solid surface.
In more detail
It arises from a competition between adhesion (attraction between liquid molecules and the tube's surface) and cohesion (attraction between the liquid's own molecules). When adhesive forces exceed cohesive forces, the liquid wets the surface, curving upward at the edges to form a concave meniscus, and surface tension pulls the rest of the liquid column up with it. The narrower the tube, the higher the liquid rises, since the upward force scales with the tube's circumference while the opposing weight scales with its cross-sectional area. This effect is quantitatively described by the Young-Laplace equation and Jurin's law, which relates rise height to surface tension, contact angle, tube radius, liquid density, and gravity.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing law | Jurin's law: h = 2γcosθ / (ρgr) |
| Key property involved | Surface tension and contact angle |
| Everyday example | Water moving up a paper towel or plant xylem |
Water rises noticeably higher in a narrow glass capillary tube than in a wide one, forming a concave meniscus, because water strongly wets glass (adhesion exceeds cohesion).
Frequently asked questions
Does capillary action work with mercury?
Mercury shows the opposite effect: it is depressed (pushed down) in a glass capillary and forms a convex meniscus, because mercury's cohesive forces exceed its adhesion to glass.
Why does a narrower tube cause a greater rise?
The upward capillary force acts along the tube's inner circumference (proportional to radius), while the weight of the raised liquid column depends on its volume (proportional to radius squared), so the rise height is inversely proportional to the radius.