Capillary
Definition and meaning of Capillary in chemistry.
Capillary refers to a narrow tube, typically under 1 mm in internal diameter, in which capillary action occurs: the spontaneous rise or fall of a liquid inside the tube driven by intermolecular forces rather than gravity or external pressure.
In more detail
Capillary action results from a competition between adhesion (attraction of liquid molecules to the tube's walls) and cohesion (attraction among the liquid's own molecules). When adhesive forces dominate, as with water in glass, the liquid wets the surface, forms a concave meniscus, and rises; when cohesive forces dominate, as with mercury in glass, the liquid is depressed and forms a convex meniscus. The height a liquid climbs is described by Jurin's law, h = 2γcosθ/(ρgr), showing that rise increases with surface tension (γ) and decreases with tube radius (r) and liquid density (ρ). The narrower the capillary, the greater the effect.
Key facts
| Field | Physical Chemistry |
|---|---|
| Governing relation | Jurin's law: h = 2γcosθ / (ρgr) |
| Typical bore size | Less than 1 mm internal diameter |
| Key forces | Adhesion vs. cohesion; surface tension and contact angle |
Dip a thin glass capillary tube into a beaker of water: the water climbs noticeably above the surrounding liquid level, forming a concave meniscus, because water strongly wets glass and its adhesive forces exceed its cohesive forces.
Frequently asked questions
Why does water rise in a capillary tube but mercury falls?
Water wets glass because its adhesive attraction to the glass surface exceeds its internal cohesive forces, pulling it upward into a concave meniscus. Mercury does not wet glass; its cohesive forces exceed adhesion to glass, so it is depressed and forms a convex meniscus.
Does capillary rise depend on the tube's material?
Yes. The contact angle between the liquid and the tube's inner surface, which depends on the material, determines whether the liquid rises (contact angle less than 90 degrees) or is depressed (contact angle greater than 90 degrees).