Capillary
Definition and meaning of Capillary in chemistry.
Capillary refers to a very narrow tube commonly used in chemistry experiments. It typically features an internal diameter that is smaller than one millimeter. Liquids can spontaneously move up or down inside this tube through capillary action.
In more detail
Capillary action results from a constant tug-of-war between two main intermolecular forces. Adhesion describes the attraction between the liquid molecules and the solid tube walls. Cohesion represents the natural internal attraction between the molecules of the liquid itself.
Water molecules are heavily attracted to the surface of a glass tube. These adhesive forces overpower the internal cohesive forces within the liquid water. The water physically crawls up the vertical glass walls to maximize this attraction.
This upward crawling motion creates a curved liquid surface called a concave meniscus. The water rises until the weight of the liquid column balances the upward pull. Mercury behaves very differently when placed inside a standard glass capillary tube.
The mercury atoms attract each other much more strongly than they attract glass. These dominant cohesive forces cause the liquid to pull away from the glass. This inward pulling action creates an outward liquid bulge known as a convex meniscus.
The exact height a liquid reaches depends on a mathematical rule called Jurin's law. A significantly narrower tube radius forces the liquid to climb to a greater height. A heavier and denser liquid will not climb as high as a lighter liquid.
Students often incorrectly assume that changes in air pressure push the liquid upward. The liquid actually climbs the walls entirely because of these tiny intermolecular attractions.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical diameter | Less than 1 millimeter |
| Driving forces | Adhesion and cohesion |
| Upward curve | Concave meniscus |
| Downward curve | Convex meniscus |
| Mathematical rule | Jurin's law |
You can observe capillary action using a thin glass tube and colored water. You simply dip the open end of the tube into a beaker of blue water. The blue water immediately shoots up the narrow inside space of the glass tube. The water level finally stops several centimeters above the water resting in the beaker. The extremely narrow space allows the adhesive forces to pull a noticeable column of water upward.
Frequently asked questions
Why does water rise in a capillary tube but mercury falls?
Water molecules stick to glass better than they stick to each other. Mercury atoms stick to each other much better than they stick to glass, so the liquid pulls away and moves downward.
Does the material of the tube matter for capillary action?
Yes. The liquid must be chemically attracted to the material of the tube. Water will not rise noticeably in a tube made of non-stick plastic because the adhesive forces are too weak.
How does the width of the tube change the liquid height?
A narrower tube allows the liquid to climb much higher. The upward pulling force has to lift a significantly smaller weight of liquid in a narrow tube compared to a wide one.