Short Answer

Capillarity is the ability of a liquid to rise or fall in a thin tube due to the combined action of cohesive forces (between liquid molecules) and adhesive forces (between liquid and tube). When adhesive forces are stronger, the liquid rises, as in the case of water. When cohesive forces are stronger, the liquid level falls, as seen with mercury.

Capillarity plays an important role in many natural and everyday processes, such as water rising in soil, ink spreading in paper, and water moving upward in plants through narrow vessels.

Detailed Explanation

Capillarity

Capillarity, also called capillary action, is the phenomenon in which a liquid either rises or falls in a narrow tube known as a capillary tube. This happens because of the balance between two important molecular forces: cohesion (attraction between similar molecules) and adhesion (attraction between different molecules).

When the adhesive force between the liquid and the tube is stronger than the cohesive force among the liquid molecules, the liquid climbs upward inside the tube. If the cohesive force is stronger, the liquid level falls. Capillarity helps explain many natural processes like water movement in plants and moisture movement in soils.

How Capillarity Works

To understand capillarity, it is necessary to understand how molecular forces influence the shape and movement of liquids inside narrow tubes.

There are two key forces:

1. Cohesive force – force of attraction between molecules of the same liquid.
2. Adhesive force – force of attraction between liquid molecules and the surface of the tube.

Depending on which force is stronger, the liquid behaves differently:

• Adhesive > Cohesive → Liquid rises in the tube
  Example: water in a glass capillary tube.
• Cohesive > Adhesive → Liquid level falls
  Example: mercury in a glass tube.

The surface of the liquid curves due to these forces and forms a meniscus (curved surface). A concave meniscus appears when the liquid rises, and a convex meniscus appears when the liquid falls.

Capillary Rise

Capillary rise occurs when adhesive forces between the liquid and tube pull the liquid upward. Water molecules stick to the glass wall because both are polar substances. This attraction pulls water upward, and the cohesive forces pull more water along behind it. Gravity tries to pull the water down, but capillary forces work against gravity.

The height of capillary rise depends on:

• Diameter of the tube
• Strength of adhesive and cohesive forces
• Surface tension
• Density of the liquid

Narrower tubes cause a greater rise because the adhesive forces act over a smaller area, making the upward pull stronger.

Capillary Fall

Capillary fall occurs when cohesive forces are stronger than adhesive forces. Mercury molecules strongly attract each other but do not stick to glass. As a result, mercury pulls itself downward inside the tube, creating a convex meniscus.

Factors Affecting Capillarity

Several factors influence how much a liquid rises or falls in a capillary tube:

1. Nature of the Liquid

Different liquids have different strengths of cohesion and adhesion.

• Water wets glass, so it shows capillary rise.
• Mercury does not wet glass, so it shows capillary fall.

2. Diameter of the Tube

The narrower the tube:

• The greater the capillary rise or fall
• The stronger the upward or downward pull

Capillary action is strongest in very thin tubes.

3. Surface Tension

Surface tension plays a major role in capillarity.

• Higher surface tension → stronger capillary action
• Lower surface tension → weaker capillary action

Water has enough surface tension to rise significantly in thin tubes.

4. Temperature

• Increase in temperature → decrease in surface tension → decreased capillarity
• Decrease in temperature → increase in surface tension → increased capillarity

Thus, capillarity is affected by the temperature of the liquid.

Examples of Capillarity in Daily Life

Capillarity is seen in many natural and everyday situations:

1. Water Rise in Plants

Plants transport water from roots to leaves using capillary action in narrow xylem vessels.

2. Oil in a Lamp Wick

Oil rises through the thin cotton fibers of a wick, allowing the lamp to burn continuously.

3. Paper Absorbing Ink

Paper has tiny pores that pull ink upward through capillarity.

4. Soil Holding Water

Water rises in soil pores, helping plants absorb moisture from underground.

5. Towel Absorbing Water

Fibers in a towel act as capillary tubes, helping soak up water quickly.

6. Test of Purity of Milk Using Lactometers

Capillary effects help level the milk inside the device.

Importance of Capillarity

Capillary action is important for:

• Survival of plants
• Absorption of water in soil
• Working of wicks in lamps and candles
• Printing and writing processes
• Movement of fluids in biology and chemistry

It also plays a role in inkjet printing, chromatography, painting, and forensic science.

Conclusion

Capillarity is the rise or fall of a liquid in a narrow tube due to the balance between cohesive and adhesive forces. When adhesive forces dominate, liquids like water rise; when cohesive forces dominate, liquids like mercury fall. This phenomenon is essential in nature and many daily activities, including water transport in plants, ink spreading, wicking of liquids, and absorption in soils. It combines the effects of surface tension, molecular forces, and tube diameter to determine the movement of liquids in narrow spaces.