Short Answer
Water rises in a capillary tube because the adhesive force between water molecules and the tube walls is stronger than the cohesive force between water molecules themselves. This stronger attraction pulls water upward along the inner surface of the tube.
Surface tension then helps other water molecules rise, causing the water level to climb higher in a narrow tube. This process is called capillary rise. The narrower the tube, the higher the water rises because the adhesive and cohesive forces act more strongly in smaller spaces.
Detailed Explanation :
Why water rises in a capillary tube
Water rises in a capillary tube due to the combined effect of adhesion, cohesion, and surface tension. These forces work together to pull the water upward against gravity. When a thin glass tube is placed in water, water climbs up the tube. This upward movement of water is called capillary rise, and it happens because water molecules are strongly attracted to the glass walls.
This phenomenon is important in nature and daily life, such as in plants absorbing water, in wicks drawing oil upward, and in paper towels absorbing liquid. Understanding why water rises in a capillary tube helps explain many natural processes.
Role of adhesion
Adhesion is the force of attraction between molecules of different substances. In this case, it is the attraction between water molecules and the glass surface of the capillary tube.
- Glass has a strong attraction to water.
- Water molecules stick to the inner walls of the tube.
- This pulls the water upward along the tube’s surface.
Because adhesion between water and glass is greater than cohesion between water molecules, the water climbs up instead of staying flat.
Role of cohesion
Cohesion is the force of attraction between molecules of the same substance. In water, cohesion is due to strong hydrogen bonding.
- Water molecules stick to each other.
- When some molecules are pulled upward by adhesion, they drag the other molecules with them.
- This chain-like action helps water rise as a unit.
Thus, cohesion supports the upward movement that adhesion begins.
Role of surface tension
Surface tension helps shape the water surface inside the capillary tube. It causes the water surface to curve and form a concave meniscus.
Surface tension acts like a stretched elastic sheet and pulls the water column upward because:
- Water molecules at the surface are pulled inward and upward.
- This upward pull helps water climb higher in the tube.
Surface tension ensures that the water surface remains stable while rising.
Why water rises higher in a narrow tube
The height of water rise depends on how narrow the tube is.
- In narrow tubes, water rises more because the adhesive force acts on a larger surface area compared to the volume of water.
- In wider tubes, water rises less because the same force is spread over a larger volume.
Thus, the smaller the diameter of the tube, the greater the height of capillary rise.
Mathematical idea (simple understanding)
Although the question does not ask for formulas, it is useful to know that the height of water rise depends on:
- Surface tension
- Density of water
- Adhesion and cohesion
- Radius of tube
Height increases when radius decreases.
Real-life examples of water rising due to capillary action
- Plants absorbing water
The tiny tubes (xylem vessels) in plant stems draw water upward from roots to leaves using capillary action. - Paper towel absorbing water
The tiny gaps between fibres act like many small capillary tubes that pull water upward. - Oil lamp wick
Wicks draw oil upward to keep the flame burning. - Soil absorbing water
Water spreads upward and sideways through tiny pores in soil. - Ink spreading in paper
Paper fibres act as capillary tubes, helping ink spread. - Cracks in walls absorbing water
Fine cracks pull water upward, sometimes causing dampness.
All these examples occur because adhesion and cohesion lift the liquid.
Why mercury does not rise
Mercury behaves differently:
- Cohesion between mercury molecules is stronger than adhesion with glass.
- Mercury falls instead of rising.
- This forms a convex meniscus.
This comparison shows how the balance between adhesion and cohesion controls the direction of the liquid movement.
Importance of water rising in capillary tubes
This property is essential for:
- Transport of water and nutrients in plants
- Movement of fluids in biological organisms
- Functioning of many household items
- Designing materials that absorb liquids
- Agricultural soil management
- Medical devices like test strips
Understanding capillary rise helps in developing better technologies in medicine, engineering, and agriculture.
Conclusion
Water rises in a capillary tube because the adhesive force between water and the tube walls is stronger than the cohesive force within water molecules. Adhesion pulls the water upward, cohesion helps drag more molecules along, and surface tension supports the rise by shaping the water surface. This upward movement is stronger in narrow tubes. Capillary rise is essential in plants, paper towels, wicks, soil, and many everyday and natural processes.