Short Answer

Adhesion is the force of attraction between molecules of different substances. It helps a liquid stick to a solid surface, such as water sticking to glass or dew drops sticking to leaves. Adhesion works together with cohesion to control how liquids spread, rise, or fall in different situations.

This force is important in many natural and everyday activities like painting, writing with ink, and water rising in plants. When adhesive forces are strong, liquids spread easily on surfaces; when they are weak, liquids form beads or rolls.

Detailed Explanation

Adhesion

Adhesion is the attractive force that occurs between molecules of different substances. It causes one material to stick to another. For example, when water sticks to a glass window, it is due to adhesion. This force is different from cohesion, which is the attraction between similar molecules. Adhesion plays an important role in how liquids behave when they come in contact with solid surfaces.

When a liquid touches a solid surface, the attraction between the molecules of the liquid and those of the solid decides whether the liquid will spread out or form droplets. If adhesion is strong, the liquid spreads. If adhesion is weak, the liquid remains in drop form.

Molecular Basis of Adhesion

To understand adhesion clearly, it is important to look at the molecular interactions involved. When two different substances come close to each other, the attractive forces between their molecules create adhesion. These attractive forces may include:

• Hydrogen bonding
• Dipole interactions
• Van der Waals forces

For example, water molecules are polar and can form hydrogen bonds with glass, which is also polar. This strong attraction causes water to spread on glass surfaces.

Adhesion in Liquids

Adhesion controls how a liquid behaves when placed on a solid surface.

• Strong adhesion → liquid spreads
  Example: water spreading on clean glass.
• Weak adhesion → liquid forms droplets
  Example: water forming beads on waxed car surfaces.

Adhesion is responsible for the wetting of surfaces. When adhesion is high, liquids wet the surface well. When adhesion is low, liquids do not wet the surface.

Difference Between Adhesion and Cohesion

Understanding adhesion becomes easier when compared with cohesion.

• Adhesion is attraction between different substances.
• Cohesion is attraction between same substances.

Both forces combine in many natural processes like capillarity.

Adhesion and Capillary Action

Adhesion plays a major part in capillarity. When adhesive forces between the liquid and the tube walls are stronger than cohesive forces, the liquid rises in the tube. Water rises in narrow plant vessels (xylem) due to strong adhesion to the walls.

Factors Affecting Adhesion

Several factors influence how strongly two substances stick to each other:

1. Nature of the Liquid

Different liquids have different molecular structures, affecting their adhesion.

• Water shows strong adhesion to glass because both are polar.
• Oil shows weak adhesion to glass because oil is nonpolar.

2. Nature of the Solid Surface

A clean, polar surface increases adhesion, while an oily or waxy surface decreases it.

Example:

• A glass cleaned with detergent shows strong adhesion with water.
• A leaf with waxy coating shows weak adhesion.

3. Temperature

Temperature affects how molecules interact.

• Higher temperature → molecules move faster → weaker adhesion
• Lower temperature → molecules move slower → stronger adhesion

This explains why warm water spreads more easily than cold water.

4. Surface Roughness

A rough surface provides more contact area, which increases adhesion.
A smooth surface reduces adhesion.

Example:

• Tape sticks better to rough cardboard than to a shiny plastic sheet.

5. Impurities

Impurities may reduce adhesion by coating the surface or interfering with molecular attraction.

Example:
Dust on a surface reduces water adhesion.

Examples of Adhesion in Daily Life

Adhesion is seen in many everyday activities:

1. Water Sticking to Glass

After washing dishes, water clings to the glass walls due to adhesion.

2. Painting a Wall

Paint adheres to walls because of strong adhesion between paint particles and wall surface.

3. Writing with Ink

Ink spreads on paper due to adhesion between ink molecules and paper fibers.

4. Cellulose and Water Interaction

Water sticks to the surface of plant cells, helping transport water in plants.

5. Adhesive Tapes

Glue works because it adheres to different surfaces.

6. Dew Drops on Leaves

Some leaves have waxy surfaces that reduce adhesion, making droplets form beads.

Importance of Adhesion

Adhesion is essential in many natural and practical processes:

• Helps plants transport water upward
• Allows liquids to wet surfaces and spread
• Helps ink, glue, and paint attach to surfaces
• Plays a role in chromatography
• Helps soil absorb water
• Important in medical bandages and tapes

Adhesion makes many everyday tasks possible and is widely used in industries, biology, chemistry, and engineering.

Conclusion

Adhesion is the attractive force between molecules of different substances. It determines whether a liquid will spread on a surface or form droplets. Adhesion works along with cohesion in processes like capillarity, surface wetting, and fluid movement. It is influenced by temperature, surface nature, and molecular structure. Understanding adhesion helps explain many natural and practical activities such as painting, writing, water movement in plants, and the working of adhesive materials.