What is cohesion?

Short Answer

Cohesion is the force of attraction between molecules of the same substance. It keeps the molecules close to each other and helps maintain the shape and volume of a liquid or solid. For example, water molecules attract each other strongly through hydrogen bonding, which is why water forms droplets.

Cohesion is responsible for many natural effects, such as surface tension, capillarity, and the round shape of water drops. It is one of the key molecular forces that control how substances behave in different states of matter.

Detailed Explanation

Cohesion

Cohesion is the attractive force that holds molecules of the same substance together. It arises due to intermolecular forces such as hydrogen bonding, van der Waals forces, and dipole interactions. Cohesion helps keep matter in a stable form by preventing the molecules from drifting apart easily. This force plays a crucial role in liquids and solids, influencing properties like surface tension, viscosity, capillarity, and the formation of droplets.

In simple words, cohesion makes similar molecules stick to each other. It is the reason water forms beads on a smooth surface and why certain liquids maintain their shape. Without cohesion, liquids would not hold together, and solids would not be rigid.

Molecular Basis of Cohesion

Cohesion comes from the attractive forces that operate between molecules of the same kind. These forces depend on the nature of the molecules:

  • Polar molecules (like water) show strong cohesion due to hydrogen bonds.
  • Nonpolar molecules (like oil) show weaker cohesion because they depend on van der Waals forces.

Cohesion helps molecules stay together, giving shape and firmness to matter.

How Cohesion Works in Liquids

In liquids, cohesion provides the internal force that allows the liquid to resist external stress. Molecules in a liquid are constantly moving, but cohesion keeps them from separating completely.

This force is visible in:

  • Formation of droplets
  • Surface tension
  • Flow behavior
  • Capillary action (when combined with adhesion)

When cohesion is strong, the liquid becomes thick and forms stable drops. When cohesion is weak, the liquid spreads easily.

Cohesion in Solids

In solids, cohesion is even stronger and keeps molecules tightly packed in fixed positions. This makes solids rigid and gives them definite shape and volume. Metals, crystalline solids, and minerals all show strong cohesion due to strong bonding forces.

Examples of Cohesion in Daily Life

Cohesion can be seen in many everyday situations:

  1. Water Droplets

Water drops on leaves or glass stay in round shape because water molecules attract each other strongly.

  1. Raindrop Shape

Raindrops form spherical shapes during rainfall due to cohesive forces among water molecules.

  1. Surface Tension

Water has high surface tension because cohesion pulls surface molecules inward, creating a tight surface film.

  1. Mercury Beads

Mercury forms round beads on surfaces due to strong cohesion caused by metallic bonding.

  1. Flow of Liquids

Honey flows slowly due to strong cohesive forces between sugar molecules, while water flows quickly due to moderate cohesion.

  1. Capillary Action

Cohesion works with adhesion to pull water upward in thin tubes, helping plants transport water from roots to leaves.

Difference Between Cohesion and Adhesion

To understand cohesion better, it is helpful to distinguish it from adhesion.

Cohesion

  • Attraction between similar molecules
  • Example: water sticking to water

Adhesion

  • Attraction between different molecules
  • Example: water sticking to glass

Both forces work together in many natural processes, such as capillarity.

Factors Affecting Cohesion

Several factors influence the strength of cohesive forces:

  1. Nature of the Molecules

Polar molecules show strong cohesion; nonpolar molecules show weak cohesion.

  1. Temperature

As temperature increases, molecular motion increases, reducing cohesion.
At lower temperatures, molecules move slowly, so cohesion increases.

  1. Impurities

Impurities may weaken or strengthen cohesion depending on their nature.

  1. Intermolecular Forces

Hydrogen bonding, dipole interactions, and van der Waals forces determine the strength of cohesion.

Importance of Cohesion

Cohesion is important in many natural and scientific processes:

  • Maintains droplet shape
  • Helps insects walk on water
  • Allows capillary rise in plants
  • Contributes to states of matter
  • Important in fluid flow and viscosity
  • Influences weather patterns and cloud formation
  • Helps determine boiling and melting behavior

Without cohesion, water would not form drops, trees could not transport water upward, and many natural processes would not function properly.

Conclusion

Cohesion is the attractive force between molecules of the same substance. It helps liquids form droplets, solids stay firm, and ensures stability in all states of matter. Cohesion influences properties like surface tension, viscosity, and capillarity. It plays a vital role in nature, everyday life, and scientific processes. By understanding cohesion, we can better explain how molecules interact and how substances maintain their shape and behavior.