Lecture Notes: Surface Tension and Related Concepts

Surface Tension

• Definition:

  • Surface tension is a property of the surface of a liquid that allows it to resist external force.
  • It is due to the cohesive forces among liquid molecules.

• Example:

  • A needle can rest on the surface of water due to surface tension.
  • It is not floating but supported by the tension.

• Cause:

  • Water molecules are attracted to each other, forming hydrogen bonds (cohesion).
  • Molecules on the surface have fewer neighbors, allowing them to form tighter bonds, leading to surface tension.

Cohesion

• Definition:

  • The attraction between like molecules.
  • In liquids, it leads to the molecules being pulled together.

• Effect on Surface Tension:

  • Molecules in the bulk are restricted by surrounding molecules.
  • Surface molecules are less restricted, forming tighter, cohesive bonds.

Practical Applications of Surface Tension

• Bile in Urine Test:

  • Surface tension is lowered by bile, indicating liver function.

• Camping and Tents:

  • Do not touch the inside surface of a rain-covered tent to prevent breaking surface tension and allowing water leakage.

• Washing Hands:

  • Detergents reduce surface tension, allowing water to spread more easily, cleaning better.

Adhesion

• Definition:

  • The attraction between different substances, such as water and container walls.

• Meniscus Formation:

  • The curved surface of a liquid in a container is due to adhesion.
  • The adhesive force pulls water up the sides of the container.

Capillary Action

• Definition:

  • The ability of a liquid to flow in narrow spaces without the assistance of external forces.

• Mechanism:

  • Arises due to adhesion between the liquid and the walls of the container.
  • The smaller the diameter of the tube, the higher the rise of the liquid.

• Importance:

  • Significant in biological systems and other practical scenarios.