Lecture Notes: Surface Tension and Related Concepts

Surface Tension

• Description: When a needle is carefully placed on water, it stays on the surface due to surface tension, not because it floats.
  • Density: The needle is denser than water and should sink if not for surface tension.
  • Breaking Surface Tension: Applying pressure breaks the tension, causing the needle to sink.

Causes of Surface Tension

• Cohesion: Water molecules are attracted to each other (hydrogen bonding), creating surface tension.
  • At Surface: Molecules at the surface can form tighter bonds as they are less restricted compared to molecules in the bulk.
  • Support Pressure: This tension allows the support of weight, like a needle.

Practical Applications

• Medical Testing: Presence of bile in urine lowers surface tension, indicating liver function.
• Camping: Touching tent fabric when wet breaks surface tension, causing leaks.
• Hand Washing: Detergents lower water's surface tension, allowing better penetration and cleaning.

Adhesion

• Definition: Water molecules are attracted to other substances, not just each other.
  • Meniscus: Water curves up at the edges in a container due to adhesion.
  • Measurement Caution: The meniscus affects accurate liquid measurement.

Capillary Action

• Mechanism: Adhesive force causes liquid to climb walls of a container, like a straw.
  • Effect: More pronounced in smaller tubes, elevating the water level inside.
  • Importance: Crucial for fluid transport in biological systems and other applications.