Lecture Notes: Water's Unique Properties and Polarity

Introduction to Water and Polarity

• Water Composition: Made of 1 oxygen atom and 2 hydrogen atoms.
• Importance: Essential to life, makes up ~60% of adult human body weight.
• Polarity Concept: Refers to the unequal sharing of electrons within a molecule.

Molecular Structure and Polarity

• Water Molecule Bonding:
  • Oxygen is larger, more protons = stronger attraction to electrons.
  • Hydrogen is smaller, fewer protons = weaker attraction.
  • Result: Oxygen attracts more electrons, behaves negatively; Hydrogen behaves positively.
• Polar Covalent Bonds: Electrons shared unequally between oxygen and hydrogen.

Hydrogen Bonding

• Inter-Molecular Attraction:
  • Negative oxygen from one water molecule attracts positive hydrogen from another.
  • Hydrogen Bonds: Special type of bond between water molecules, also possible with other polar or ionic substances.

Cohesion and Adhesion

• Cohesion: Water's ability to stick to itself.
• Adhesion: Water's ability to stick to other substances.

Surface Tension and Insect Locomotion

• Surface Tension:
  • Created by hydrogen bonds forming a thin film on water surface.
  • Enables light insects to walk on water.
  • Human weight breaks the bonds, causing sinking.

Ice Density and Freezing

• Density Anomaly:
  • Ice is less dense than liquid water due to hydrogen bonds keeping molecules farther apart in ice.
  • Ice is ~9% less dense, allowing it to float.
• Freezing from Top Down:
  • Lakes freeze from the top down, allowing aquatic life to survive winter.

Conclusion

• Unique Properties of Water: Result from its polarity and hydrogen bonding.
• Significance: The polarity of water makes it special from cellular functions to global ecosystems.