Understanding Water: A Key Molecule

Basics of Chemistry Recap

• Covalent vs Ionic Bonds
  • Covalent bonds involve sharing of electrons.
  • Ionic bonds involve transfer of electrons.
• Polar vs Nonpolar Covalent Bonds
  • Polar covalent bonds have unequal sharing leading to partial charges.
  • Nonpolar covalent bonds have equal sharing of electrons.

Water: An Essential Molecule

• Water makes up a significant portion of living organisms (e.g., a fetus is 90% water).
• Vital for biological processes and ecosystems.

Properties of Water

Polarity and Hydrogen Bonding

• Water is a polar molecule.
  • Oxygen is partially negative, hydrogen is partially positive.
• Hydrogen bonds
  • Allow water molecules to bond with each other.
  • Demonstrated by water beading on non-polar surfaces like wax.

Cohesion and Adhesion

• Cohesion
  • Attraction between water molecules due to hydrogen bonding.
  • Leads to surface tension (e.g., water insects walking on water).
• Adhesion
  • Attraction between water molecules and other materials.
  • Causes phenomena like the meniscus in a graduated cylinder.
  • Important in plant water transport through capillary action.

Temperature Moderation

• High Specific Heat
  • Water absorbs a lot of heat before it increases in temperature.
  • Stabilizes climates and aquatic environments.
• Evaporative Cooling
  • Absorbs heat when evaporating, cooling the environment.
  • Mechanism behind sweating and panting.

Density and Ice

• Ice Floats on Water
  • Ice is less dense due to hydrogen bonding creating a lattice structure.
  • Crucial for aquatic life survival in cold climates.

Solvent Properties

• Excellent Solvent
  • Dissolves many substances due to its polarity.
  • Interacts with ionic and polar substances, aiding in dissolving salts.

Hydrophilic vs Hydrophobic

• Hydrophilic (Water-loving)
  • Substances that dissolve in water.
• Hydrophobic (Water-fearing)
  • Substances that do not dissolve in water (e.g., oil).

Ionization and pH

• Water Ionization
  • Forms H⁺ and OH⁻, impacting pH.
• pH Scale
  • Measures acidity based on H⁺ concentration.
  • Acids: High H⁺ concentration (pH < 7).
  • Bases: Low H⁺ concentration (pH > 7).

Importance in Biology

• Biological Molecules
  • Hydrogen bonds critical for structure and function.
  • pH changes can disrupt molecular interactions.

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Next Focus

• Upcoming lectures will discuss the molecules of life.