Chemistry Lecture: Importance of Water for Life

Structure of Water

• Bonds: Polar covalent bonds
• Polarity: Electrons spend more time near the electronegative oxygen atom
  • Partial negative charge around oxygen
  • Partial positive charges around hydrogen

Properties of Water

• Adhesion: Attraction between different substances
  • Example: Water rises in a glass pipette due to attraction to charged glass
• Cohesion: Attraction between like substances (water to water)
  • Hydrogen Bonds: Slightly positive hydrogen of one molecule to slightly negative oxygen of another
  • Up to four hydrogen bonds per water molecule
  • Explains why water forms beads and has surface tension

Surface Tension

• Water molecules at the surface form hydrogen bonds with molecules next to and below them
• This creates a “skin” on the water surface
• Example: Water strider walking on water

Heat of Vaporization

• Definition: Amount of heat required for liquid to become gas
• High Heat of Vaporization: Due to hydrogen bonds
  • Water molecules need energy to break hydrogen bonds and vaporize

Freezing and Ice Formation

• Temperature Effects:
  • At 100°C: Molecules are highly energetic, in gaseous state
  • At 50°C: Molecules less energetic, closer together, form/break hydrogen bonds slower
  • At 0°C: Molecules slow down, form maximum hydrogen bonds, create crystal lattice
• Density of Ice: Ice is less dense than water, floats on water
  • Important for life in aquatic environments

Solutions and Solubility

• Solution: Mixture of solvent (water) and solute (substance that dissolves in water)
• Polarity and Solubility:
  • Polar substances (hydrophilic) dissolve in water
  • Non-polar substances (hydrophobic) do not dissolve

Concentration and Molarity

• Molarity: Number of moles of solute per liter of solution
  • Example: 1 molar solution of potassium chloride (KCl) = 74.5 grams/liter

Acids and Bases

• Acids: Add hydrogen ions (H+) to a solution
• Bases: Reduce hydrogen ions in a solution
• pH Scale: Measures acidity/basicity
  • pH < 7: Acidic
  • pH > 7: Basic
  • pH = 7: Neutral (pure water)
• Calculating pH:
  • Negative logarithm of hydrogen ion concentration
  • Example: [H+] = 0.001 (10^-3), pH = 3 (acidic)