Chapter 3: Water and Life

Properties of Water

Water has four emergent properties due to hydrogen bonding, making it essential for life:

1. Cohesion of Water

   • Cohesion: Water molecules stick together due to hydrogen bonding.

   • Adhesion: Water molecules stick to other substances (e.g., plant cell walls).

   • Surface Tension: A measure of how difficult it is to break the surface of a liquid. Water has high surface tension due to hydrogen bonds, allowing insects like spiders to walk on water.

2. Ability to Moderate Temperature

   • Heat vs. Temperature:

     • Heat: The total kinetic energy of molecules in a substance.

     • Temperature: The average kinetic energy of molecules.

   • Specific Heat: Water has a high specific heat (1 cal/g/°C), meaning it resists temperature changes. This helps stabilize ocean and body temperatures.

   • Heat of Vaporization: Water has a high heat of vaporization, meaning it requires a lot of energy to change from liquid to gas. Evaporative cooling (e.g., sweating) helps organisms maintain temperature.

3. Expansion Upon Freezing

   • Why does ice float?

     • Water expands when frozen due to the arrangement of hydrogen bonds, making ice less dense than liquid water.

     • If ice sank, bodies of water would freeze solid, making life impossible.

4. Versatility as a Solvent

   • Solution: A homogeneous mixture of solute dissolved in a solvent.

   • Solute vs. Solvent:

     • Solvent: The substance that dissolves another (e.g., water).

     • Solute: The substance being dissolved (e.g., salt).

   • Hydrophilic vs. Hydrophobic:

     • Hydrophilic substances (water-loving) dissolve in water (e.g., salt, sugar).

     • Hydrophobic substances (water-fearing) do not dissolve in water (e.g., oils, fats).

Avogadro’s Number

• 1 mole (mol) = 6.02 × 10²³ molecules.

• A mole represents an exact number of molecules, just like a dozen represents 12 objects.

Acids vs. Bases

• Acid: A substance that increases H⁺ (hydrogen ion) concentration in a solution.

  • Acidic solutions have pH < 7.

• Base: A substance that reduces H⁺ concentration.

  • Basic solutions have pH > 7.

• pH Scale: Measures H⁺ concentration; a lower pH means higher acidity.

Ocean Acidification & Acidic Precipitation

• Ocean Acidification:

  • CO₂ from fossil fuels dissolves in seawater, forming carbonic acid.

  • This lowers ocean pH, reducing carbonate availability for coral reefs and marine organisms.

  • The ocean has become 30% more acidic in the last 200 years.

• Acidic Precipitation:

  • Caused by sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) from burning fossil fuels.

  • Reacts with water in the atmosphere, forming acid rain (pH < 5.2).

  • Damages aquatic ecosystems and soil chemistry.