Chapter 3 Chemistry of Water

Chapter 3: Water and Life

Structure of Water

• Chemical Composition: Water (H2O) consists of two hydrogen (H) atoms and one oxygen (O) atom.

• Polar Covalent Bonds: In water, electrons in polar covalent bonds are closer to the oxygen atom than to hydrogen, making water a polar molecule. This polarity results in uneven charge distribution.

  • Hydrogen Bonding: Polar water molecules can form hydrogen bonds with each other due to their polarity.

Properties of Water

• Water exhibits four key properties that facilitate life:

  1. Cohesive Behavior

  2. Ability to Moderate Temperature

  3. Expansion upon Freezing

  4. Versatility as a Solvent

1. Cohesion of Water Molecules

• Hydrogen bonds create cohesion among water molecules, which contributes to a high surface tension.

• Surface Tension: The measure of how difficult it is to stretch or break the surface of a liquid.

2. Moderation of Temperature by Water

• Water can absorb and release heat effectively without substantial temperature change.

• Kinetic energy: Associated with the random motion of atoms, contributes to thermal energy in water.

  • Specific Heat: Water has a specific heat of 1 cal/(g · °C).

  • To heat 1 liter of water from 30ºC to 40ºC:

    • 1L = 1000g

    • Calculation: 10ºC × 1000g = 10,000 calories.

3. Expansion upon Freezing

• Water is less dense as a solid (ice) than as a liquid, allowing ice to float.

• Importance: If ice sank, aquatic ecosystems would freeze solid, threatening life on Earth.

4. Water: The Solvent of Life

• Water is a versatile solvent due to its polarity, allowing it to dissolve a variety of substances.

  • Hydration Shells: When ionic compounds are dissolved in water, they are surrounded by a shell of water molecules.

  • Water also dissolves large polar molecules, such as proteins, if they contain ionic and polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with an affinity for water (e.g., salts, sugars).

• Hydrophobic: Substances that repel water (e.g., oils).

Concentration of Solutions

• Molarity (M): Number of moles of solute per liter of solution.

  • Example: A 5M glucose solution contains 5 moles of glucose per liter.

• Molecular Mass: Sum of the masses of all atoms in a molecule (e.g., glucose = 180 daltons).

Acids and Bases

• Acid: Increases H+ concentration in a solution (e.g., strong acids fully dissociate in water).

• Base: Reduces H+ concentration (e.g., strong bases fully dissociate).

The pH Scale

• Definition: pH = -log[H+].

  • Neutral solution: [H+] = [OH-] = 10^-7, pH = 7.

  • Acidic solutions: pH < 7.

  • Basic solutions: pH > 7.

Buffers

• Buffers minimize changes in H+ and OH- concentrations in solutions.

  • Most buffer solutions include a weak acid and its corresponding base, helping maintain stable pH levels.

Ocean Acidification

• Impact: Human activities, particularly fossil fuel combustion, increase CO2 levels, leading to ocean acidification as CO2 reacts with seawater to form carbonic acid.

• Consequence: Decreased carbonate ions available for calcification in marine organisms, potentially altering entire ecosystems, particularly coral reefs.