Chapter 3: Water and Life – Properties, Chemistry, and Biological Importance

Chapter 3: Water and Life

Overview

This chapter explores the unique properties of water, its chemical structure, and its critical role in supporting life on Earth. Students will learn about water's physical and chemical characteristics, its behavior as a solvent, and its importance in biological systems, including its role in acids, bases, and buffer systems.

Water Properties

Importance of Water for Life

• Water is critical for life: All known forms of life depend on water for survival and biological processes.

• Polarity: Water is a polar molecule, meaning it has a partial positive charge on one side (hydrogen) and a partial negative charge on the other (oxygen).

• Physical and chemical properties: Water exhibits unique behaviors due to its molecular structure and hydrogen bonding.

Water Supports All of Life

• Three physical states: Water is the only natural substance that exists in all three states—solid, liquid, and gas—under Earth's normal conditions.

• Emergent properties: Water's unique properties, such as cohesion, high specific heat, and solvent abilities, make Earth habitable.

• Molecular structure: The bent shape and polar covalent bonds of water molecules allow them to interact with each other and other substances via hydrogen bonds.

Chemical Structure and Bonding in Water

Polarity and Hydrogen Bonding

• Polar covalent bonds: In water, electrons are shared unequally between oxygen and hydrogen, making oxygen slightly negative (δ-) and hydrogen slightly positive (δ+).

• Hydrogen bonds: The partial charges allow water molecules to form hydrogen bonds with each other, which are weaker than covalent bonds but crucial for water's properties.

• Dynamic bonding: Hydrogen bonds in liquid water constantly form, break, and reform, giving water its fluidity and cohesion.

Emergent Properties of Water

1. Cohesion and Adhesion

• Cohesion: The attraction between water molecules due to hydrogen bonding. This helps transport water in plants against gravity.

• Adhesion: The attraction between water molecules and other substances, such as plant cell walls, aiding in capillary action.

• Surface tension: Water has a high surface tension, making it difficult to break the surface. This allows small organisms to "walk on water."

2. Moderation of Temperature

• Thermal energy: The kinetic energy associated with the random movement of atoms or molecules.

• Specific heat: The amount of heat required to change the temperature of 1 g of a substance by 1°C. Water's specific heat is high (1 cal/g°C), allowing it to resist temperature changes.

• Heat of vaporization: The heat required to convert liquid water to gas. Water's high heat of vaporization enables evaporative cooling, stabilizing temperatures in organisms and environments.

Equation:

$\text{Specific heat of water} = 1 \ \text{cal} / (\text{g} \cdot ^\circ\text{C})$

3. Expansion Upon Freezing

• Ice is less dense than liquid water: As water freezes, hydrogen bonds stabilize and keep molecules further apart, making ice float.

• Biological significance: Floating ice insulates water below, protecting aquatic life in cold climates.

4. Versatility as a Solvent

• Solution: A homogeneous mixture of two or more substances.

• Solvent: The dissolving agent (water in aqueous solutions).

• Solute: The substance being dissolved.

• Hydration shell: Water molecules surround and isolate ions or polar molecules, allowing them to dissolve.

• Hydrophilic substances: Attracted to water (e.g., salts, sugars).

• Hydrophobic substances: Repel water (e.g., oils, fats).

Acids, Bases, and pH

Dissociation of Water

• Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-).

• In pure water at 25°C: $[H^+] = [OH^-] = 10^{-7} \ \text{M}$

• The product of these concentrations is always $10^{-14}$ at 25°C.

Equation:

$[H^+][OH^-] = 10^{-14}$

Acids and Bases

• Acids: Substances that increase the concentration of H+ in a solution.

• Bases: Substances that reduce the concentration of H+, often by increasing OH-.

• Strong acids/bases: Dissociate completely in water.

• Weak acids/bases: Partially dissociate, acting as reversible proton donors or acceptors.

The pH Scale

• pH: A measure of hydrogen ion concentration, defined as $\text{pH} = -\log_{10}[H^+]$

• pH < 7: Acidic; pH > 7: Basic; pH = 7: Neutral

• Each pH unit represents a tenfold change in [H+].

Equation:

$\text{pH} = -\log_{10}[H^+]$

Buffers

• Buffers: Substances that minimize changes in pH by accepting or donating H+ ions.

• Example: The bicarbonate buffer system in blood helps maintain pH near 7.4.

Concentration and Molarity

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

• Molecular weight: The sum of atomic weights in a molecule (e.g., sucrose C12H22O11 = 342 Da/molecule).

• Avogadro's number: $6.02 \times 10^{23}$ molecules per mole.

Definitions and Types of Mixtures

• Solvent: The liquid in which a solute dissolves.

• Solute: The substance dissolved in a solvent.

• Solution: A homogeneous mixture of solute and solvent.

• Salts: Formed from the reaction of acids and bases (e.g., HCl + NaOH → NaCl + H2O).

• Electrolytes: Substances that form ions in water and conduct electricity (e.g., salts, acids, bases).

• Nonelectrolytes: Substances that dissolve in water but do not form ions (e.g., sugar).

• Heterogeneous mixtures: Not uniform throughout (e.g., living organisms).

• Homogeneous mixtures: Uniform throughout (e.g., salt water solution).

Summary Table: Key Properties of Water

Property	Description	Biological Importance
Cohesion	Attraction between water molecules	Enables water transport in plants
Adhesion	Attraction between water and other substances	Helps water climb plant cell walls
High Specific Heat	Resists temperature change	Stabilizes climate and body temperature
Expansion upon Freezing	Ice is less dense than liquid water	Insulates aquatic environments
Versatile Solvent	Dissolves many substances	Facilitates chemical reactions in cells

Example: Water as a Solvent

When table salt (NaCl) dissolves in water, the Na+ and Cl- ions are surrounded by water molecules, forming hydration shells. This allows ions and polar molecules to be transported in biological fluids.

Additional info:

• Water's role in hydrolysis and condensation reactions is essential for building and breaking down biological macromolecules.

• Ocean acidification, caused by increased CO2 absorption, threatens marine life by reducing carbonate ion availability for shell-building organisms.