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

Water and Life

Introduction

Water is fundamental to all known forms of life. Its unique physical and chemical properties make it indispensable for biological processes, from cellular structure to ecosystem function. This chapter explores the molecular structure of water, its emergent properties, and its role in supporting life on Earth.

Water Properties

Water is Critical for Life

• Polarity: Water is a polar molecule, meaning it has an uneven distribution of charge due to the difference in electronegativity between oxygen and hydrogen atoms.

• Physical and Chemical Properties: Water exhibits unique behaviors such as cohesion, adhesion, high specific heat, and solvent capabilities.

• Acid/Base Behavior: Water can act as both an acid and a base, participating in important chemical reactions in biological systems.

Water Supports All of Life

• Water is the only natural substance on Earth that exists in all three physical states of matter: solid, liquid, and gas.

• Its emergent properties make Earth habitable for life.

• The structure of the water molecule allows it to interact with other molecules, facilitating a wide range of biological processes.

Structure and Chemistry of Water

Molecular Structure and Polarity

• Water (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom.

• The oxygen atom is more electronegative, pulling shared electrons closer and creating a partial negative charge near the oxygen and a partial positive charge near the hydrogens.

• This polarity enables water molecules to form hydrogen bonds with each other and with other polar molecules.

Hydrogen Bonding

• Hydrogen bonds are weak attractions between the partially positive hydrogen of one water molecule and the partially negative oxygen of another.

• These bonds are transient, constantly forming and breaking, but collectively they give water its unique properties.

Emergent Properties of Water

Four Key Properties

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

• Cohesion: Hydrogen bonds hold water molecules together, allowing for surface tension and the transport of water in plants.

• Adhesion: Water molecules can also stick to 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."

Moderation of Temperature

• Water absorbs heat from warmer air and releases stored heat to cooler air, helping to stabilize temperatures in organisms and environments.

• Kinetic Energy: The energy of motion; in water, this is reflected as thermal energy.

• Temperature: Represents the average kinetic energy of molecules.

• Heat: Thermal energy transferred from one body to another.

Units of Heat

• Calorie (cal): The amount of heat required to raise 1 g of water by 1°C.

• Kilocalorie (kcal): 1,000 calories; used in food energy.

• Joule (J): SI unit of energy; 1 cal = 4.184 J.

Specific Heat of Water

• Definition: The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C.

• Water's specific heat is 1 cal/(g·°C), which is higher than most substances.

• This high specific heat is due to hydrogen bonding: heat is absorbed to break bonds and released when bonds form.

• As a result, water resists temperature changes, stabilizing climates and living organisms.

Evaporative Cooling

• Evaporation: The transformation of a substance from liquid to gas.

• Heat of Vaporization: The amount of heat a liquid must absorb for 1 g to be converted to gas.

• Evaporative cooling occurs as the surface of a liquid cools during evaporation, helping organisms regulate temperature.

Expansion Upon Freezing

• Water is less dense as a solid than as a liquid, so ice floats.

• In ice, hydrogen bonds are stable and hold molecules further apart, creating a crystalline structure.

• This property insulates bodies of water, protecting aquatic life in cold climates.

Versatility as a Solvent

• Water is known as the "universal solvent" because it can dissolve a wide variety of substances.

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

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

• Solute: The substance that is dissolved.

• When ionic compounds dissolve, each ion is surrounded by a hydration shell of water molecules.

• Large polar molecules, such as proteins, can also dissolve if they have ionic and polar regions.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances that have an affinity for water ("water-loving").

• Hydrophobic: Substances that do not have an affinity for water ("water-fearing"), often nonpolar molecules like oils.

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-] = $1 \times 10^{-7}$ M.

• The product of these concentrations is always $[H^+][OH^-] = 1 \times 10^{-14}$ at 25°C.

Acids and Bases

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

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

• Strong acids and bases dissociate completely in water; weak acids and bases reversibly release and accept hydrogen ions.

The pH Scale

• pH is a measure of hydrogen ion concentration, defined as $\mathrm{pH} = -\log_{10}[H^+]$.

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

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

Table: pH Values of Some Aqueous Solutions

Solution	pH
Battery acid	1
Lemon juice	2
Vinegar	3
Tomato juice	4
Black coffee	5
Pure water	7
Seawater	8
Household bleach	13
Oven cleaner	14

Buffers

• Buffers are substances that minimize changes in concentrations of H+ and OH- in a solution.

• They help maintain a stable pH in biological systems, crucial for cellular function.

• Example: The bicarbonate buffer system in blood.

Acidification and Environmental Impact

• Human activities, such as burning fossil fuels, increase atmospheric CO2, some of which dissolves in oceans, forming carbonic acid and lowering ocean pH (ocean acidification).

• This process reduces carbonate ion concentration, affecting marine organisms that build shells and skeletons from calcium carbonate.

Concentration and Molarity

Calculating Concentrations

• 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.

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

Definitions and Types of Mixtures

• Solvent: The liquid in which a substance dissolves.

• Solute: The dissolved substance.

• Solution: A homogeneous mixture of solvent and solute.

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

• Electrolytes: Salts, acids, or bases that form ions in water and conduct electricity.

• Nonelectrolytes: Substances like sugar that dissolve in water but do not form ions.

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

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

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the table of pH values and the definition of molarity.