Water and Life

Introduction

Water is essential for all known forms of life. Its unique chemical and physical properties make it indispensable for biological processes and the maintenance of life on Earth. This chapter explores the molecular structure of water, its emergent properties, and their significance for living organisms.

Structure of Water

Polar Covalent Bonds and Molecular Polarity

The water molecule (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. The electrons in these bonds are shared unequally, resulting in a polar covalent bond.

• Oxygen is more electronegative than hydrogen, pulling shared electrons closer and giving oxygen a partial negative charge (δ−) and hydrogen a partial positive charge (δ+).

• This unequal sharing makes water a polar molecule, with an uneven distribution of charge.

Example: The polarity of water allows it to interact with other polar molecules and ions, making it an excellent solvent.

Hydrogen Bonding

The polarity of water molecules enables them to form hydrogen bonds with each other. These are weak attractions between the partially positive hydrogen atom of one molecule and the partially negative oxygen atom of another.

• Hydrogen bonds are individually weak but collectively strong, giving water many of its unique properties.

Example: Hydrogen bonding is responsible for water's high surface tension and its ability to moderate temperature.

Emergent Properties of Water

Overview

Water exhibits four key properties that make it vital for life:

• Cohesive behavior

• Ability to moderate temperature

• Expansion upon freezing

• Versatility as a solvent

Cohesion and Adhesion

Cohesion refers to the attraction between water molecules due to hydrogen bonding. Adhesion is the attraction between water molecules and other substances.

• Cohesion results in high surface tension, making it difficult to break the surface of water.

• Cohesion helps transport water and dissolved nutrients against gravity in plants.

• Adhesion between water and plant cell walls helps counteract gravity, aiding in water movement from roots to leaves.

Example: Water droplets forming beads on a surface demonstrate cohesion; water climbing up a paper towel shows adhesion.

Moderation of Temperature

Water can absorb or release large amounts of heat with only slight changes in its own temperature, due to its high specific heat.

• Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• For water, the specific heat is 1 cal/(g·°C).

• Hydrogen bonding causes water to resist temperature changes: heat is absorbed to break bonds and released when bonds form.

Example: Coastal areas experience milder climates because large bodies of water absorb and release heat slowly.

Expansion Upon Freezing

Unlike most substances, water is less dense as a solid (ice) than as a liquid.

• At 0°C, water molecules form a crystalline lattice, keeping them farther apart than in liquid water.

• This makes ice about 10% less dense than liquid water, allowing it to float.

Example: Floating ice insulates the water below, protecting aquatic life during cold periods.

Versatility as a Solvent

Water is known as the universal solvent due to its ability to dissolve a wide variety of substances.

• Its polarity allows it to surround and separate ions and polar molecules.

• Water forms hydration shells around dissolved ions, facilitating their dispersion.

Example: Table salt (NaCl) dissolves in water as Na+ and Cl− ions become surrounded by water molecules.

Summary Table: Properties of Water

Additional info: The above table summarizes the main emergent properties of water and their significance for life.