Chapter 3 – Water and Life

Key Terms and Definitions

This section introduces essential vocabulary for understanding chemical bonds, water's properties, and their biological significance.

• Electronegativity: The attraction of an atom for the electrons of a covalent bond.

• Polar covalent bond: A covalent bond between atoms with different electronegativities, resulting in unequal sharing of electrons and partial charges.

• Nonpolar covalent bond: A covalent bond between atoms with similar electronegativities, resulting in equal sharing of electrons and no partial charges.

• Polar molecule: A molecule with an uneven distribution of charges, such as H2O.

• Nonpolar molecule: A molecule with an even distribution of charges.

• Hydrogen bond: A weak chemical bond formed when a slightly positive hydrogen atom in a polar covalent bond is attracted to a slightly negative atom in another molecule or region of the same molecule.

• Ion: An atom or group of atoms that has gained or lost electrons, thus acquiring a charge.

• Anion: A negatively charged ion.

• Cation: A positively charged ion.

• Hydrophilic: Having an affinity for water; water-loving.

• Hydrophobic: Repelling water; water-fearing.

• Solute: A substance that is dissolved in a solution.

• Solvent: The dissolving agent of a solution (water is the universal solvent).

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

• Cohesion: The linking together of like molecules, often by hydrogen bonds.

• Adhesion: The clinging of one substance to another, such as water to plant cell walls.

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

• Acid: A substance that increases the hydrogen ion concentration of a solution.

• Base: A substance that reduces the hydrogen ion concentration of a solution.

• pH: A measure of hydrogen ion concentration, ranging from 0 (acidic) to 14 (basic).

Chemical Bonds and Electronegativity

Polar and Nonpolar Covalent Bonds

Chemical bonds are formed by the sharing or transfer of electrons between atoms. The type of bond depends on the difference in electronegativity between the atoms involved.

• Electronegativity is the tendency of an atom to attract electrons in a bond. The greater the difference in electronegativity, the more polar the bond.

• Polar covalent bonds occur when atoms have an electronegativity difference between 0.4 and 2.0. Electrons are shared unequally, resulting in partial positive and negative charges on the atoms.

• Nonpolar covalent bonds occur when atoms have similar electronegativities (difference less than 0.4), resulting in equal sharing of electrons and no partial charges.

• Example: In a water molecule (H2O), oxygen is more electronegative than hydrogen, so the shared electrons spend more time near the oxygen atom, giving it a partial negative charge and the hydrogens partial positive charges.

Ionic and Hydrogen Bonds

Other important types of chemical bonds include ionic and hydrogen bonds.

• Ionic bonds form when one atom donates an electron to another, resulting in oppositely charged ions (cations and anions) that attract each other.

• Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom.

• Relative strength: Ionic bonds > Covalent bonds > Hydrogen bonds (weakest).

• Example: Table salt (NaCl) forms when sodium donates an electron to chlorine, creating Na+ and Cl- ions.

Structure and Polarity of Water

Water Molecule Structure

A water molecule consists of two hydrogen atoms and one oxygen atom, forming a bent or V-shaped structure. The oxygen atom is more electronegative, pulling shared electrons closer and creating a polar molecule.

• Partial charges: Oxygen has a partial negative charge (δ-), and hydrogens have partial positive charges (δ+).

• Polarity: The unequal sharing of electrons makes water a polar molecule, allowing it to form hydrogen bonds with other polar molecules.

• Diagram: The oxygen atom is at the center with two hydrogens at an angle, showing partial charges (δ- on O, δ+ on H).

Hydrogen Bonding in Water

The polarity of water molecules allows them to form hydrogen bonds with each other and with other polar molecules.

• Each water molecule can form up to four hydrogen bonds with neighboring water molecules.

• Hydrogen bonds are responsible for many of water's unique properties, such as high cohesion, surface tension, and specific heat.

• Example: In ice, water molecules are held in a rigid lattice by hydrogen bonds, making ice less dense than liquid water.

Properties of Water Essential to Life

Cohesion, Adhesion, and Surface Tension

Hydrogen bonding gives water several important properties that are essential for life.

• Cohesion: Water molecules stick to each other, aiding in the transport of water in plants.

• Adhesion: Water molecules stick to other substances, such as cell walls, helping water move against gravity in plants.

• Surface tension: Water has a high surface tension, allowing small insects to walk on its surface.

Thermal Properties: Specific Heat and Evaporative Cooling

• High specific heat: Water can absorb or release large amounts of heat with little temperature change, helping to stabilize temperatures in organisms and environments.

• Evaporative cooling: As water evaporates, it removes heat from surfaces, helping organisms regulate body temperature.

• Equation: Where is heat absorbed or released, is mass, is specific heat, and is temperature change.

Density and Expansion Upon Freezing

• Water expands when it freezes, making ice less dense than liquid water. This causes ice to float, insulating aquatic life in winter.

• Biological importance: If ice sank, bodies of water would freeze from the bottom up, threatening aquatic ecosystems.

Water as a Solvent

• Water is a versatile solvent due to its polarity, dissolving many ionic and polar substances.

• Hydrophilic substances: Substances that dissolve easily in water (e.g., salts, sugars).

• Hydrophobic substances: Substances that do not dissolve in water (e.g., oils, fats).

• Example: Table salt (NaCl) dissolves in water as the polar water molecules surround and separate the ions.

Acids, Bases, and pH

• Acids: Increase the concentration of hydrogen ions () in solution.

• Bases: Reduce the concentration of hydrogen ions, often by accepting or releasing .

• pH scale: Measures the concentration of hydrogen ions; .

• Range: 0 (most acidic) to 14 (most basic), with 7 being neutral.

Classification and Comparison of Chemical Bonds

Summary of Water's Life-Supporting Properties

• Water moderates temperature due to high specific heat.

• Displays cohesion and adhesion, aiding in water transport in plants.

• Expands upon freezing, causing ice to float and insulate aquatic life.

• Dissolves many polar molecules, making it the universal solvent.

• Dissociates into and , allowing for acid-base chemistry essential to life.

Additional info: Water's ability to form hydrogen bonds is directly linked to its polarity, which is a result of the difference in electronegativity between oxygen and hydrogen. This underlies all of water's unique properties that are essential for life.