The Chemical Context of Life & Water and Life: Study Notes

Chapter 2: The Chemical Context of Life

Introduction to Matter and Elements

All living organisms are composed of matter, which is anything that has mass and occupies space. Understanding the chemical basis of life begins with the study of elements, compounds, and the structure of atoms.

• Matter: Anything that has mass and takes up space.

• Element: A pure substance consisting of only one type of atom (e.g., Oxygen, Carbon).

• Compound: A substance formed by the chemical combination of two or more elements in fixed ratios (e.g., H2O).

Elements Essential for Life

• Four elements make up about 96% of living matter:

  • Carbon (C)

  • Hydrogen (H)

  • Oxygen (O)

  • Nitrogen (N)

• The remaining 4% includes:

  • Calcium (Ca)

  • Phosphorus (P)

  • Potassium (K)

  • Nitrogen (N) (Note: Nitrogen is also in the main 4%)

• Trace elements: Required in minute quantities but essential for life (e.g., Iron, Iodine).

Atoms and Subatomic Particles

• Atom: The smallest unit of an element that retains its properties.

• Subatomic particles:

  • Protons: Positively charged, found in the nucleus.

  • Neutrons: No charge, found in the nucleus.

  • Electrons: Negatively charged, orbit the nucleus in electron shells.

• Atomic nucleus: Contains protons and neutrons.

• Electron cloud: Region around the nucleus where electrons are found.

Atomic Number, Mass Number, and Isotopes

• Atomic number: Number of protons in an atom.

• Mass number: Sum of protons and neutrons.

• Atomic mass: Average mass of all isotopes of an element.

• Isotopes: Atoms of the same element with different numbers of neutrons.

  • Radioactive isotopes: Unstable, emit radiation; used in medical imaging, dating fossils, and tracing chemical pathways.

Energy Levels and Electron Configuration

• Energy: The capacity to do work.

• Potential energy: Stored energy due to position or structure.

• Electron shells: Energy levels where electrons are found.

• Valence electrons: Electrons in the outermost shell; determine chemical reactivity.

• Full valence shell: Atoms are stable when their outer shell is full.

• Orbitals: Specific regions within shells where electrons are likely to be found.

The Periodic Table

• Organizes elements by atomic number and properties.

• Allows determination of atomic number, atomic mass, and the number of protons, neutrons, and electrons for each element.

Chemical Bonds and Molecular Formulas

• Chemical bonds: Forces that hold atoms together in molecules.

• Structural formula: Shows arrangement of atoms (e.g., H–O–H for water).

• Molecular formula: Shows the number and type of atoms (e.g., H2O for water).

Types of Chemical Bonds

• Covalent bonds: Atoms share electrons.

  • Can be single, double, or triple bonds.

  • Electronegativity: Atom's ability to attract shared electrons.

  • Nonpolar covalent bond: Electrons shared equally.

  • Polar covalent bond: Electrons shared unequally, creating partial charges.

  • Examples: H2O (polar), O2 (nonpolar)

• Ionic bonds: Electrons are transferred from one atom to another, forming ions.

  • Cation: Positively charged ion.

  • Anion: Negatively charged ion.

  • Ionic compounds: Formed from cations and anions (e.g., NaCl).

• Van der Waals interactions: Weak attractions between molecules due to transient charges.

Molecular Recognition

• Molecules recognize each other by shape and surface charge (molecular fit).

Chemical Reactions

• Involve the making and breaking of chemical bonds.

• Reactants: Starting substances.

• Products: Substances formed.

• Example:

  • 2H2 + O2 → 2H2O

Chapter 3: Water and Life

Polarity of Water

Water is a polar molecule, meaning it has an uneven distribution of charge, which gives rise to its unique properties essential for life.

• Polar molecule: Has regions of partial positive and negative charge due to unequal sharing of electrons.

Properties of Water

• Cohesion: Water molecules stick to each other due to hydrogen bonding.

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

• Adhesion: Water molecules stick to other substances (e.g., water to glass).

• Capillary action: The movement of water up narrow tubes, due to cohesion and adhesion working together.

• High specific heat: Water can absorb or release a large amount of heat with only a slight change in its own temperature.

• Evaporative cooling: As water evaporates, it removes heat from surfaces (e.g., sweating).

Thermal Properties of Water

• Kinetic energy: Energy of motion.

• Heat: Total thermal energy transferred from one body to another.

• Temperature: Average kinetic energy of molecules.

• Calorie: Amount of heat needed to raise 1 g of water by 1°C.

• Specific heat: Amount of heat required to change the temperature of 1 g of a substance by 1°C.

• Heat of vaporization: Amount of heat needed to convert 1 g of liquid to gas.

Density of Water and Ice

• Ice is less dense than liquid water because hydrogen bonds stabilize and keep molecules further apart in ice.

• Example: Ice floats on water, insulating aquatic life in winter.

Solutions and Solubility

• Solution: Homogeneous mixture of two or more substances.

• Solvent: The dissolving agent (e.g., water).

• Solute: The substance being dissolved (e.g., salt).

• Aqueous solution: Solution where water is the solvent.

• Relationship: A solute dissolves in a solvent to form a solution (e.g., saltwater).

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances attracted to water (e.g., sugar, salt).

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

Molecular Mass

• Molecular mass: The sum of the masses of all atoms in a molecule.

Acids, Bases, and pH

• Hydrogen ion (H+): Higher concentration in acids.

• Hydronium ion (H3O+): Formed when H+ associates with water.

• Hydroxide ion (OH-): Higher concentration in bases.

• Acid: Substance that increases H+ concentration in solution.

• Base: Substance that reduces H+ concentration or increases OH- concentration.

• pH scale: Measures hydrogen ion concentration; ranges from 0 (most acidic) to 14 (most basic).

• pH calculation: $pH = -\log_{10}[H^+]$

• Examples:

  • pH of 7: Neutral (pure water)

  • pH < 7: Acidic (e.g., lemon juice)

  • pH > 7: Basic/alkaline (e.g., bleach)

Buffers

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

Acid Precipitation

• Acid precipitation: Rain, snow, or fog with a pH lower than 5.6.

• Effects: Harms plants and aquatic life, lowers soil and water pH.

• Human contribution: Burning fossil fuels releases sulfur dioxide and nitrogen oxides, which form acids in the atmosphere.

Table: Key Chemical Concepts

Term	Definition	Example
Element	Pure substance of one type of atom	Oxygen (O)
Compound	Two or more elements chemically combined	Water (H2O)
Covalent bond	Atoms share electrons	H2O, O2
Ionic bond	Atoms transfer electrons, forming ions	NaCl
Hydrogen bond	Attraction between polar molecules	Between water molecules
Acid	Increases H+ in solution	Lemon juice
Base	Reduces H+ or increases OH-	Bleach
Buffer	Resists changes in pH	Bicarbonate in blood

Summary

• Life depends on the unique chemical and physical properties of water and the elements that compose living organisms.

• Understanding atomic structure, chemical bonds, and the behavior of acids and bases is fundamental to biology.

• Water's polarity and ability to form hydrogen bonds underlie its role as the solvent of life and its importance in biological processes.