Chapter 2: Chemistry of Life: Water and Mixtures

Chemistry of Life: Water and Mixtures

Introduction

This section explores the chemical properties of water and mixtures, emphasizing their biological significance in Anatomy & Physiology. Understanding these concepts is essential for grasping how substances interact within the human body.

Mixtures and Compounds

Definition and Distinction

• Mixture: A combination of two or more substances that are physically blended but not chemically combined. Each component retains its own properties.

• Compound: A substance formed when two or more elements are chemically bonded together, resulting in new properties distinct from the original elements.

• Example: Air is a mixture (mainly nitrogen and oxygen), while water (H2O) is a compound.

Biological Relevance

• Body fluids are complex mixtures of chemicals, primarily dissolved or suspended in water.

• Water constitutes 50–75% of body weight, varying with age, sex, and fat content.

Properties of Water

Molecular Structure and Bonding

• Water molecules are V-shaped and contain polar covalent bonds.

• The polarity of water allows for hydrogen bonding between molecules.

• These bonds give water unique properties essential for life.

Key Properties of Water

• Solvency: Water is known as the universal solvent due to its ability to dissolve a wide range of substances, facilitating metabolic reactions.

• Cohesion: The tendency of water molecules to stick to each other, resulting in surface tension.

• Adhesion: The tendency of water molecules to stick to other substances, reducing friction around organs.

• Surface Tension: The force that causes the surface of water to act like a stretched elastic membrane, allowing small objects (like insects) to rest on it.

• Chemical Reactivity: Water participates in many chemical reactions, including hydrolysis and dehydration synthesis.

• Thermal Stability: Water can absorb and retain heat, helping to stabilize body temperature.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances that dissolve in water; typically polar or charged (e.g., sugars, salts).

• Hydrophobic: Substances that do not dissolve in water; typically nonpolar or neutral (e.g., fats).

Types of Mixtures

Solutions, Colloids, and Suspensions

Mixtures in the body can be classified based on the size of their particles and their behavior in water.

Type	Particle Size	Appearance	Membrane Passage	Separation on Standing	Example
Solution	< 1 nm	Clear	Passes through most membranes	No	Salt water, glucose in blood
Colloid	1–100 nm	Cloudy	Too large to pass through semipermeable membranes	No	Proteins in blood plasma
Suspension	> 100 nm	Cloudy or opaque	Cannot penetrate membranes	Yes	Blood cells in plasma
Emulsion	Varies	Cloudy	Varies	Yes	Fat in breast milk

Acids, Bases, and pH

Definitions

• Acid: A proton donor; releases H+ ions in water.

• Base: A proton acceptor; accepts H+ ions or releases OH- ions.

pH Scale

• The pH scale measures the concentration of hydrogen ions (H+) in a solution.

• Scale ranges from 0 (most acidic) to 14 (most basic); 7 is neutral.

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

• Each unit change represents a tenfold change in H+ concentration.

• Normal blood pH is 7.35–7.45 (slightly basic).

• Acidosis: Blood pH < 7.35; can lead to coma or death.

• Alkalosis: Blood pH > 7.45; also dangerous.

Buffers

• Buffers are chemical solutions that resist changes in pH, helping maintain homeostasis in the body.

Concentration of Solutions

• Concentration can be expressed in various units, such as moles per liter (molarity), percent solutions, or weight/volume.

• Understanding concentration is important for interpreting physiological and clinical data.

Example Table: pH Values of Common Substances

Substance	pH
Lemon juice	2
Grapefruit juice	3
Urine	5–8
Pure water	7
Household ammonia	11

Additional info: The notes above expand on the brief points in the original slides, providing definitions, examples, and context for each concept. The tables are reconstructed and clarified for academic use.