Module 2.4 Inorganic Compounds: Water, Acids, Bases, and Salts

Module 2.4 Inorganic Compounds: Water, Acids, Bases, and Salts

Chemistry of Life: Inorganic vs. Organic Compounds

Understanding the distinction between inorganic and organic compounds is fundamental in anatomy and physiology, as it underpins the chemical basis of life.

• Inorganic compounds generally do not contain carbon bonded to hydrogen. Examples include water, acids, bases, and salts.

• Organic compounds do contain carbon bonded to hydrogen. These include carbohydrates, lipids, proteins, and nucleic acids.

• Hydrocarbon chains and rings are common structures in organic molecules, with carbon atoms forming the backbone and hydrogen atoms attached.

Example: Hydrocarbon chain (C-C-C...) and hydrocarbon ring (hexagonal structure) are basic representations of organic molecules.

Water: Properties and Biological Importance

Water (H2O) constitutes 60–80% of the mass of human cells and is essential for life due to its unique physical and chemical properties.

• High heat capacity: Water can absorb significant amounts of heat without a substantial change in temperature, helping regulate body temperature.

• Evaporation: Water carries heat away from the body when it evaporates (liquid to gas), aiding in cooling mechanisms such as sweating.

• Cushioning and protection: Its relatively high density allows water to cushion and protect body structures (e.g., cerebrospinal fluid around the brain).

• Lubrication: Water acts as a lubricant between adjacent surfaces, reducing friction (e.g., synovial fluid in joints).

Water as a Universal Solvent

Water serves as the body's primary solvent, often called the universal solvent because it dissolves many solutes, either entirely or partially.

• Polarity: Water is a polar covalent molecule with an oxygen pole (partially negative, δ-) and a hydrogen pole (partially positive, δ+).

• Water molecules interact with solutes, surround them, and keep them apart, facilitating chemical reactions and transport.

Hydrophilic and Hydrophobic Molecules

The ability of water to dissolve substances depends on the nature of the solute.

• Hydrophilic solutes: These have fully or partially charged ends and dissolve readily in water. Examples include ionic compounds (e.g., NaCl) and polar covalent molecules (e.g., glucose).

• Hydrophobic solutes: These lack charged ends and do not dissolve in water. Examples include nonpolar covalent molecules such as oils and fats.

• "Like dissolves like": Water dissolves ionic and polar covalent solutes but not nonpolar substances.

Example: Sodium chloride (NaCl) dissolves in water, while methane (CH4) does not.

Acids and Bases: Hydrogen Ion Dynamics

The study of acids and bases centers on the behavior of hydrogen ions (H+) in solution.

• Water molecules can dissociate into positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-): $H_2O \rightleftharpoons H^+ + OH^-$

• Acids: Proton donors; increase the concentration of hydrogen ions in solution.

• Bases (alkalis): Proton acceptors; decrease the concentration of hydrogen ions in solution.

• Acids and bases are defined by their behavior with respect to hydrogen ions.

Example: Hydrochloric acid (HCl) is an acid because it donates H+ ions; sodium hydroxide (NaOH) is a base because it accepts H+ ions.

pH Scale and Its Biological Significance

The pH scale is a convenient way to represent hydrogen ion concentration in a solution, ranging from 0 (most acidic) to 14 (most basic).

• Neutral solution: pH = 7; equal numbers of hydrogen ions and hydroxide ions.

• Acidic solution: pH < 7; hydrogen ions outnumber base ions.

• Basic (alkaline) solution: pH > 7; base ions outnumber hydrogen ions.

Example: Blood pH is tightly regulated between 7.35 and 7.45.

Buffers and Homeostasis

Buffers are chemical systems that resist changes in pH, preventing large swings when acids or bases are added. They are essential for maintaining homeostasis in body fluids.

• Buffers typically consist of a weak acid and its corresponding anion.

• The major buffer system in the body is the carbonic acid–bicarbonate buffer system:

$H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

Example: This buffer system helps maintain blood pH within the narrow range necessary for physiological function.

Salts and Electrolytes

Salts are compounds formed from the combination of a metal cation and a nonmetal anion, held together by ionic bonds. When dissolved in water, salts dissociate into ions, forming electrolytes capable of conducting electrical current.

• Ionic compounds: Hydrophilic and readily dissolve in water.

• Electrolytes: Essential for nerve impulse transmission, muscle contraction, and maintaining fluid balance.

Table: Comparison of Hydrophilic and Hydrophobic Molecules in Water

Type of Molecule	Interaction with Water	Example
Hydrophilic (Ionic/Polar)	Dissolves readily	NaCl, Glucose
Hydrophobic (Nonpolar)	Does not dissolve	Oils, Methane