Chemical and Physical Foundations of Biological Systems Section: Content Category 5A

Unique nature of water and its solutions
In order to fully understand the complex and dynamic nature of living systems, it is first necessary to understand the unique nature of water and its solutions. The unique properties of water allow it to strongly interact with and mobilize many types of solutes, including ions. Water is also unique in its ability to absorb energy and buffer living systems from the chemical changes necessary to sustain life.

The content in this category covers the nature of solutions, solubility, acids, bases, and buffers. The topics and subtopics are below:

Topic Level Key:

The abbreviations found in parentheses indicate the course(s) in which undergraduate students at many colleges and universities learn about the topics and associated subtopics. The course abbreviations are:

BC = first-semester biochemistry
GC = two-semester sequence of general chemistry

Please note topics that appear on multiple content lists will be treated differently. Questions will focus on the topics as they are described in the narrative for the content category.


Acid/Base Equilibria (GC, BC)

  • Brønsted–Lowry definition of acid, base
  • Ionization of water
    • Kw, its approximate value (Kw = [H+][OH−] = 10−14 at 25°C, 1 atm)
    • Definition of pH: pH of pure water
  • Conjugate acids and bases (e.g., NH4+ and NH3)
  • Strong acids and bases (e.g., nitric, sulfuric)
  • Weak acids and bases (e.g., acetic, benzoic)
    • Dissociation of weak acids and bases with or without added salt
    • Hydrolysis of salts of weak acids or bases
    • Calculation of pH of solutions of salts of weak acids or bases
  • Equilibrium constants Ka and Kb: pKa, pKb
  • Buffers
    • Definition and concepts (common buffer systems)
    • Influence on titration curves


Ions in Solutions (GC, BC)

  • Anion, cation: common names, formulas and charges for familiar ions (e.g., NH4+ ammonium, PO43− phosphate, SO42− sulfate)
  • Hydration, the hydronium ion


Solubility (GC)

  • Units of concentration (e.g., molarity)
  • Solubility product constant; the equilibrium expression Ksp
  • Common-ion effect, its use in laboratory separations
    • Complex ion formation
    • Complex ions and solubility
    • Solubility and pH


Titration (GC)

  • Indicators
  • Neutralization
  • Interpretation of the titration curves
  • Redox titration

To support your studies, see the following video tutorials below from the Khan Academy MCAT Collection. The videos and associated questions were created by the Khan Academy in collaboration with the AAMC and the Robert Wood Johnson Foundation.