Counterintuitive Examples, Discrepant Events

1. In a way, equilibrium systems are themselves discrepant events. The presence of all reactants and products in a reaction vessel is not expected by students who have generally been exposed earlier only to reactions that “go to completion.”

2. Students sometimes get the idea that higher temperatures drive reactions towards completion. If dissolution is treated as an equilibrium (Solute + Solvent Solution), for example, increasing the temperature is thought to increase the concentration of the solution. Although this is most often true with solid/ liquid combinations, there are exceptions (e.g., Li 2 SO 4 ). The solubility of NaCl is hardly affected by temperature at all. The solubility of a gas decreases with increased temperature. Opening a warm can of soda pop amply demonstrates this fact (see also Demonstration 2 in Solubility and Precipitation module).

3. Solubility equilibria are an excellent chance to challenge student ideas about physical vs. chemical changes. Ask students if a reaction resulting in a precipitate is a physical or chemical change [M + (aq) + A ­ (aq) Æ MA (s)]. Then ask if the dissolution of an ionic salt is a physical or chemical change [MA(s) Æ M + (aq) + A ­ (aq)]. Then show them that both reactions can be represented as one equilibrium reaction.

Pictures in the Mind

These diagrams represent molecular activity during a reversible reaction. Figure 1 is just one example from the Teacher’s Resource Manual, Concept Mastery section; Chemistry: the Study of Matter , Prentice-Hall, Dorin, Demmin, and Gabel, pp. CM-31 and 32. Have students label each drawing. Which drawing(s) represent the system at equilibrium (B and C) and which represent the system not at equilibrium (A and D)? X(g)

Analogies and Metaphors

1. In a football game, the number of players on the field is constant although exchange of players (substitution) changes actual persons.

2. Connected fish bowl analogy . Two fish tanks are connected by a tube large enough to allow passage of fish. A number of fish are placed in one of the tanks. At equilibrium, the number of fish in each tank will eventually become unchanged.

3. Two jugglers analogy.

Figure 2. Two jugglers analogy.

4. Drinking fountain line:

5. Picture a number of horses and wranglers in a corral. As each wrangler mounts a horse, the wrangler is bucked off. The equilibrium is: Horse + Wrangler Mounted wrangler Consider the effect (a la LeChatelier) of adding horses or wranglers.

Tips for the Teacher

Language of Chemistry

  1. A full-service crime laboratory consists of specific units. These include (in addition to the unit's function):
  2. a. Physical Science Unit : Applies principles of chemistry, physics, and geology (e.g., identification of glass, soil samples, or tire marks) to the identification and comparison of crime-scene evidence.

    b. Biology Unit: Examines blood, body fluids, hair, fibers, and botanical specimens.

    c. Firearms Unit: Examines firearms, bullets, cartridge cases, shotgun shells, tool marks, latent prints, and ammunition of all types.

    d. Document Examination Unit : Examines handwriting and typewriting on questioned documents; analyzes paper and ink, indented writings, obliterations, erasures, and burned or charred documents.

    e. Toxicology Unit : Examines body fluids and organs for drugs and poisons.

    f. Other units include the Identification Unit (photography, fingerprint, polygraph, and voice-print analysis), and Evidence Collection Unit.

  3. . The Evidence Collection Unit dispatches trained personnel to the crime scene to retrieve evidence for laboratory examination. Physical evidence can be anything from massive objects to microscopic traces. The presence of many items of evidence is obvious, but some can only be detected by close laboratory scrutiny. Many examples of physical evidence were described above.
  4. Forensic science is the application of science to criminal and civil laws.
  5. Chemistry, biology, physics, and geology are useful for determining evidential value of crime scene and related evidence.
  6. Forensic scientists may be called to testify in civil cases (e.g., water quality or product liability).
  7. The microscope is the most important tool in some crime laboratories. TLC is an important technique and GC is an important instrument.
  8. Forensic scientists often deal with small amounts of the evidential sample, often necessitating, as the first choice, examination by a nondestructive laboratory test.
  9. . Samples examined by forensic chemists are usually impure. Tests will show the presence of "something" in a lot of "garbage."
  10. Forensic science laboratories are seldom on the forefront of technology in chemistry (or science). Attorneys and judges feel comfortable with what has already been accepted in the courts. On occasion, evidence will be accepted if it provides information that cannot be obtained otherwise (e.g., DNA profiling).
  11. Confiscated street drugs may require extraction, isolation, and spectral analysis for identification. Extraction of a suspected compound from a mixture is often pH dependent (many of these compounds contain basic groups), dependent on the solubility of the suspected compound in the extraction solvent, and the resulting crystalline form depends on whether the substance is precipitated in neutral, acid, or base form. This may also be true of some pharmaceutical samples.

Common Student Misconceptions

  1. "Forensic chemistry can solve all crimes." Forensic chemistry is only one of the sciences important in crime solving. The principles and techniques of biology, geology, and physics are applied to the many types of evidence. Mathematics and psychology are also important areas.
  2. "Crimes can be solved easily." Despite the fact that many crimes as portrayed on television and in the movies are solved within 1-2 hrs, most crimes require a team effort involving deductive reasoning and sophisticated scientific techniques over a period of time. Some crimes are never solved.
  3. "Perfect fingerprints can be taken off anything." Generally, smooth surfaces (glass, table tops, etc.) will yield good fingerprints; a rough surface (stone wall) will not. The quality of a fingerprint is also determined by the amount of skin oil.
  4. "Identical twins have the same fingerprints." Fingerprints are individualized. In addition, other types of physical evidence that can be individualized include jigsaw matches, footwear and tire tread prints, striations present in tool marks, and ballistic evidence.