Counterintuitive Examples and Discrepant Events

  1. Can you prepare a saturated solution of bromine in carbon tetrachloride? Ethyl alcohol in water?
    [No, bromine, which is ordinarily a liquid, is infinitely soluble in carbon tetrachloride; thus it is impossible to form a saturated solution of these two liquids. Ethyl alcohol is also infinitely soluble in water.]
  2. When sugar dissolves in water, why is the resulting solution clear? Why has the sugar become “invisible?"
    [You cannot see anything that is smaller than the wavelength of visible light (about 10-5 cm). When solid, visible sugar dissolves, it releases individual sugar molecules. These have a diameter of about 10-7 cm. Even when surrounded by water molecules, their size is less than the wavelength of visible light. Since lightwaves pass through them, they are, indeed, “invisible.”]
  3. Why do the particles in a true solution never settle?
    [Particles in solution have extremely low mass. A sugar molecule, for example, has a mass of about 5 x 10-22 g. Although even a particle this small is affected by gravity, it is constantly bombarded by water molecules. As it begins to fall, water molecules below hit it and tend to drive it upward, keeping it in solution. Particles large enough to overcome this bombardment will settle, and will not form a true solution.]
  4. 4. Do all solids that lose their physical state in aqueous solution dissolve in water?
    [No. Salts dissolve, but metals react with acids and then dissolve because salts are formed.]

Analogies

  1. Ions separating from compounds and recombining to form precipitates may be compared to couples (cations + anions) going to a “dance.” During the dance, couples exchange partners to form “new combinations.”
  2. Increase in entropy during the dissolving process is somewhat like a small group of students going into a large auditorium, where they have a tendency to spread out, rather than bunch together.

Pictures in the Mind

The Dissolving Process

  1. Draw a picture of the water molecule, showing its positive and negative “ends.”
  2. Draw a picture of a sodium chloride crystal, showing its positive and negative ions.
  3. Draw a picture of several water molecules attracted to a negative chloride ion.
  4. Draw a picture of several water molecules attracted to a positive sodium ion.

The Precipitation Process

  1. Draw a picture of two different ions in a solution.
  2. Draw a picture of a different solution with two different ions.
  3. Draw a picture of all these ions in the two solutions mixed together.
  4. Draw a picture to show how a precipitate might form from a new combination of these ions.

 


(page 10 & 11)

TABLE OF CONTENTS TOPIC OVERVIEW CONCEPT/SKILLS DEVELOPMENT LINKS/ CONNECTIONS APPENDIX