Sample pre-lab quiz for Crystallization Experiment.

Besides the types of questions below, questions on handling of wastes and on lab etiquette will be included on all prelab quizzes.

1) When a solution is allowed to crystallize slowly, impurities are excluded from the growing crystal structure because:

  1. the molecules in the crystal lattice are cooler than the solvent and impurities would have too high a melting point to join the lattice
  2. the molecules in the crystal lattice are in equilibrium with the molecules in solution and molecules unsuitable for the lattice (impurities) are likely to remain in solution
  3. impurities could never join the lattice because they are always liquid at room temperature
  4. during slow crystallization, impurities are prevented from joining the crystal structure because of their ionic strength

2) Norite is used to:

  1. remove colored impurities
  2. aid in the crystallization process
  3. aid in the filtering process
  4. reduce the melting point of a compound

3) In this lab you will heat your recrystallization solvent with a steam bath to get all the crude solids in solution. Why is it preferable to use a steam bath to heat organic solutions as opposed to a heating mantle, hot plate or Bunsen burner?

  1. The temperature of a steam bath can be easily controlled to prevent excessive boiling of organic solvents.
  2. Bunsen burners apply excessive heat and will easily decompose the fragile organic compounds that are being heated.
  3. Most organic solvents are quite flammable and sources of flame or high flameless heat can cause them to catch fire or explode in the presence of air.
  4. Water (steam) is generally immiscible with organic solvents so a small amount of water in the solvent that is being used will not affect the recrystallization process.

4) When you are finished with your experiment, the recrystallized products are to be placed:

  1. in the acetone waste bottle
  2. down the sink
  3. in the recovery jar
  4. in the organic waste container
  5. in the aqueous waste container

5) Recrystallized product is most efficiently collected by:

  1. vacuum filtration
  2. gravity filtration
  3. solvent evaporation
  4. extraction

6) The ideal solvent for the crystallization of a particular compound is one that

  1. dissolves a moderately large amount of the compound when hot
  2. does not react with the compound
  3. boils at temperature below the compound's melting point
  4. dissolves only a small amount of compound when cool
  5. all of the above

7) A melting point range:

  1. begins at the temperature at which the first droplet of liquid is observed in the solid sample and ends at the temperature at which the sample becomes a clear liquid
  2. begins at the temperature at which the first droplet of liquid is observed in the solid sample and ends at the temperature at which the sample begins to boil
  3. begins at the temperature at which the solid sample becomes a clear liquid and ends at the temperature at which it begins to boil
  4. begins at the temperature at which the solid sample begins to melt and ends when it decomposes

8) Name the following glassware items.

Answers.