Chemistry is Crystal Clear

Root, Linda                        Everett Elementary
                                   650-4550
                          

Objectives:

Students will be able to:
     -Identify crystals
     -Understand how crystals are formed
     -Understand that crystals have six basic shapes
     -Compare crystals of various substances
     -Observe that crystals can be recovered by various means


Apparatus Needed:

     Something porous:  piece of brick
                        lump of charcoal
                        piece of cement

     Table salt                           Saucepan
     Water                                Hot plate
     Laundry bluing                       Several tall instant
     Household ammonia                       coffee jars
     Sugar                                Pencil
     Alum (available at drugstores)       Paper clip
     Salol (Phenyl salicylate)            Several petri dishes 
     Food coloring                        Overhead projector
     Cotton thread                        Hand lenses
     Several shallow                      Poster paper
        glass bowls                       Large and small
     Craft wire                               styrofoam balls

Recommended Strategy:

1.  Before the lesson is presented, grow one or more crystal gardens, 
    and recover a variety of crystals from solutions.  You may use salt, 
    sugar, alum, or other compounds of your choice.  (See Borgford and 
    Summerlin, Chemical Activities, American Chemical Society) 
 
2.  To introduce the lesson, display the crystals you have grown, along 
    with saucers or petri dishes of salt, sugar, and alum granules.  Do 
    not identify the items by name, merely by number.  Allow the children 
    about ten minutes to observe the items on display in detail with a hand 
    lens, comparing and contrasting them as they study them.  A discussion 
    of their observations should follow.    

3.  Review the three states of matter-solids, liquids, and gases- and 
    their properties.  Present the concept of amorphous and crystalline 
    solids.  A transparency showing the structure of each should be 
    projected on the screen during the discussion.  

4.  Narrow the discussion to salt crystals.  Show a transparency and a 
    model of a unit cell of salt and a salt lattice.  (The model can be  
    made with the craft wire and the styrofoam balls.)  Discuss how the 
    lattices form.  Refer to the items on display, reminding them that what 
    they are seeing are lattices, not individual units. 

5.  Discuss the six basic shapes of crystals, while displaying an 
    example of each shape on a transparency.  Relate these examples to the 
    crystals the children have already observed from the examples. 

6.  Review the definition of a saturated solution, and discuss how each 
    of the sample items was made.  Discuss the fact that crystals are 
    formed through cooling and evaporation.  Explore the concept that 
    crystals can be recovered from vapors, molten materials, and 
    solutions.  Give examples of each in nature.  (Snowflakes, diamonds, 
    and salt crystals would be an example of each.)  The slower the 
    evaporation or cooling process, the larger the crystal. 

7.  To visually demonstrate crystal formation, heat a small amount of 
    phenyl salicylate in a petri dish on a low setting on the hot plate 
    until the crystals melt.  Put the petri dish on the overhead.  Seed it 
    with a pinch of phenyl salicylate and watch as the liquid cools, and 
    the crystals begin to form.  The process can be repeated many times 
    with the same petri dish.  Simply warm it again until the crystals 
    disappear, and repeat the process. 

8.  Orally review the concepts taught.

Additional Activities

Children can grow their own crystals and record their observations.

Children can compare the growth rates of various crystals, and
graph their results.

Children can investigate the effects of light on various crystals.

Children can investigate the properties of a variety of crystals.

Additional References

Brown and Forsyth, The Crystal Structure of Solids, Crane and Russak,
N.Y. 1973.

Holden and Singer, Crystals and Crystal Growing, Anchor Books,
Doubleday and Co.  1960.