NAME
DATE
Period
Crystal Formation
What is a crystal? How do crystals form? What factors affect the number, size and shape of crystals?
Materials:
1 glass petri dish
1” pc. copper wire
1 plastic divided petri dish with lid
2 watch glasses
silver nitrate solution
steel wool
100% and 50% concentrations of salt solutions
1 petri dish with crushed ice
tongs
salol crystals
PART A: Define a Crystal
1. Place a 1 cm piece of copper wire (that you have cleaned with steel wool) in a clean glass petri dish.
2. Put the slide on the stage of the stereoscope. (Set magnification to 3X)
3. PUT GOGGLES ON. From the dropper bottle marked silver nitrate solution, put 1-2 drops of the dilute
silver nitrate solution on the copper wire and immediately watch what happens.
CAUTION: Do not spill the silver nitrate solution or get any of it on your clothes or hands!
4. In the space, draw an up-close sketch of the
crystal growth on the copper wire.
5. Dispose of the wire and silver in the trash container.
Clean and dry the petri dish.
a. Are there flat, plane surfaces that might suggest a crystal arrangement of atoms? ___________
b. The pattern you have drawn is called a dendritic pattern and is made of silver. Is there a regular,
repeating pattern to the growth?
Describe it:
c. Define ‘CRYSTAL pattern’
d. Could the silver be called a “crystal”? Explain
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Part B: Concentration and Crystal Formation
Predict: How will the CONCENTRATION of a solution affect the amount of crystals that form?
1. Obtain 2 petri dishes. Add 5.0 ml of the 10% salt solutions to one dish. Label it 10%.
Then add 5.0 ml of the 100% concentration solution to the other dish that you have labeled 100%.
2. Allow them to evaporate overnight. Draw a close-up sketch of the SIZE, SHAPE and NUMBER of crystals
below.
10% concentration solution
100% concentration solution
a. Which solution made MORE crystals?
b. Write a summary statement comparing the concentration of the solution and the number of crystals.
Part C: Space and Crystal Size.
Predict: Do you expect the SIZE of the salt crystals to be larger or smaller in the bigger dish space?
1. Add 5.0 ml of the 100% salt solution to one QUARTER of a petri dish.
2. Allow it to completely evaporate, then draw the shape and size of the crystals in the space below.
a. How did the size, shape and number of crystals in the
SMALL QUARTER SPACE compare with the
100% concentration in the LARGE SPACE in Part B?
b. Write a statement comparing the size and shape of the
crystals in the large space to those in the smaller space.
3. Describe how (by what process) did the crystals formed in parts B and C??
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Part D: Cooling Rate and Temperature
Predict: How will the rate of cooling (slow vs. fast) affect the size of the crystals that form?
_______________________________________________________________________________________
1.
2.
3.
4.
Place a tiny scoop of salol (phenyl salicylate) into the center of a small watch glass.
Using tongs, carefully set the watch glass on hot plate set a temp. #1. The salol will quickly melt.
When the salol is melted, remove the watch glass using tongs and place it on the stereoscope stage.
Watch the crystal growth. * It will be necessary to “start” the crystal growth by dropping one crystal of
salol onto the liquid melt. (This is called “seeding”) If the seed crystal also melts try another crystal
until the liquid cools enough to allow growth to begin.
5. Repeat Steps 2 and 3 but this time set the watch glass on a dish of ice to COOL the melt quickly.
Place the watch glass on the stereoscope stage and observe the size of the crystals that formed.
6. Sketch a diagram of the SIZE and SHAPE of the salol crystals that cooled slowly compared to the
crystal that cooled quickly on the ice.
Crystals that cooled SLOWLY
Crystals that cooled FAST.
a. Describe the shape of the crystals when the melt cooled slowly:
b. Which sample had crystals that were larger and well-formed?
c. How do the results compare to your prediction?
d. Write a statement that compares the size of the crystals and the rate at which they cooled.
e. The crystals were not able to form until they cooled to 43°C. Write a statement relating the
temperature of a melt and crystal formation.
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Part E: Cooling Rate and Crystal Size
Early in Earth’s history, the crust was produced by the cooling of magma. As the atoms of the different
elements that make up the magma cool and slow down, they group themselves into a regular order to form solid
crystals. When the magma has cooled to about 500°C, most of the minerals have crystallized into various rocks
with different composition and crystal size.
1. Using a stereoscope (set to magnify 1X), look at the samples of the rock granite, basalt rock and obsidian
rock. Remember, they were once molten liquid minerals.
Rock
Size and shape of crystals
Ranking of cooling rate
(1= fastest, 3 = slowest)
Granite
Obsidian
Basalt
a. Are the mineral crystals in granite large enough to see with the unaided eye?
b. What does this say about the TIME that the minerals had to cool and crystalize?
c. Are the crystals in the basalt rock the same size as the crystals in granite?? What does this say about
cooling rate of the basalt?
d. Does obsidian appear to have any individual crystals?
e. Of the 3 rocks, which cooled at a fastest rate? Explain.
f. Where did the minerals in granite cool that allowed them for form such large, well-defined crystals?
(pg. 122-123)
Cooling Temperatures of Minerals
g. According to the chart of Cooling
Temperatures of Minerals, do ALL
minerals cool and crystalize at the
SAME temperature?
Give an example using 2 minerals
from the chart:
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