Microscopic Observation of Crystal Growth
Objective: Science Standards:
To observe crystal nucleation and Science as Inquiry
growth rate during directional Physical Science
solidification . - position and motion of objects
- properties of objects and materials
Science Process Skills: Unifying Concepts and Processes
Observing Change, Constancy, & Measurement
Communicating
Investigating
CONTENTS
Activity Management
Assessment
Crystal Growth
Crystal Growth Worksheet
MATERIALS AND TOOLS
Bismarck brown Y
Mannite (d-mannitol)
HOCH2(CHOH)4CH20H
Salol (Phenyl salicylate)
C13H10O3
Microprojector
Student microscopes (instead of a
microprojector)
Glass microscope slides with
cover glass
Ceramic bread-and-butter plate
Refrigerator
Hot plate or desktop coffee cup
warmer
Forceps
Dissecting needle
Spatula
Eye protection
http://quest.arc.nasa.gov/space/teachers/microgravity/13observ.html
Activity Management
The mannite part of this activity should be done as a demonstration, using a microprojector or
microscope with a television system. It is necessary to heat a small quantity of crystalline
mannite on a glass slide to 168C and observe its recrystallization under magnification. The
instructions call for melting the mannite twice and causing it to cool at different rates. It is better
to prepare separate samples so they can be compared to each other. The slide that is cooled
slowly can easily be observed under magnification as crystallizes. You may not have time to
observe the rapidly chilled sample properly before crystallization is complete. The end result,
however, will be quite apparent under magnification. If students will be conducting the second
part of theactivity, it is suggested that you prepare several sets of mannite slides so they may be
distributed for individual observations. The salol observations are suitable for a demonstration,
but because of the lower melting temperature (48C), it is much safer for students to work with
that the mannite. A desktop coffee cup warmer is sufficient for melting the salol on a glass slide.
Because of the recess of the warmer's plate, it is best to set several large metal washers on the
plate to raise its surface. The washers will conduct the heat to the slide and make it easier to pick
up the heated slide with forceps. Point out to the students that they should be careful when
heating the salol because overheating will cause excessive evaporation and chemical odors, and
will increase the time it takes for the material to cool enough for crystallization to occur. The
slide should be removed from the hot plate just as it starts melting. The glass slide will retain
enough heat to complete the melting process.
Only a very small amount of bismarck brown is needed for the last part of the activity with salol.
Only a few dozen grains are needed. Usually just touching the spatula to the chemical causes
enough particles to cling to it. Gently tap the spatula held over the melted salol to transfer the
particles. It will be easier to do this if the salol slide is placed over a sheet of white paper. This
will make it easier to see that the particles have landed in the salol.
If students are permitted to do individual studies, go over the procedures while demonstrating
crystallization with the d-mannitol. Have students practice sketching the crystallized mannitol
samples before they try sketching the salol.
Refer to the chemical notes below for safety precautions required for this activity.
Notes On Chemicals Used: Bismarok Brown Y
Bismarck brown is a stain used to dye bone specimens for microscope slides. Because bismarck
brown is a stain, avoid getting it on your fingers. Bismarck brown is water soluble.
Mannite (d-mannitol) HOCH2(CHOH)4CH20H Mannite has a melting point of approximately
168C. It may be harmful if inhaled or swallowed. Wear eye protection and gloves when handling
this chemical. Conduct the experiment in a well-ventilated area.
Salol (phenyl salicylate) C13H10O3
It has a melting point of 43 degrees C. It may irritate eyes.
Observations of Salol
1. Repeat the procedure for mannite (steps 1-4) with the salol, but do not use glass cover
slips. Use a desktop coffee cup warmer to melt the salol. It may be necessary to add a
seed crystal to the liquid on each slide to start the crystallization. Use a spatula to carry
the seed to the salol. If the seed melts, wait a moment and try again when the liquid is a
bit cooler. (If the microprojector you use does not have heat filters, the heat from the
lamp may remelt the salol before crystallization is completed.)
2. Prepare a new salol slide and place it on the microprojector stage. Drop a tiny seed
crystal into the melt and observe the solid-liquid interface.
3. Remelt the salol on the slide and sprinkle a tiny amount of bismarck brown on the melt.
Drop a seed crystal into the melt and observe the motion of the bismarck brown granules.
The granules will make the movements of the liquid visible. Pay close attention to the
granules near the growing edges and points of the salol crystals.
Assessment:
Salol Lab #2
Today we are going to be studying igneous rocks. We are going to make two igneous rocks and
explore how they are formed. Take a look at rocks A-D in your box. (A-Granite, B- Rhyolite, C-
Gabbro, D-Basalt) You can remove these four rocks and put them on your desk.
1. What observations can you make about these rocks?
2. Color—What do you think makes the color in these rocks? The minerals in them.
3. Look at granite. You can see crystals of individual minerals. Do you see crystals in all
four of the rocks? What ideas do you have about why you can see crystals well in some
but not in others?
We are going to find out. Put the salol on hotplate to melt and give each team two pieces of
wax paper and one bowl.
I have melted these minerals to make magma. I am going to put this magma on your wax paper
and I want you to observe what happens. Put small amount of salol on each team’s wax paper
(not on ice yet).
What did you see? So the rocks crystallized from molten magma. This is how igneous rocks are
formed. You have to have a melt and as that melt cools, the minerals crystallize out to form
rocks. What do you think would happen if the magma cooled at a lower temperature?
Put a piece of ice in each team’s bowl. Put your wax paper over your ice so that when I put the
magma in your ice will be directly under it to cool it quickly.
1. What did you observe this time? Which cooled and crystallized faster: the magma at
room temperature or the one on ice?
2. How does the one that crystallized faster compare to the one that had more time to cool?
3. So the amount of time that a magma has to cool can determine the size of the crystals.
Which magma has larger crystals, one that cools quickly or one that cools slowly?
4. A magma that cools slowly has larger crystals that you can see with your naked eye. One
that cools quickly has smaller crystals. Why do you think this is? The more time a crystal
has to grow the larger it can get.
Concept Introduction:
1. Compare the rocks you made with the four igneous rocks in your box (A-D). Which do
you think cooled slowly? Granite, Gabbro. Why? Larger crystals
2. Which do you think cooled quickly? Rhyolite, Basalt. Why? Smaller crystals make
minerals hard to see.
3. Can you think of any place on earth where magma pushes through the crust to become
lava and cools quickly? Volcanoes, spreading ridges
4. If it cools quickly, will you be able to see the minerals easily? No.
5. So which rocks in your set came from volcanoes or spreading ridges? Rhyolite, Basalt.
The lava to make these rocks extrudes from, or comes out of, the earth. Remember that below the
solid crust there is a molten mantle with magma in it. Sometimes it makes its way to the surface
of the earth. Because it extrudes, these rocks are called extrusive, or rocks that were formed
outside of the earth’s crust.
Rocks that don’t explode out of the crust cool slowly. Remember it is much hotter as you get
closer to the mantle. Intrusive rocks stay inside the crust until the rock has completely
crystallized. We can’t see them as they form. We have to wait millions of years until these huge
pockets of rock are exposed by plate tectonics and erosion. The mountains at Yosemite are
actually huge pockets of intrusive rock that cooled inside the earth and were later exposed.
6. Which of your rocks are intrusive rocks? Granite, Gabbro
Application:
So we’ve seen that the crystal size in a rock depends on whether it formed intrusively (inside the
earth) or extrusively (outside the earth). Crystal size separates the rocks into two groups. What
else would separate the rocks into groups?