Most of the following material has been gathered from Elizabeth Woods, Crystals A Handbook for Teachers. The reader is referred to that source for more detailed procedure and further experiments.
Growing crystals is one of the more interesting and pleasurable parts of this curriculum. This is a hands-on experience where the students grow their own crystals from solution. We grow crystals from salt solution because they are readily available and the process is not dangerous. Growing a crystal from a melt, for example, would be much more dangerous.
A. Equipment and Procedure
Salt (table salt, sodium
chloride, NaCl)
Sugar
Water
Borax
Alum(Ammonium alum)
Equipment Cup, glass or other container Measuring cup Teaspoon, thread or thin string Magnifying glass Tweezers or forceps Microscope slides Candle or match flame Source of heat to boil water refrigerator
B. Sodium Chloride
Growing the crystals and observing their growth. Put 3 teaspoonfuls of salt into—cup water. Stir well. Let the mixture stand overnight. Next morning the mixture will appear clear. Pour clear solution into a shallow glass or cup being careful not to stir up any material from the bottom. This process is called
decanting
. Let clear solution stand, uncovered, for a few days.
The solution is now
saturated
. As the saturated solution stands and the solvent evaporates, it may become
supersaturated
. In the case of a supersaturated solution, the addition of the tiniest bit of
solute
(crystal) will cause
precipitation
of the excess solute in the bottom of the vessel.
As the first crystals appear examine them with a magnifying glass. Watch it change from day to day.
Question What forces cause the solution to solidify in the form of little cubes? It is an amazing fact that out of a formless solution such ordered form should come out!
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a. What to do with the crystal
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1. An exhibit could be made showing the development at various stages. The sequence from small to large will show how the crystal keeps its form.
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2. Break them. Tap the crystals with a hammer or something heavy. This may cause them to break along plane surfaces parallel to the surface. These may still be broken into smaller pieces and retain their cubic form. This is called
cleavage
.
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3. Place a crystal on a glass slide and place a drop of water on it, watching it with a magnifying glass as it dissolves in the water.
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a .If you allow the water to once again evaporate the salt crystals will again appear.
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b. Lessons from this section
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1. A salt crystal grows by adding salt to itself from the water solution of salt that surrounds it and that it grows with shiny faces which are at right angles to each other, provided its growth is not obstructed.
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2. Cleavage shows that within the crystal one direction is not like every other. The kind and arrangement of atoms, ions or molecules in a crystal determine its shape and other properties.
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3. Decanting was used as a method of separating a liquid from a solid.
C. Borax
Borax forms crystals very quickly. It is used as a cleaning agent and sold in paper boxes just like soap. It is harmful if swallowed. Borax is much more soluble in hot water than in cold water.
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1. Growing the crystals and observing their growth.
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Add one teaspoonful of borax to 1/2 cup of very hot water, stirring the mixture until the borax has dissolved completely. After this has cooled, many beautiful little crystals will grow.
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2. Do the same things you did with the sodium chloride crystals to the borax crystals. One crystal could be taken out of solution, dried, tied with a thread and used for a seed to be suspended in a satiated solution. It is best to have the seed near the bottom of the container. Compare the shape of the borax crystals with that of the salt crystals.
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3. Lessons from this section:
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Some crystals, such as borax, grow faces that do not meet at right angles. Borax and salt have different shapes
D. Sugar
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1. Growing the crystals
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Sugar forms a thick syrup and the molecules do not move very well making it hard to form the proper crystal pattern. It is important to let the solution stay warm thus allowing the molecules proper mobility. To promote growth the following directions should be observed: heat one cup of sugar and 1/2 cup of water gently, with constant stirring, until all the sugar dissolves and the solution is clear. Put this in a jar and cover it but do not screw it down tightly. The jar must be kept warm for many days. It could be placed over the pilot light of a gas stove. A little water escapes because the lid is not tight. Slowly crystals will grow with the beautiful form of the water crystals.
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2. What to do with the crystals
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All the suggestions made with the salt and borax crystals could also be made with the sugar crystals. You can also compare the crystals.
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3. Lessons from this section
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Sugar crystals differ in the rate of growth, and in shape. The particles that form the crystals must be free to move to allow them to get together to form the crystals. The sugar crystal solution is so viscous(thick) that it slows down this motion making it harder for these crystals to grow.
E. Alum(Ammonium alum)
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1. Growing the crystals
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Put 4 teaspoonfuls of alum powder in 1/2 cup of hot water. Stir. After the powder dissolves the solution will clear. Put a light cover, such a piece of paper, over the container, to keep the dust out. As the water evaporates, beautiful alum crystals will appear. Compare these crystals with the others.
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2. What to do with the crystals
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a. Arrange a growth sequence exhibit.
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b. Break them to find out if they have cleavage.
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c. Use one as a seed crystal.
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3. Lessons from this section
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Alum crystals are unlike salt or sugar crystals in several different ways. They grow large more quickly. They show a different shape. They do not show cleavage. The alum crystals have a new property: color. Each substance has its own form and properties.