Growing Crystals

Crystals are solids that form with a regular pattern of molecules connected together. A collection of atoms called the unit cell repeats over and over again, so that the crystal has a visible form similar to its atomic structure, the shape taken by its microscopic parts. The underlying structures of crystal forms are called crystal systems. There are six common crystal systems (see figure). Most soluble (able to dissolve in liquid) household solids, such as salt and sugar, will form crystals under the right conditions.

Minerals are chemical elements and compounds that occur naturally as products of inorganic processes. Common mineral crystals include quartz, copper, diamond, amethyst, and garnet. Crystals of gold are sometimes found in quartz.

Materials

• miscellaneous solids, including baking soda, sugar, and salt
• microscope or magnifying glass
• metric measuring cup
• distilled water (available at a supermarket)
• saucepan
• small spoon
• stove or hot plate (requires adult help)
• clear drinking glass
• small stainless steel washer or other weight
• 15-to-20-cm string
• stirring rod (or a pencil or chopstick)
• notebook and pencil
• postal or metric scale

Procedure

1. Begin with baking soda (sodium bicarbonate). Study this powder under the microscope or magnifying glass to see whether any crystal form is detectable.
2. Dissolve as much baking soda as you can in 200 ml of distilled water in a saucepan by adding the baking soda gradually with a small spoon and stirring.
3. Have an adult heat the solution gently until you can see vapor wisping off the top, but not to a full boil.

The solute is the solid that you are dissolving. The solvent is the liquid in which it dissolves. Together they form a solution. The more solute you add, the more concentrated the solution becomes. When no more solute will dissolve, the solution is saturated.

4. Remove the solution from the heat and dissolve as much more baking soda as you can, again adding the powder slowly.
5. Allow the solution to cool, then pour it into a clear drinking glass.
6. Tie a small stainless steel washer to one end of a 15-to-20-cm string. Tie the other end of the string to the middle of a stirring rod. Rest the rod across the mouth of the glass and adjust the string so that it hangs down into the solution.
7. Put the glass in a place where it will not be disturbed. The less vibration, the better.

A very solid surface would be the poured concrete floor of a garage or basement.



8. As the solvent evaporates, crystals will form on the string. They become visible after several days. Some regularity to the crystal form should become evident. When the crystals are large and well formed, remove the string, allow it to dry, and sketch the crystals as they appear under the microscope or magnifying glass.

The string provides a surface for the solute to crystallize on. Crystals tend to form around impurities and intrusions. Sometimes as solutions cool, the solutes remain dissolved beyond the quantity that would ordinarily dissolve. These solutions are called supersaturated, and a sudden disturbance or addition of a particle can trigger the solute to immediately precipitate (separate from the solution).

9. Repeat your experiment with other solids, such as sugar and salt. Compare your findings.
   1. How does the crystal form vary among the different solutes?
   2. How does their solubility compare?

   Sugar is much more soluble than salt. A saturated solution of sugar water will contain many times more sugar by weight than a saturated one of salt.

   3. How does their rate of crystallization compare? Compare the weight of different strings that have the same length and growing conditions.

References

Crystals and Crystal Gardens You Can Grow by Jean Stangl (Danbury, Conn.: Franklin Watts, 1990).

Crystals for Kids (Little Angel Books) by Leia A. Stinnett (Flagstaff, Ariz.: Light Technology Publications, 1997).

Grow Your Own Crystals (book and crystal kit) by David Packard (Mahwah, N.J.: Troll, 1995).

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