Crystals are really neat structures and show up in a surprising variety of places in nature. This is a great crystal growth experiment to try with your kids and will help them learn about these fascinating solids.
Minerals are considered crystalline. A crystalline structure is characterized by a certain internal arrangement of atoms and molecules. The external shape of a mineral may or may not reflect its internal structure. In the case of crystals, the external form reflects its internal structure.
Rocks are composed of mineral grains. Some sedimentary rocks are composed of or may contain minerals that crystallize from concentrated salt water, these include rock salt (halite), and gypsum. Igneous rocks include those that solidify from a molten material. The crystal size in igneous rocks is controlled by their rate of cooling. Slowly cooling molten material causes crystals to grow to a visible size and have a coarse grained texture while molten material that cools quickly will result in quite small crystals.
Alum (approx. 4 ounces) – Alum is available in the spice section of most grocery stores.
4 Tablespoons salt
4 glass containers – One pint mason jars or beakers work well.
3 feet of thread
A hot plate
Magnifiers (microscope or magnifying glasses)
Samples of mineral crystals (quartz, halite, or calcite)
Samples of igneous rocks (granite or basalt)
Part 1 – Crystal Shapes
In this activity, you will grow crystals from both alum and salt. Have students observe each substance and not that they are similar in color. The alum will likely be granulated and finer than the salt. Have the students make predictions of whether crystals formed from these two substances will be similar or different from each other in color, size, and shape. Heat two cups of water (approx. 500 ml) and dissolve four ounces of alum in the water. When the alum is dissolved pour about one inch into a clean jar and set aside uncovered. In about an hour, small crystals should begin to form in the bottom of the jar. Pour the remainder of the solution into a jar and cover it. Now heat one cup of water and dissolve as much salt as possible in the water. There will probably be some salt that doesn’t dissolve. Pour about one inch of the solution into a clean jar and set aside uncovered. In about an hour, very small crystals of salt should begin to form. Once the crystals have formed in both solutions, have the students examine them and compare the color, size, and shape of the crystals. After examining with a magnifying glass, have the students compare their predictions to the result.
Part 2 – The Effect of Cooling on Growth of Crystals
In this activity, you will grow alum crystals under various conditions, one jar will be cooled while the other will sit at room temperature. Have the students make predictions about what effect the cooling will have on the shape or size of the crystals. Remove two crystals from the alum solution that was made in part one to use as “seed crystals.” Tie the seed crystals to separate pieces of thread, each about a foot long. This can be a bit tricky so it may be useful to notch the crystals on each side with a small knife. Tie the free end of the thread to a pencil and prop the pencil across the top of the jar, do this with each crystal so it is hanging suspended near the bottom of the jar. Take your reserved alum solution from part one and divide it equally between the two jars. Make sure the solution is not too hot as it can dissolve the crystals. Place one of the jars in ice water, and set the other jar where it can cool at room temperature. The crystals will now need several hours to form completely. Once the crystals have formed, have the students observe and compare the shape and size of the crystals grown under the different conditions and compare their predictions with the result.
Results and Discussion:
Students should have observed that the shapes of the salt and alums crystals are quite different. The salt crystals are cubical while most of the alum crystals are variants of octahedrons. Students should have also noticed that the salt crystals are much smaller than the alum crystals. This is due to a function of the kinetics of crystallization. At this point, show the students some different mineral crystals if available, to demonstrate the variety of possible shapes.
This part of the exercise can have variable results but ideally the crystals grown at room temperature will be larger than the crystals grown while cooling on ice. Larger crystals result from having a greater time for the crystallization to occur. At this point, show students how igneous rocks formed from the cooling of magma such as granite have bigger crystals because they cool slowly, while crystals in rocks such as basalt are smaller because they cool rapidly.
It is a good idea to experiment with this exercise before attempting it with students. You can also add another part to the experiment by mixing together the salt and alum in a water solution and having students observe what type of crystals form. It is also a great time of construct a lab sheet for students to fill out.
Bonus: Check out this video of crystals growing. Cool!