How do Crystals Form?
Grade Level: 7th to 10th; Type: Geology
Make three different saturated solutions and see how different minerals form crystals over time.
How do crystals form? Do some minerals form crystals faster or more easily than others? How are crystals formed by different minerals the same? How are they different?
- One bottle of alum (aluminum potassium sulfate, a pickling spice)
- One container of salt One container of sugar
- Disposable gloves
- Very warm or hot water 500-mL beaker or glass measuring cup
- Three large glasses or jars (that can hold at least 12 ounces)
- Six coffee stirrers or craft sticks
- Three pipe cleaners
- Three eight-inch pieces of string
- Three stickers or pieces of masking tape and a pen
- Put on your disposable gloves and fill the beaker or measuring cup with 400 mL of hot water. Sprinkle a little alum in the water and stir it with one of the stirrers until it dissolves completely. (Safety note: alum is a skin and eye irritant; always use gloves when you handle it, and don’t let it get in your eyes! MSDS at http://www.sciencelab.com/msds.php?msdsId=9922860)
- If all of the alum dissolves, add a little more and stir again. Keep doing this until it won’t dissolve any more. Now you have a saturated solution.
- Make a circle out of one of the pipe cleaners, small enough to fit into one of the glasses without touching the sides, and tie it to a fresh stirrer with one of the pieces of string. Set the stirrer across the top of the glass so the pipe cleaner dangles inside.
- Write “Alum” on your sticker or tape and stick it to the outside of the glass.
- Carefully pour the alum solution over the pipe cleaner and set the glass where nobody will disturb it and it won’t get too warm (you don’t want the water to evaporate too fast). Wash the beaker or measuring cup thoroughly.
- Repeat steps 1-5 with the salt and then the sugar, using a fresh stirrer each time and labeling each solution as appropriate.
- Over the next several days, keep checking your three solutions. Notice whether they form crystals at different rates or about the same rate. Also, look closely at the shapes of the crystals. Do they look the same or different in each solution?
- After the crystals stop growing, you can take them out of their solutions to dry and display them!
Terms/Concepts: saturated solution; crystal formation; mineralogy
References: Dig It!: Over 40 Experiments in Geology, by Lockwood DeWitt and B. K. Hixson, pp. 60-61 (Loose in the Lab Science Series, 2003).
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