What You Need:
- Alum (you can often find this in the spice aisle of the grocery store)
- Clean beaker or jar
- Saucer or shallow dish (a petri dish works, too)
- Pencil
- Fishing line
What You Do:
1. In one beaker, slowly add alum to 1/4 cup of very hot tap water, stirring to dissolve. Keep adding the alum until no more will dissolve: this is a saturated solution. Pour a little bit of this solution into a shallow dish or saucer and let it sit undisturbed overnight. Make sure you only pour the clear solution, not any of the undissolved material. (You can pour it through a coffee filter if necessary.)
2. The next day you should see small crystals growing in the dish. When they look to be a good size, carefully pour off the solution.
3. Make another saturated alum solution with about 1/2 cup of hot water. Pour the solution into a clean beaker or jar; avoid pouring any undissolved material.
4. Remove the biggest and best-looking of the small crystals from the saucer to use as your seed crystal.
5. Get someone to help you tie the fishing line to the seed crystal. This can be tricky; a pair of tweezers will help! If you need to, you can score a groove in the crystal to hold the line in place.
6. Tie the other end of the fishing line to a pencil, then set the pencil across the top of the jar so the seed crystal is suspended in the alum solution without touching the sides or bottom of the jar. (You may need to adjust the length of the fishing line: just wrap it around the pencil until it is the right length.) Note: if your seed crystal starts to dissolve, that means your solution isn’t saturated enough. Remove the seed quickly and add more alum to the solution, filtering off any undissolved particles.
7. Cover the jar with a paper towel to keep out the dust and let your crystal grow until you are happy with its size. When you take it out of the solution, set it on some plastic wrap to dry.
8. Note: if you see other crystals growing in the jar, transfer the solution and seed crystal to another clean jar.
What Happened:
The small crystals that formed in the saucer grew because of nucleation. A few alum molecules found each other in the solution and joined together in a crystal pattern. Other alum molecules continued to join them until enough molecules gathered to become a visible crystalline solid. (Chemists call that a crystal “falling out of” the solution.) If you left these crystals in the solution they would continue to grow, but they wouldn’t get very big because they would all be competing for the remaining alum molecules in the solution. Instead, you took one crystal and used it as the only nucleation site in the solution. It was the primary site for the alum molecules to join together, so the crystal could grow quite large!
Going Further
There are many factors that affect crystal growth, so crystal projects are great for scientific experimentation and science fairs. For example, many crystals are formed because of evaporation. Temperature and humidity are two things that affect the rate of evaporation, so you could design an experiment to see how different temperatures and humidity levels affect the rate of crystal growth and the size of crystals. Another aspect to test is the rate at which your solution is cooled: if you make a solution with boiling water, will crystals grow better if it’s allowed to come to room temperature slowly or if it’s cooled in the refrigerator?
There are many substances that can be used to grow crystals. You could design an experiment to test which one grows larger crystals under the same conditions.
Try alum, Epsom salt, table salt, sugar, baking soda, or non-household chemicals like copper sulfate (blue crystals) and potassium ferricyanide (red crystals) – adult supervision is required for the last two.
