It only takes a spark to start a fire, and it only takes one atom to act as a seed for crystal formation. Under the right conditions, the atoms in alum will join together like puzzle pieces to form large crystals. I posted a few years ago about how to grow a large alum crystal, but this experiment is even more fun. It’s also easier for young kids, since it takes less small-motor coordination.
Alum is also called potassium aluminum sulfate. It’s used in pickling and in found in baking powder. You can grow beautiful alum crystals at home with a few jars of alum, water and any object you don’t mind covering with glue. We made fake geodes by breaking eggs in half and washing them out, but we also encrusted a grape stem and a plastic shark.
To do this experiment, you’ll need glue, 3/4 cup alum from the spice section of the grocery store (4 or 5 small jars should do it,) water and whatever you want to coat with crystals. It takes three days to complete.
On day one, paint glue on the objects you want to grow crystals on. If you’re making “geodes”, apply a thin layer of glue to the inside of an eggshell that’s been cut in half, washed out and dried. Then, sprinkle a little alum powder on the glue and let it dry overnight. We heavily coated our object with alum, but might have grown larger crystals if we’d used less. Each alum particle acts as a seed for crystal growth. The closer together they are, the less room your crystals will have to grow.
On day two, dissolve 3/4 cup alum in 2 cups of water by boiling. This step requires adult supervision. Make sure all the alum dissolves (it may still look a little cloudy) and let the solution cool. This is your supersaturated alum solution.
After about 30 minutes, when the solution is cool enough to be safely handled, gently immerse your object in the alum solution. For color, you can add a large squirt of food coloring. Let your project sit overnight to grow crystals.
On day three, gently remove your object from the alum solution and let it dry. How does it look? Draw it or take a picture to put in your science notebook!
Crystals are geometric networks of atoms. Imagine a three dimensional chain-link fence, and you’ll get the picture. Certain crystals will only grow in certain shapes. For example, diamonds are always cube-shaped when they form. Whether the atoms have joined to form a small diamond, or a large one, it will always be in the shape of a cube!
Some crystals, like alum, will form from supersaturated solutions, like the one you used in this experiment. A supersaturated solution is one that is forced to hold more atoms in water (or another solute) than it normally would. You can make these solutions using heat or pressure. Crystals can form when a supersaturated solutions encounters a “seed” atom or molecule, causing the other atoms to come out of the solution and attach to the seed.
What else could you try? Could you do the same experiment with salt, or sugar crystals? How do you think the color gets incorporated into the crystal? Do you think the food coloring disrupts the shape? Will larger crystals grow if you let your object sit in the solution longer?
You can read more about crystals and gems here.