Growing crystals is easy, fun, and, a lot of times, edible and pretty to look at. It’s also a simple way to introduce younger kids to some basic elements of chemistry. It allows the opportunity to practice skills like careful measuring and the use of some great chemistry vocabulary that will come in handy later in the science classroom.
Crystal growing in all its forms relies on one basic principle: the creation of solutions. For crystal growing purposes, a solution is made of a crystalline solid like salt or sugar (the solute) and a compatible liquid, usually water (the solvent). The liquid solvent evaporates from the solution, leaving the solid solute behind. This is what the crystal is composed of, the left behind remnants of the solute after the solvent is gone. Another method creates flash crystals through the use of supersaturation and seed crystals.
Basic evaporation tends to be the easiest. It can be done with household ingredients such as cane sugar, table salt (un-iodized tends to work better), and baking soda. Crystal growth can take up to a week, however, and the container needs to be left in an undisturbed spot with little temperature variation. To start the process, it’s best to not just make a solution, but make a saturated solution. Saturated means that all possible solute has been dissolved in the solvent. The solvent is carrying as much solute as it can. For different materials, this is a different amount. There are myriad resources online that give these amounts, but there’s also a way to figure it out. At room temperature, mix the solute into the solvent little by little by little until no matter how hard its stirred, there’s a little bit left on the bottom. Pour the solution into a new container through a coffee filter, making sure there isn’t any residue or tiny crystals at the bottom. From here, the next step is evaporation.
To grow crystals on a string, tie a piece of string to a pencil and place it over the container so that the string dangles into the glass. Crystals need something rough to stick to, so the courser the string the better. Rough twine is recommended for best results. Crystals can also be grown very easily on something porous like a piece of ceramic pot or a charcoal briquette. Simply place the object in a dish of the solution in such a way that it is about halfway submerged. The solution flows into the pores in the object and spreads out along the exposed part of the object through something called capillary action. Think of the way a sponge absorbs water. Even if only half of the sponge is made wet, the entire thing becomes damp. The solvent in the solution (the liquid) evaporates, leaving the solute behind on the object as crystals. As evaporation continues, the crystals grow on one another, creating a little crystal patch. The object can then be removed and put on display. Depending on what solute is used, the crystals can be very delicate and might require an acrylic sealing spray that can bought at any hobby store.
Flash crystals are a little trickier to make but are almost immediate and neat to watch. They can be made most easily and dramatically with sodium acetate, among others (most assuredly non-edible and only obtainable from a chemical supply company). Cane sugar can also be used, but it’s less reliable. Flash crystals require a supersaturated solution. Supersaturated means that more solute than the solvent “allows” is in solution. This is possible by first heating the solution gently then adding just a little bit more solute than is needed for normal saturation. Heat typically increases a solvent’s ability to dissolve. Remove the solution from heat, and let it cool slowly. Then, take just a few crystals of the solute and drop it into the naturally cooled solution. Very shortly, crystals will form on the top of the solution. Feel the container. There is a temperature change depending on whether that particular reaction is endothermic or exothermic (absorbs heat or releases heat). After a while, all the crystallization will take place, and the new crystal can be scooped from the solution.
Done right, crystal growing is a great introduction to chemistry and physical science. The process creates not just cool little crystals that can last for a long time, it creates awesome memories for years to come.