In our book, it describes two ways that crystals can form from solution - when water evaporates and leaves the mineral ions behind, or when the solution is so concentrated with dissolved ions that they form in the solution. We decided to model these two phenomena in our classroom in the simplest way - by showing salt and sugar crystals.
Salt crystals are incredibly easy to show from solution and they look pretty striking when they are done. For our demonstration, we mixed a solution of salt and warm water, then simply poured it into a shallow pan. As the water evaporates away overnight, the slow process allows the salt ions time to re-connect into cubic crystals.
Depending on several variables in the solution, the crystals can be tiny cubic growths, or much larger. Below are some images of both smaller and larger crystals.
Demonstrating crystals forming in solution is a little trickier. We decided to make rock candy in the classroom. In a pot or saucepan, mix two parts sugar to one part water and bring the solution to a boil. This is the only way to ensure that the sugar is completely dissolved. We used a simple lab burner to do so. Once the solution has boiled, you can suspend either a string with a weight on the end or a popsicle stick into the solution. We created a cross-shaped apparatus out of two popsicle sticks and a rubber band to make sure that the stick did not touch the bottom or get stuck to the sides.
The salt crystals are typically ready overnight depending on the depth of the water. I will usually move onward with whatever lesson is scheduled for that day, then allow the students a few minutes at the end of class to analyze their salt crystals and record some data into their lab packets.
The sugar crystals typically take 4-7 days, but some small crystals can be seen in those first few days. Students will also do the regularly-scheduled lesson for that day, then record their observations on the last day of class. Technically, these sugar crystals are edible after a quick rinse and a few minutes to dry.
Often, the sugar crystals can be a bit tricky and tend to form along the sides of the cup or beaker more so than along the stick or string. Either way, crystals will form and they are typically large enough for the kids to pull them out and examine them. Below is an image of sugar crystals that formed predominantly along the inside of the cup.