Making rock candy, which is characterized by the formation of large sugar crystals, is an easy -- and delicious -- way to observe science at work in the kitchen. It takes approximately one week to produce fully formed sugar crystals, and you can check the crystals' progress daily. This is a fun project for kids, but supervision is required: Sugar syrup is very hot and should only be handled by an adult.
Sugar and water are boiled together to create a syrup. The syrup is placed in a clean glass jar, and a string or wooden stick is suspended in the jar. As the water evaporates over the course of several days, the remaining sugar will adhere to the string or wooden stick, forming large crystals.
Let It Sit
Sugar’s natural tendency is to bond with other sugar molecules and form crystals. The water molecules in the syrup prevent this from happening. As the water evaporates from the syrup over the course of several days, there is less interference with the sugar’s natural ability to bond, so sugar crystals begin to form.
Rock candy develops into its distinct geometric pattern, in part, because of the conditions under which the sugar is cooled. The more slowly a sugar syrup cools, the more time its molecules will have to cluster together and form organized crystals. The syrup used to make rock candy is boiled so it won’t cool immediately. The fact that the syrup is left undisturbed for several days also has a role in promoting larger crystals, since stirring would cause the crystals to break apart and become smaller.
The Right Stuff
The type of sugar used to make rock candy also contributes to the candy’s distinct appearance. White granulated table sugar is the best sugar for making rock candy because it is made up entirely of sucrose -- a sugar that is good at forming crystals. Brown sugars, including maple sugar, are primarily sucrose but also include some glucose and fructose, which inhibit crystal formation. Brown sugars can be used to make rock candy, though the crystals will take longer to form. Sugars that cannot be used to make rock candy include powdered sugar, due to the inclusion of cornstarch; artificial sweeteners, due to their lack of sucrose; and sugar syrups such as corn syrup or honey, because they are primarily fructose and glucose.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- Keys to Good Cooking: A Guide to Making the Best of Foods and Recipes; Harold McGee
- Photo Credit Comstock/Stockbyte/Getty Images