Making Rock Candy

Given the time of year and all that candy in the aisles at your local grocery, the science department thought it might be nice to try and make your own candy at home.


Making rock candy is a lesson in supersaturated solutions, evaporation, crystalline structure, seeding, and deposition. Great vocabulary words for any budding scientist and all well worth the google search.

There are a number of recipes, but this is my favorite from “The Accidental Scientist, Science of Cooking” http://www.exploratorium.edu/cooking/candy/recipe-rockcandy.html

Gather:

• 4 cups sugar
• 2 cups water
• a small saucepan
• a wooden spoon
• a candy thermometer
• a small, clean glass jar
• a measuring cup
• cotton string
• a weight to hang on the string (such as a screw or galvanized washer)
• waxed paper
• a pencil (to suspend the string in the jar)

Directions:

1. Heat the water in the saucepan over medium-high heat until it comes to a boil.

2. Completely dissolve the sugar in the boiling water, stirring continuously with the wooden spoon until the solution grows clear and it reaches a rolling boil.

3. Remove the solution from the heat, and then carefully pour it into the jar. Cover the jar with a small piece of waxed paper.

4. Tie the weight to one end of the string, and then tie the other end to the middle of the pencil. The string should be about two-thirds as long as the jar is deep. Dip the string into the sugar solution, remove it, lay it on a piece of waxed paper, straighten it out, and let it dry for a few days.

5. Gently suspend the prepared string in the solution and let sit at room temperature, undisturbed, for several days. You can check each day to see how much your crystals have grown. It’s tempting, but don’t touch the jar until the experiment is finished—it usually takes about seven days.

6. At the end of the week, the crystals on your string should be clearly defined, with sharp right angles and smooth faces of various sizes. In the field of crystallography, these are called monoclinic crystals. Their shape is determined by the way the individual sugar molecules fit together, which is similar to the way the shape of a pile of oranges is determined by the shape of the individual oranges and the way they stack together.

Additions:

• Try adding food coloring or flavoring to your sugar syrup before making the rock candy.

~Dean Whitfield
Math and Science Chair