Making Rock Candy: A Supersaturated Treat

Michael Kraft is from Plano, Texas and is currently pursuing a bachelor’s degree in chemistry at the University of Texas at Dallas. He is also the president of the Chemistry Student Association, a student chapter of the ACS.

Bulk Rock Candy Production

In preparation for a National Chemistry Week event, the Chemistry Student Association at University of Texas at Dallas decided to give out rock candy samples in honor of Imperial Sugar, one of the event’s sponsors. It took a few dye spills and hot sugar burns to work out the details, but I’m confident to say that the following procedure is a solid way to make lots of rock candy, and we suggest that your student chapter give it a try!

This recipe makes about a 1/2 lb of rock candy.

Materials:

  • Tall steel pot
  • Tool for straining
  • Colander
  • Clear storage shoe box containers
  • Many pounds of sugar
  • Dye
  • Stirring spoon
  • A kitchen
  • A large, clean space where crystals can develop (non – laboratory residential area)
  • Oven mitts
  • Safety glasses

Procedure:

Warning: This is only a general outline. Be sure to start growing the crystals at least one month in advance, to allow time to fine tune everything. Also note that there’s a risk of getting sugar and dye on your work surfaces.

1.    Wash the pot and storage boxes thoroughly.

2.    The mixing ratio is 3 cups sugar: 1 cup water: about 3 cups of fluid are produced. Each shoe box will be filled with about 6 cups of fluid.

3.    Measure out as many cups of water as you’ll need into the pot, and start boiling it. Have the oven mitts on hand, and use them whenever you need to move hot objects.

4.    After the water is hot, add the sugar. Continue applying heat, and stir until the liquid becomes clear. Warning: When the sugar solution boils, it will foam everywhere. Don’t fill the pot more than about two thirds full to prevent the scalding sugar syrup from foaming over.

  • In this step, a supersaturated solution of sugar is being created. The sugar grains are made of countless individual sugar molecules, all attracted to one another via intermolecular forces. Normally, the attraction and hydrogen bonding between water and sugar are enough to break apart these grains. When the solution approaches saturation, however, there aren’t enough water molecules to break apart the sugar grains. The addition of heat makes up for this energy deficit, and the sugar grains dissolve via an endothermic dissociation. This shifts the equilibrium towards more dissociation.
  • Unlike a traditional supersaturated solution, though, the solute (sugar) can melt at temperatures close to boiling point. More and more sugar can be added to the water, until a caramel syrup of melted sugar is formed.

5.    You need to let the sugar solution cool. I prefer to place it in a sink full of water, to help suck away the extra heat. Regardless of how you cool it, expect to wait at least 2 hours.

Below, note that the spoon can be easily seen through the solution. When saturating the water with sugar, the solution will be cloudy at first. After everything dissolves and the liquid cools, it will become transparent.
6.    Once the solution is lukewarm, pour the syrup into the shoe box containers to a height of about 2.5 inches.

7.    Allow the solution to sit for one week.

• In this step, the equilibrium shifts to the formation of crystals. The attraction between the sugar molecules is greater than their attraction to water, and there isn’t enough ambient heat in the solution to keep the molecules from reforming.

8.    Break a hole in the surface of the sugar crust that forms on the surface, preferably in the corner. Pour the syrup out of the storage box into the steel pot, using the strainer to keep the crystals from falling in too.

  • This syrup can be resaturated; it seems like one cup of sugar is lost from each unit of 3 cups sugar: 1 cup water. I encourage others to undertake further experimentation on this issue.

9.    Twist the plastic container to break up the sheets of crystals. Pour the crystals into the colander, and rinse them under cold water until all the syrup has washed away. Shake the colander to get rid of as much water as possible.

10.    Using hot water, clean out the plastic container, and then dry it thoroughly.

11.    Pour the wet crystals into the container. Apply enough dye to make the crystals a deep, rich color and mix well.

12.    Allow the container to sit for at least a week to dry with the lid off. At least once a day put the lid on the container and shake it; this allows the water pockets to be exposed and to dry.

  • I built a “drying chamber” by tilting a box fan so it would blow over the shoe boxes filled with crystals. I only did this to dry some at the last minute, but the idea might prove useful for others.

13.    After the drying week, there should be no detectable traces of moisture in the box. Using the stirring spoon, break up the crystals into the desired size. Warning: Note that the crystals will shatter and fly everywhere when you strike them, so try to partially cover the mouth of the container if you can. NEVER break the crystals with your bare hands, as they are very sharp and can pierce the skin.  You should wear wear eye protection for this step.

  • We were giving these out in small, 2 x 2 inch bags, so I tried to make the crystals somewhere around eraser head size.

14.    Place crystals into a bread bag and seal. Pretend you’re Walter White if you made blue crystals.

I hope you find this guide helpful, and perhaps a useful tool for planning your next event! The sugar crystals were a big hit at our booth, and we handed out countless baggies of rock candy to the museum’s guests.

4 thoughts on “Making Rock Candy: A Supersaturated Treat

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