This program can be completed as a cooking program and/or STEM program. Recommended for children in grades 5th and 6th due to the science.
Rock candy
To make most types of candies, you always start by dissolving sugar in boiling water. This forms a sugar syrup, which you can cool down by taking it off the burner. But how you cool down the syrup can make all the difference.
If you want to make rock candy, you need to let the syrup slowly cool down over many days until big sugar crystals form. But if you want to produce fudge, you need to continuously stir the syrup after an initial cooling period, so when the sugar crystals form, they stay small and do not grow too much. If you want to make cotton candy and glass candy, you need to cool the syrup quickly to keep crystals from forming.
Supplies:
Water
Sugar
Mason Jar
String
Pencil
Food coloring
Scissor
Paper Towel
Microwave or burner
Directions to make rock candy:
Pour about 3 cups of granulated cane sugar into the large glass container.
Add 1 cup of water to the sugar. Watch what happens as the water bubbles through all that sugar. There’s a lot going on in the container already. Use the heavy spoon to thoroughly stir the water (a solvent) and the sugar (a solute) together to make a solution. It will be very viscous (thick) and heavy because there’s a lot more sugar than water in there. Stir it well!
An adult must help with this Step! You need to give the water some help with all that sugar so warm up the water. If the container is microwave-safe, put the solution in the oven and heat it for two minutes on high. (You can use a cooktop to heat the solution if you prefer.) Heat the solution to the boiling point.
An adult must help with this Step! Move the stirred solution to the microwave (or cooktop) again and heat it on high for another two minutes. Don’t let the solution boil over.
Add 3-7 drops of any food coloring to the mixture and stir it in thoroughly.
Pour the colored solution into the smaller glass container.
Tie new clean string to the middle of the pencil. Use the scissors to cut off a length longer than the small container is tall. Lay the pencil on top of the small container and trim the string so it’s about 2/3 the height of the container. You want it shorter than the container.
Holding the pencil, lower the string into the solution and let it soak for a short time. You want the solution to soak through the string. Lay the pencil and soaked string on a piece of wax paper so the string is perpendicular to the pencil. Allow the solution to cool to room temperature and the straight string to dry completely.
As it cools, the solution becomes more viscous (having a thick, sticky consistency between solid and liquid; having a high viscosity) so it might be a trick to push the dried string into it again. You may have to use a slow steady pressure to get it to sink deeply into the solution.
You’ll need to allow the string to soak in the solution for a week, too.
A paper towel over the container will keep dust and goobers away from your candy – uh, your science experiment.
When you’re ready for the big reveal, lift the pencil and pull the string loaded with crystals out of the jar. Lay them on some wax paper and look closely at what grew on the string. Of course, a taste test will have to be a part of your analysis.
The Science Behind Rock Candy
Why does the candy form on the stick? Why is having the right temperature so important?
The Cool Chemistry Of Rock Candy
Candy chemistry isn’t just dependent on how you heat up your sugar mixture; how you cool it down is important too. Rock candy is made by heating up sugar water to the hard-ball stage, then slowly cooling it for several days in order to allow huge crystals of sugar to form. Part of what allows those big crystals to grow is a fundamental chemistry concept known as Le Chatelier’s principle, which basically says that when conditions are shifted inside a system at equilibrium, the system will respond in an attempt to restore equilibrium. In this case, the decreasing temperature of the sugar solution provokes crystallization.
“A decrease in temperature causes a system to generate energy, in an attempt to bring the temperature up,” science writer Tom Husband wrote in an in-depth look at rock candy for the American Chemical Society. Sucrose molecules join together to form crystals and “because the formation of chemical bonds always releases energy, more sucrose molecules will join the crystal in an attempt to increase the temperature.”
Credit: World Science Festival. (n.d). The Cool Chemistry of Rock Candy. Retrieved from http://www.worldsciencefestival.com/2014/10/sugary-secrets-candy-making-chemistry/.
What makes the crystals grow?
Two different methods will contribute to the growth of the crystals on the string. You have created a supersaturated solution by first heating a saturated sugar solution (a solution in which no more sugar can dissolve at a particular temperature) and then allowing it to cool. A supersaturated solution is unstable—it contains more solute (in this case, sugar) than can stay in a liquid form—so the sugar will come out of solution, forming what's called a precipitate. This method is called precipitation.
The other is evaporation—as time passes, the water will evaporate slowly from the solution. As the water evaporates, the solution becomes more saturated and sugar molecules will continue to come out of the solution and collect on the seed crystals on the string. The rock candy crystals grow molecule by molecule. Your finished rock candy will be made up of about a quadrillion (1,000,000,000,000,000) molecules attached to the string.
Credit: The Science of Cooking. (n.d.) Rock Candy. Retrieved from https://www.exploratorium.edu/cooking/candy/rock-pop.html
Note: Make worksheet explaining the science of rock candy for kids to take home.
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