With Valentine’s Day fast approaching, you may be looking for an easy, fun, do-it-yourself activity to do with your significant other or friends. Or, you may be interested in an edible science experiment that teaches some basic chemistry principles. Either way, making your own rock candy is a great way to learn more about crystal formation.
Ingredients & equipment
For this recipe, you will need:
- 12 cups (about 4 lbs) of granulated sugar
- 4 cups of water
- A large pot or saucepan
- A metal whisk
- Wooden skewers, chopsticks, or popsicle sticks to grow the rock candy on
- 4 16 oz mason jars or similar wide-mouth glasses
- Tape or large clips to hold the skewers in place
- Oven mitts
- (Optional) food coloring and/or flavors
Essentially, this recipe involves making a supersaturated sugar syrup and slowly cooling it to generate large, crunchy sugar crystals. (More on what supersaturation means later.)
Directions
1. Combine 12 cups of sugar and 4 cups of water in a large pot. This recipe can be scaled-up or scaled-down from this quantity, which produces 4 rock candies. Simply add 3 cups of sugar for each cup of water.
2. Put this pot on the stove on medium-high heat and stir continuously with a metal whisk until boiling.
3. While one person monitors the stove, have another person prep your glass jars to prevent thermal shock by running them under hot water. At this stage, you can add a few drops of food coloring or flavoring to the bottom of each jar as desired.
4. Dip your wooden skewers in a glass of water and roll lightly in some spare sugar. Set these sticks aside to dry. These sugar granules will be our seed crystals (more on that later).
5. Once the sugar mixture is boiling, continue to stir until all of the sugar is dissolved before removing from heat.
6. Carefully pour approximately 1.5 cups of sugar syrup into each glass jar. Be careful (and kids, get adult supervision!), since this sugar mixture is very hot and can scald skin.
7. Suspend the sugar-coated skewers in the syrup, securing to the sides of the jar with tape. You want the bottom of the stick to be at least 1 inch from the bottom of the jar, since crystals will form along the bottom of the jar as well and they will merge with the crystals on the stick if they’re too close together.
8. Now, all you have to do is leave the jars undisturbed for several days.
While you’re waiting, let’s talk about the science that’s going on!
Dissolving Crystals
As mentioned previously, rock candy consists of large sugar crystals, but what causes these crystals to grow to this large size? First, let’s zoom in on the structure of sugar. Sugar is a polymer composed of repeating units of sucrose, a simple molecule shown here:
In the small grains of sugar we’re starting with, there is an orderly arrangement of sucrose molecules that extends in all three dimensions. These sucrose molecules are attracted to each other by intermolecular forces, which results in the repeating pattern and formation of the small crystal. When we pour our cups of sugar granules into water in step 1, the water molecules will bind to the sucrose molecules through intermolecular forces and pull the sucrose away from the crystal and into solution – the sugar dissolves in the water.
Generally speaking, only a certain amount of material can be dissolved in a given amount of water at a given temperature. Any extra material added to the system will not dissolve and will remain as a solid. Once the water cannot dissolve any more material, the solution is said to be saturated. To understand why this occurs, we need to understand the two processes that occur during the mixing of water and sugar. The obvious process is the break-down of sugar crystals into dissolved sugar. However, at the same time, the sugar molecules which have been pulled away from the larger crystal can also re-crystallize. This recrystallization occurs because the sugar molecules, once in solution, are constantly moving around. When these dissolved sugar molecules encounter another sugar molecule, they can bind together again via intermolecular forces and form a new sugar crystal. A nice animation of this process can be found here.
Supersaturation
As you might imagine, when there are more sugar molecules present in the solution, they bump into each other more often and recrystallizing becomes more likely. However, when there are only a few sugar molecules in solution, they are very unlikely to bump into each other and form a new crystal. When the solution is saturated, the rates of dissolution and crystallization are equal – the constant dissolving and recrystallizing balance each other out. However, you can temporarily speed up dissolution by increasing the temperature of the water. This is why we heat up the sugar and water mixture when making rock candy – so that all of the sugar crystals will dissolve. Once this heated solution cools back to room temperature, however, the solubility of the sugar-in-water goes back down, lower than the actual amount of sugar in the water – this is what’s called a supersaturated solution. Supersaturated solutions are unstable, so as the temperature decreases, sucrose molecules will fall out of solution and end up forming the larger crystals we associate with rock candy. The longer and slower the cooling process is, the larger the sugar crystals will be!
Nucleation
Lastly, let’s talk about why we rolled the wooden skewers in sugar before placing them in the sugar solution to form rock candy. I mentioned previously that a sucrose molecule will recrystallize when it encounters another sucrose molecule. When we roll the wooden skewers in sugar ahead of time, we stick un-dissolved sugar crystals in the exact place where we want more crystals to grow. We’ve essentially created a landing site for the sugar crystals to form along the wooden skewers. Otherwise, the crystals would form everywhere in the solution and we wouldn’t have a nice candy-coated stick to carry around.
So what does this all have to do with nanotechnology? As we described in this blog post, one way of making nanoparticles involves nucleation, or growing crystals around a specific site like a bubble, metal atom, or sugar crystal! Synthesis of certain nanomaterials involves recrystallization as a crucial step, such as formation of zinc oxide nanorods from recrystallization of zinc acetate (learn more here). Sugars and sugar-based materials can also be nanoparticles themselves. We in the CSN sometimes work with these sugar-based nanomaterials – including phytoglycogen, cellulose, and alginate.
The Results
Now that we’ve talked about the science behind it, let’s get back to the recipe. It may take a few days for sugar crystals to start to form on your skewers. Be sure to check on your crystals every day. After several days, or whenever you’re satisfied with your crystal growth, remove the wooden skewers from the sugar syrup and lay them out to let the excess syrup drip off. Once dry, the rock candy is ready to eat! Let’s take a look at my final product several days after I set up my supersaturated solutions:
You’ve now done your own recrystallization! This is a fun, easy, edible DIY that would be a great Valentine’s Day activity. To minimize waste, the leftover sugar syrups can be used to flavor your morning coffee or tea, or to make more candy. If you do end up making rock candy, be sure to share some photos with us in the comments or on social media!
Acknowledgements
Special thanks to CSN alum Dr. Laura Kesner, who started researching rock candy before her graduation and shared some preliminary notes to help with this post.
References
- Husband, T. (2014, October). The Sweet Science of Candymaking. American Chemical Society. https://www.acs.org/education/resources/highschool/chemmatters/past-issues/archive-2014-2015/candymaking.html
- Bayline, J. L., Tucci, H. M., Miller, D. W., Roderick, K. D., & Brletic, P. A. (2018). Chemistry of Candy: A Sweet Approach to Teaching Nonscience Majors. Journal of Chemical Education, 95(8), 1307–1315. doi: 10.1021/acs.jchemed.7b00739
- Rock Candy Recipe – Crystallization of Sugar – The Sci Guys: Science at Home. (n.d.). http://www.youtube.com. Retrieved February 13, 2024, from https://youtu.be/VpOU0Fo7QfU?si=25SSeHlKo-00Y3gS
- DIY Science Experiment How To Make Colorful Sugar Crystal Rock Candy | CaptainScience. (n.d.). http://www.youtube.com. Retrieved February 13, 2024, from https://youtu.be/–0thOrYP58?si=6fcRvljXzi6wdMih
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