Sugar crystals are called rock candy because these hard crystals are edible. Sugar (sucrose) crystals are one of the few types of crystals you can grow and eat. You can eat the natural clear crystals or you can color and flavor them.
Rock Candy Materials
You only need a few common kitchen materials for this crystal project:
- 3 cups sugar (sucrose)
- 1 cup boiling water
- food coloring (optional)
- flavoring (optional — good choices include cherry, peppermint, and cinnamon)
- clean glass or plastic jar
- wooden skewer or cotton string
You don’t need string or a skewer to grow sugar crystals, but they are good support structures for making rock candy sticks. Crystals also grow on pipe cleaners, but they probably aren’t food-friendly. The crystals also grow on the bottom and sides of their container, forming a confection that is known as Misri in India and Iran.
You can use different kinds of sugar, but avoid powdered sugar because it often contains anti-caking agents. Anti-caking agents don’t dissolve, so the tiny particles attract crystal growth. While this sounds like a good things, it means you’ll get a mass of small crystals rather than large crystals.
Grow Sugar Crystals
The procedure is really easy.
- Dissolve the sugar in the boiling water. Heat the sugar solution on the stove, or in the microwave if you have trouble getting the sugar to dissolve. Once the sugar dissolves, remove it from heat. If you keep heating it, it hardens and burns.
- Add a few drops of food coloring and flavor, if desired.
- Let the solution cool a bit before pouring it into your jar. You don’t want to get burned!
- Pour the sugar solution into a jar. Place a wooden skewer into the jar or else hang a string into the middle of the jar, tied to a pencil or butter knife.
- Place the container somewhere it won’t be disturbed. If you like, cover the jar with a paper towel or coffee filter to allow evaporation while keeping the crystal solution clean.
- It takes a few days to get good crystal growth. If you see crystals forming on the top of the jar, you can remove them and eat them. If you leave them, these crystals will compete with your stick or string for sugar and will reduce the size of your crystals.
- Remove the crystals and enjoy them! If you want to store the crystals before eating them, keep them in an airtight container so humidity in the air won’t make the rock candy sticky.
If you enjoyed growing sugar crystals, how about trying another easy crystal-growing project?
How Long Does It Take to Grow Sugar Crystals?
Sugar crystals may appear overnight or it may take several days (weeks) for growth. Sugar crystal growth depends on three main factors:
- Solution Concentration: Crystals don’t start growing until you have a saturated solution, where no more sugar dissolves in the water. Whether or not the liquid is saturated depends on the amount of sugar in the water and the temperature. If you aren’t seeing crystal growth, try lowering the temperature. Refrigeration might do the trick!
- Temperature: Temperature determines when crystals start growing and how big they are. Slow cooling resulting in slower growth, but larger crystal formation. Similarly, quick cooling promotes crystal growth, but you get a mass of smaller crystals. Ideally, slowly cool the hot sugar water down to room temperature (or lower). Quick cooling doesn’t yield great results. Putting the liquid in a window causes temperature fluctuations, so crystals dissolve when it’s warm and form when it’s cool. Avoid the window.
- Humidity: Humid air contributes water to the liquid and slows crystal growth. Dry air evaporates water and aids crystal growth. If you aren’t seeing crystals, try improving air circulation or reducing humidity. The easiest solution is turning on a fan.
About Sugar Crystals
The white granular sugar you buy at the store is sucrose (C12H22O11), which is a disaccharide consisting of glucose and fructose subunits. Raw sugar is usually golden or brown and contains some natural impurities, but still crystallizes. However, while pure sugar crystals are colorless, raw sugar crystals retain their color.
Sucrose contains covalent bonds and crystallizes using these bonds, too. The molecule crystalizes in the monoclinic space group. The resulting crystal is hard, brittle, and rigid. Examples of other crystals that contain covalent bonds are quartz and diamond.
References
- Beevers, C. A.; McDonald, T. R. R.; Robertson, J. H.; Stern, F. (1952). “The crystal structure of sucrose”. Acta Crystallogr. 5 (5): 689–90. doi:10.1107/S0365110X52001908
- Hynes, R. C.; Le Page, Y. (1991). “Sucrose, a convenient test crystal for absolute structures”. Journal of Applied Crystallography. 24 (4): 352. doi:10.1107/S0021889891002492
- Richardson, Tim (2002). Sweets: A History of Candy. Bloomsbury. ISBN 978-1-58234-229-0.