Rock Candy Experiment

How to Make Rock Candy

Your child is a full of creativity and wonder from the first moment, whether they are doing gravity experiments by throwing their toys down the stairs or making art projects on their bedroom walls. We also want to help you build those skills with your kids at home, which is why we created our Experiment of the Month series. This month, we’ll be teaching you and your little SciGenius how to make rock candy at home. Keep reading to learn more and contact SciGenius today to explore all the educational programs we can offer!


What You Need

  • 2 cups of water
  • 4 cups of granulated white sugar
  • Tall, cylindrical glasses or jars
  • A ball of string or wooden skewers
  • Food color and flavoring (optional)


The Process


1. Bring water to a boil

Pour the two cups of water into a pot and increase the heat until it starts to boil. If you’re performing this experiment with older kids, they may be able to participate in this step, but with young children it’s safer if they simply watch while an adult operates the stove. Use purified water if you can, because impurities in tap water can prevent sugar crystals from growing properly.


2. Stir in the sugar

Add the sugar to the boiling water in small increments, about half a cup at a time. Stir with a spoon to help it dissolve into the water. It may take longer for the sugar to dissolve as you add more and more, because the water will start to become saturated. Try to stir until the water becomes clear again and the sugar has been totally dissolved. If it still hasn’t dissolved after a few minutes, increase the heat. Hot water has a higher saturation point that cooler water, and the sugar should dissolve more easily.


3. Remove from heat

Let your sugar solution rest and cool for about 15–20 minutes. If there is any remaining sugar that hasn’t completely dissolved, you need to remove it from the solution. You can try pouring the sugar solution through a strainer to separate out the liquid. Undissolved sugar can disrupt the growth of your rock candy in a similar way to impure tap water: the crystals will attach and grow on the undissolved sugar rather than the string or stick you use.


4. Add flavoring and color (optional)

If you want to add any flavors or colors, stir it into your cooling sugar solution. You will probably only need a few drops of flavoring, but if you want obvious color in your rock candy, you will probably need to add more food coloring than you may think you need. It should be a strong color, almost dark. If you don’t have artificial flavorings on hand, you can try fruit juices from lemon, limes, or oranges, or you can try adding Kool Aid to your sugar solution for both color and flavor.


5. Pour solution into a heat-safe container

The container you use should be glass, because plastic may melt when it comes into contact with the hot sugar solution. Make sure that your class is also clean and without dust particles, which is another substance the sugar crystals may try to stick to rather than the string or stick. Fill the container almost to the top. You can cover the glass with parchment or wax paper to prevent dust from settling.


6. Create “seed crystals”

To create rock candy like your kids might see in the store, you need to create “seed crystals.” These small, “starter” crystals will give the sugar a surface to cling to and help them grow. One way of making seed crystals is to wet your stick or string and roll it in granulated sugar. If you choose this method, make sure that your material with the seed crystals is completely dry before dipping it into your sugar solution. You can also dip your material into the sugar solution for a moment and take it out to dry. A few small sugar crystals will form as the water evaporates. If you choose to use a string rather than a skewer, you will need to use a natural fiber string like twine or cotton, which will have enough texture for the sugar crystals to cling to.


7. Lower your stick or string into the sugar solution

If you use a string…
On one end of the string, tie something with a little weight that can act as an anchor. A paperclip or a small metal washer should work fine. This will make sure your string hangs straight down and doesn’t touch the sides of the glass. On the other end of the string, tie it to something wide enough to keep it from slipping into the glass, like a pencil or popsicle stick. If you’re using a pencil, you may want to tape it to the glass to keep it from rolling. The string should be about two-thirds the depth of the glass to give your crystals plenty of room to grow; your weight shouldn’t touch the bottom.


If you use a wooden skewer…
If you choose a stick rather than a string, you should still try to ensure that it doesn’t touch the bottom or sides of your container. A clothespin is a good option for helping suspend your skewer. Make sure the skewer is held securely by the clothespin – you may have to pinch it closer to the spring rather than using the groove of the pin – and balance the clothespin across the mouth of your glass.


8. Cover the glass

Choose the material you use to cover the glass carefully. Evaporation is an important part of this process, so you don’t want to seal the glass with something like plastic wrap. Use a breathable material like a paper towel.


9. Store the glass somewhere safe and quiet

The crystals need time to form, and vibrations for music or footsteps can cause them to fall of the stick or string. If you want to grow small crystals quickly, you can leave the glass in a sunny place and the water will evaporate more quickly. If you are aiming for larger crystals, let your experiment rest in a cool dark place. The water will evaporate more slowly, but it will give the crystals time to grow nice and big.


10. Check on your experiment after a week or two

Your science buddy may get impatient, but you want to give the crystals time to grow undisturbed. Touching or tapping the glass can keep them from growing or cause existing crystals to fall off. After a week or two, you should see big, smooth crystals have formed on your string or skewer. If a crust has formed on the surface of the solution, try to break it away without breaking the crystals you grew on the skewer or string. A butter knife can work well for that process. Carefully remove your rock candy from the glass and lay it on wax paper to dry. If the candy is stuck to the bottom, run hot water over the bottom of the glass. It should loosen up the sugar and let you remove the rock candy without damage.


The Science

When you first dissolved the sugar in boiling water, you created a supersaturated solution. As we mentioned earlier, hot water has a higher saturation point than cool water, so the boiling water absorbed more sugar than it would have if the water had come straight from the tap. As the water cools, the saturation point drops again, and it can’t hold as much sugar. The sugar will precipitate out of the solution and crystalize on whatever surface it can find, which hopefully means the string or stick you place in the middle of the glass. That’s why using purified water and clean glasses is so important — the sugar may cling to those tiny impurities and crystallize there instead. If that happens, you still made rock candy! It may just be a little harder to eat.

Check back every month for more engaging experiments, and if you want to help your child keep growing and learning, explore SciGenius’ STEAM programs! We travel wherever you need us to offer amazing science and art opportunities, led by passionate educators. Talk to your child’s school about our integrated enrichment programs for young kids or explore all the amazing topics your kids can learn, from LEGO robotics for kids kindergarten through third grade to digital animation for high schoolers. Give them a better outlet for their curiosity and innovative spirit with STEAM educational programs from SciGenius! STEAM education offers an integrated approach to teaching science, technology, engineering, art, and math with the goal of helping your child develop skills that will help them build a bright future. SciGenius offers all kinds of programs for kids from kindergarten to high school, including after-school programs and summer camps, with more qualified educators per student than any other STEAM program in the area.  Contact SciGenius today to learn more!

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