Fun Easy STEM Rock Candy!

Rock candy is a timeless classic, who doesn’t love sugar on a stick? It turns out making it at home is actually a really fun and easy science project you can do with your kids using principles like solutes, solvent, viscosity, evaporation, precipitation and supersaturated solutions. Not to mention, if done correctly, there is a sweet reward waiting for you at the end!

Wood candy sticks or skewers
White granulated sugar (a lot!)
Water
Mason jars or other glass container large enough to hold your sticks
Food coloring (if desired)
Candy flavor (if desired)
Clothes pin or other way to suspend the wooden stick in the jar

A notebook for making observations (optional)

This project is going to take about a week to complete. You will use your notebook to record observations each day. Before you start take a minute to write down what changes you expect to see today, in a few days, and at the end of the week. Later you can compare your predictions to your observations!

To prep your sticks you need to dip 2/3 of the stick in water and then coat it with white sugar.

The Science: Sugar molecules crystallize in a solution when they bump into each other and stick together. This phase is called nucleation. Once a tiny crystal forms it serves as a nucleation point and other sugar molecules stick to it and increase the size of the crystal. The sugar on the stick, called a seed crystal, serves as the nucleation point in this experiment.

Advanced experiment: try a variation with one stick seeded and one without. What do you think will happen?

For this step you need to add as much sugar as possible to your water. The amount of water and sugar you use depends on how many jars you want to fill. For this experiment I wanted to fill two jars and needed ~4 cups of solution. The amount of sugar you are going for is somewhere between as 2:1 and 3:1 sugar:water ratio. Start with 1:1 in a large pot and heat on the stove. Slowly add more sugar ~1 cup at a time letting each cup dissolve completely until no more sugar is dissolving in your hot water and instead it just sinks to the bottom.

The science: The solution you are making on the stove is supersaturated, meaning it contains more sugar (solute) that water (solvent) and is therefore unstable. This is important for the next steps which is the crystal formation.

Very carefully divide your supersaturated sugar solution amongst your glass containers. This is the time to add any food coloring or flavors to you jars as well. I didn’t add any flavor and I chose pink and blue (because that was what I had available).
Attach your sticks so that they are not touching the bottom, clothespins are the go-to for this, but I didn’t have any so I improvised.

Now it’s time for the magic to happen. Set your jars aside in a bright/warm area. I recommend covering loosely with a paper towel; this allows the water to still evaporate but also keeps dust out of your candy. You’ll have to let them do their thing for about a week.

Come back and check on your jars every day, and record your observations in your notebook. What changes do you see in terms of crystal growth? Has the amount of liquid changed?

The science: According to exploratorium.edu, two different methods will contribute to the growth of the crystals on your stick: precipitation and evaporating. You have created a supersaturated solution by heating a saturated sugar solution. As I mentioned earlier, a supersaturated solution is unstable—it contains more solute (in this case, sugar) than can stay in a liquid form—so as the water cools 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 stick. The rock candy crystals grow molecule by molecule. After 1 week, your rock candy will be made up of about a quadrillion (1,000,000,000,000,000) molecules attached to the stick

After about a week there should be enough large crystals on your stick that it’s time to remove them from the jar. It’s ok if sugar crystals have also formed on the sides or formed a thin layer on top of the solution, you can just crack through that and get your STEM reward!

While you are munching on your sweet treat take a minute to reflect back on your notes. Did your initial predictions turn out to be correct? Did anything unexpected happen? What are some variations you might try next time?

I hope you enjoyed this experiment as much as I did!