Whenever I teach introductory chemistry, one of my favorite activities is to create rock candy. Rock candy is formed by allowing a supersaturated solution of sugar and water to crystallize onto a surface suitable for crystal nucleation, such as a string or stick.
STEM Club kids were delighted the day I told them we were making rock candy in class. Sadly, our “experiment” didn’t turn out as expected.
Words to Know
Mixture :: A mixture is simply a combination of two or more substances that do not react to form something new. For example, mud, cake batter, milk, salad, latex paint, black top (asphalt and gravel). Mixtures are combinations of compounds that can be separated by mechanical or physical processes. If the atoms can only be separated with chemical reactions, they are complex molecules, not mixtures.
Solution :: A special kind of mixture called a solution is where mixing occurs at the molecular level. Examples are sea water, Kool-Aid, antifreeze, seltzer water, and gasoline.
In a solution, one or more substances are dissolved into another substance. In the example of Kool-Aid, the sugar and mix are dissolved into water. The substance that gets dissolved is referred to as the solute. The substance that dissolves a solute is referred to as the solvent. A solvent is usually a liquid but can also be a solid or a gas. In this example, Kool-Aid is the solute and water is the solvent.
All solutions are mixtures, but not all mixtures are solutions. Solutions are homogeneous mixtures (uniform in composition or character).
Saturated Solution :: A saturated solution is one in which no more solute can be dissolved.
Supersaturated Solution :: The physical properties of a compound can change when other substances or compounds are added. The melting or boiling point can increase or in some cases decrease. Raising the temperature of a compound (water for example) will enable you to dissolve more sugar into the solution. This creates a super-saturated solution.
How to Make Rock Candy
- 2 cups water
- 4 cups granulated sugar
- 1/2-1 tsp flavoring extract or oil (optional)
- food coloring (optional)
- glass jar
- wooden skewer and a clothespin
1. Wash a glass jar thoroughly with hot water to clean it. Wet the skewer, and roll it in granulated sugar. This base layer will give the sugar crystals something to “grab” when they start forming. Set the skewer aside to dry while you prepare your sugar syrup.
2. Place the water in a medium-sized pan and bring it to a boil. Begin adding the sugar, one cup at a time, stirring after each addition. Heating the water before adding the sugar allows more sugar to dissolve thus producing larger crystals.
You will notice that it takes longer for the sugar to dissolve after each addition. Continue to stir and boil the syrup until all of the sugar has been added and it is all dissolved. Remove the pan from the heat.
3. If you are using colors or flavorings, add them at this point. If you are using an extract, add 1 tsp of extract, but if you are using flavoring oils, only add ½ tsp. Add 2-3 drops of food coloring and stir to ensure even, smooth color.
4. Allow the sugar syrup to cool for approximately 10 minutes, then pour it into the prepared jar. Suspend the skewer into the solution about 1 inch from the bottom.
5. Carefully place your jar in a cool place, away from harsh lights, where it can sit undisturbed. Cover the top loosely with plastic wrap or paper towel.
7. You should start to see sugar crystals forming within 2-4 hours. If you have seen no change to your skewer or thread after 24 hours, try boiling the sugar syrup again and dissolve another cup of sugar into it, then pour it back into the jar and insert the string or skewer again.
8. Allow the rock candy to grow until it is the size you want. Once it has reached the size you want, remove it and allow it to dry for a few minutes, then enjoy or wrap in plastic wrap to save it for later.
Our Results & What We Learned
As we were underway, my thought was, “I don’t want a dozen jars of sticky sugar syrup on my counter for the next week or so. We may as well put all the skewers into the same jar – one for every student.”
I clearly didn’t think this through. As the water evaporated, the crystals grew and expanded. Eventually, they merged and even began to grow onto the sides of the jar. I was unable to remove the skewers and a few even broke off from the tension as I tried to pull them out.
I tried using a knife to break apart the crystals. I even placed the jar into a water bath in an attempt to reheat the solution and melt the crystals. Can you guess what happened?
Yep! The jar broke!! Even though it was a pressurized Kerr canning jar. Yikes!
We thereby learned it is best to suspend a single skewer into a glass of sugar solution to avoid crystal conglomeration. Keep an eye on the progress – don’t let it grow too large. You don’t want the crystals to reach the bottom or you’ll have difficulty removing the skewer.
Rock candy is a common ingredient in Chinese cooking, and many households have rock candy available to marinate meats and add to stir fry. It is used to sweeten Chrysanthemum tea and Cantonese dessert soups. It is also an important part of the tea culture of East Frisia and Tamil cuisine in India.
If you enjoyed this activity and would like to expand on the concepts introduced here, I encourage you to check out my 10-weekmultidisciplinary, hands-on chemistry curriculum, Physics Logic: Cool Chemistry.