Rock Candy: An Edible Study of Crystallization
Making rock candy is a safe way to introduce students to solutions and crystal growth—and you can make it a tasty treat at the same time! This activity helps students visualize how a supersaturated solution grows the extra-large crystals of sucrose needed to make rock candy. Table sugar (sucrose) and water are the only required ingredients. Food coloring and flavoring are optional ingredients to enhance the taste of the candy. Using the instructions below, this activity can be completed at home or outside of the chemistry lab; it requires the use of food-grade materials and equipment.
Three types of solutions can be made with a given solute and solvent: unsaturated, saturated, and supersaturated. An unsaturated solution has not reached its limit of solubility for a solute at a given temperature. A saturated solution has reached its limit of solubility for a solute at a given temperature. Any additional solute just falls to the bottom of the container without dissolving. A supersaturated solution holds more dissolved solute than it normally would at a given temperature. When making rock candy, this is done by raising the temperature to the boiling point of water, dissolving a lot of sugar, and then allowing the solution to cool undisturbed. If undisturbed at room temperature, the solution retains the extra dissolved solute at the higher temperature; however, if more crystals of the solute are added, they also act as seed crystals (nucleation sites for crystal formation). As the solution evaporates slowly in a dust-free environment (dust can also become nucleation sites), the crystals grow larger as excess dissolved solute crystallizes.
Use an oven mitt when transferring the hot sugar solution to the glass jar. Pour with caution because the sugar solution can cause serious burns if spilled on skin. Check to see if any of your students are allergic to any food coloring or flavorings; they are optional and can be omitted from the activity.
Materials (per student)
- Wooden Skewer
- 2 Clothespins
- Tall Drinking Glass or Glass Jar
- 4 c Table Sugar
- 2 c Water
- Small Saucepan
- Wax Paper
- Paper Towel
- Food Coloring (optional)
- ½ to 1 tsp Flavoring Extract or Oil (optional)
- Stove or Burner
- Oven Mitt
- Place a wooden skewer vertically in the jaws of 2 connected clothespins. Put in the glass container, adjusting the height of the skewer so that it is 1 cm above the bottom of the container with the clothespins resting on top (see Figure 1). Mark the skewer with a pen just below the clothespins.
Figure 1 Skewer held with clothespins 1 cm
above bottom of glass container.
- Place a piece of wax paper on a table, pour a teaspoon of table sugar in the center, and spread it out to make an even layer.
- Remove the skewer from the glass container. Moisten the skewer with water below the pen mark.
- While still wet, roll the skewer in the sugar on all sides so that an even coating of sugar adheres to the skewer. Lay the skewer aside on wax paper to dry. These sugar crystals are the seed crystals upon which the dissolved sugar molecules from the supersaturated solution will crystallize.
- Pour 2 cups of water into a saucepan and heat to boiling. Add the sugar, 1 cup at a time, stirring after each addition. Continue heating and stirring until all 4 cups of sugar have been added. Any crystals on the sides of the saucepan can be pushed down into the syrup while heating. Once all the sugar dissolves, you should have a thick syrup of supersaturated sucrose.
- If using food coloring, add 2 to 3 drops. If using a flavoring, add 1 tsp of extract or ½ tsp of oil. Stir thoroughly. Pour the warm syrup solution into the glass container.
- Carefully reconnect the skewer in the 2 clothespins. Attach the clothespins just above the mark you made with your pen.
- Slowly lower the skewer into the center of the sugar solution and allow the 2 clothespins to rest on the top of the glass container.
- Make a small hole in the center of a paper towel and slide the paper towel over the skewer and down to the mouth of the glass container to cover the opening and prevent dust from entering.
- In 5 to 7 days, you should see large crystals of sugar forming on the seed crystals on the skewer.
- Before eating the rock candy, place the skewers on a piece of wax paper and use a hand magnifier or stereomicroscope to view the crystals. Describe their shape and draw or photograph them with your cell phone camera.
- Remove the skewer from the clothespins and taste your rock candy, or wrap in plastic wrap to save for later.
Infographic: Rock Candy for Rookies
- Compare the size of the crystals when the hot, supersaturated sugar solution is forced to cool very quickly in a refrigerator vs. cooling gradually at room temperature.
- Another method of collecting crystals is to use a string soaked in water and rolled in granulated sugar. After drying, attach a paper clip as a weight and suspend the string from a pencil or clothespins into the supersaturated sugar solution. Crystals will collect on the string after 5 to 7 days.