How to make rock candy

This activity is part of our Science Lab series. Check out the ANU Science Lab for more experiments! In this activity, you will grow your own rock candy sugar crystals on a wooden skewer to explore supersaturated solutions, seed crystallization, and crystal growth. Watch the video above to see how it is done.

Rock candy sugar crystal materials & ingredients

For this activity you will need:

• Wooden skewer or cotton string
• Peg
• Glass jar
• 2.5 cups white sugar
• 1 cup water
• Small saucepan and stove (for dissolving the sugar)
• Food colouring or flavouring extracts (optional)
• Fork and clean empty jar (for harvesting)

Safety Note: Boiling water and hot sugar syrup can cause severe burns. Adult supervision is required when heating the mixture on the stove.

How to grow rock candy sugar crystals: Step by step instructions

Step 1: Prepare the supersaturated solution and seed skewer

1. Wet a wooden skewer with water and roll it in granulated sugar to create seed crystals, then leave it to dry completely.
2. Heat water in a saucepan and gradually stir in sugar until no more sugar will dissolve, creating a hot supersaturated syrup. Stir in food colouring if desired.
3. If you want to make more or less supersaturated solution, just make sure to maintain the sugar to water ratio of 2.5 parts sugar to 1 part water.

Step 2: Suspend the skewer in the jar

1. Carefully pour the warm sugar solution into a clean glass jar.
2. Lower the dried seed skewer into the centre of the jar, using a peg placed across the jar rim to hold it suspended. Ensure the skewer tip sits well above the bottom of the jar so the growing crystals do not fuse to the base of the glass.

Step 3: Grow and harvest your crystal

1. Place the jar in an undisturbed location for several days to allow large sugar crystals to grow along the skewer.
2. If a crystal crust forms on top, use a fork to gently loosen the skewer from the rim.
3. Lift the skewer out, place it into a clean empty jar, and allow it to drip dry for a few hours before examining or eating your rock candy!
4. If your skewer was placed too close to the bottom and fused to the jar base, simply submerge the jar in warm water so the sugar dissolves back into solution to clean out the container.

Rock candy sugar crystals: The science explained

This kitchen chemistry demonstration illustrates solubility, supersaturation, and crystallisation:

• The maximum amount of sugar that can dissolve in water increases with temperature. Heating water allows far more sugar molecules to dissolve than cold or room-temperature water.
• As the hot sugar syrup cools back down to room temperature, the liquid contains more dissolved sugar than it can stably hold, resulting in a supersaturated solution.
• Dissolved sugar molecules naturally leave the cooling liquid to form solid structures. The pre-coated sugar seeds on the skewer act as nucleation sites, providing a surface where dissolved sucrose molecules attach and build up in orderly repeating geometric patterns over time.

Science fair projects & taking it further: Experimenting with variables

• Compare growing crystals in the refrigerator against growing them at room temperature or wrapped in an insulated towel. How does cooling speed affect overall crystal size?
• Compare refined white table sugar against raw sugar or brown sugar to see how impurities influence the rate of crystallisation.
• Compare a rough wooden skewer against a smooth plastic rod or a cotton string to discover which material produces the largest crystals.

Enjoyed this experiment? Explore more hands-on activities on the Science Lab ANU YouTube channel.