Rock Candy
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely
Categories: Crystals, Water and Solubility, Food Science and Nutrition
Alternative titles: Homemade Sugar Crystal Sticks
Summary
Dissolve sugar into hot water until it forms a supersaturated solution, then suspend sugar-coated sticks in jars to grow edible sugar crystals over several days. Color or flavor can be added to make decorative, tasty rock candy.
Procedure
- Shape or cut wooden skewers so they fit your jars without touching the bottom; clip a clothespin to each skewer so it can hang centered.
- Wet each skewer and roll it in granulated sugar to make a seed layer; set aside to dry completely.
- Heat water in a saucepan and stir in sugar until it dissolves; continue adding sugar a little at a time until no more dissolves and the liquid looks slightly cloudy (about a 3:1 sugar to water ratio by volume).
- Optional: Add a few drops of food coloring and a small amount of candy flavoring; bring just to a gentle simmer, then remove from heat.
- Let the syrup cool until it is hot but not boiling to reduce thermal shock to glass.
- Pour the hot syrup into clean, heat-safe jars, one color per jar if making multiple colors.
- Lower a fully dried, sugar-coated skewer into each jar and use the clothespin to keep it suspended in the center without touching sides or bottom.
- Place jars where they can sit undisturbed; loosely cover tops with paper to keep out dust.
- Observe crystal growth daily; if a surface crust forms, gently break it with a clean utensil so crystals can continue to grow on the stick.
- After 5–7 days, lift sticks out, let excess syrup drip off, and place them on a clean surface to dry before eating.
Links
Rock Candy Experiment - Emily's Science Lab:
Watchable by stmartins.sa.edu.au. Make Your Own Rock Candy! | The Science of Cooking! - SciShow Kids:
📄 Rock Candy Experiment - Play Learn Grow: https://www.growingajeweledrose.com/2015/02/rock-candy-experiment.html
Variations
- Compare different starting sugar concentrations (2:1 vs 3:1) and record how growth rate and crystal size change.
- Test food coloring in the solution versus painting finished crystals lightly with gel color.
- Try cotton string tied to a pencil instead of a skewer and compare crystal coverage.
- Grow larger crystals by transferring sticks to a fresh hot supersaturated solution after several days.
Safety Precautions
- Hot syrup can cause burns; adults should handle heating and pouring, and use heat-safe jars on a stable surface.
- Keep jars out of reach while hot; allow to cool before children observe closely.
- Use only food-safe containers, utensils, colorings, and flavorings if the candy will be eaten.
- Do not handle sticks with bare hands after dipping if you intend to eat them; use clean tools and good hygiene.
- Cover jars to prevent contamination by dust or insects; discard any batch that shows mold or off smells.
Questions to Consider
- Why do crystals grow as the solution cools and sits? (The hot solution holds more dissolved sugar; as it cools and evaporates, it becomes supersaturated and sugar deposits as crystals.)
- Why coat the stick with sugar first? (The rough sugar layer provides nucleation sites so crystals start on the stick instead of the jar walls.)
- What controls crystal size? (Higher supersaturation and faster cooling favor many small crystals; slower cooling and stable conditions favor fewer, larger crystals.)
- What happens if the stick touches the jar wall or bottom? (Crystals can bridge to the glass, fusing the stick in place and reducing even growth.)
- Why is the 3:1 ratio recommended? (It reliably achieves supersaturation at kitchen temperatures, promoting steady crystal growth without needing extreme evaporation.)