Sugar Crystal Rock Candy

Growing edible sugar crystals on a string

Crystallization Beginner (Middle School) 25 min ~$4.00

Objective

Grow large sugar (sucrose) crystals on a string to create rock candy, demonstrating supersaturation and nucleation with an edible product.

Background

Rock candy is made by suspending a string in a supersaturated sugar solution and waiting for crystals to grow. Sugar is unusual in that it is extremely soluble in water (about 2 kg per liter at 100°C) and forms large monoclinic crystals with well-defined facets.

Safety Warnings

  • Sugar syrup is extremely hot — burns risk
  • Supervise children during the boiling step

Required PPE

goggles

Materials

  • Granulated sugar (500 g)
    White sugar
  • Water (200 mL)
  • Cotton string (30 cm)
    Unbleached
  • Food coloring (5 drops)
    Optional
  • Flavoring extract (1 tsp)
    Optional

Equipment

Saucepan Stove or hot plate Tall glass jar Pencil Clothespin

Procedure

1

Heat 200 mL water to boiling in the saucepan. Gradually dissolve 400-500 g sugar, stirring continuously.

10 min Hot syrup — burns risk
2

Remove from heat. Add food coloring and flavoring if desired.

2 min
3

Wet the cotton string with water and roll in granulated sugar to create nucleation sites. Let dry for a few hours.

3 min
4

Pour the cooled (but still warm) sugar solution into the glass jar.

2 min
5

Suspend the prepared string from a pencil across the top of the jar so it hangs in the solution without touching the sides or bottom.

3 min
6

Cover loosely and place in a stable location. Check daily for 5-7 days. Crystals should grow along the string.

5 min

Expected Results

After 5-7 days, large sugar crystals (1-3 cm) should form along the string, creating rock candy. Crystals are monoclinic with flat, angled faces. The result is edible.

Cleanup

Excess sugar solution can be reheated and reused. Wash all equipment with warm water.

Details

Category
Crystallization
Difficulty
Beginner (Middle School)
Duration
25 min
Est. Cost
$4.00
Steps
6
Materials
5

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Frequently Asked Questions

What is the objective of Sugar Crystal Rock Candy?
Grow large sugar (sucrose) crystals on a string to create rock candy, demonstrating supersaturation and nucleation with an edible product.
How difficult is Sugar Crystal Rock Candy?
This experiment is rated as Beginner (Middle School). It takes approximately 25 minutes to complete.
What safety precautions are needed for Sugar Crystal Rock Candy?
Key safety precautions include: Sugar syrup is extremely hot — burns risk; Supervise children during the boiling step.
What materials are needed for Sugar Crystal Rock Candy?
The main materials required are: Granulated sugar, Water, Cotton string, Food coloring, Flavoring extract.
What results should I expect from Sugar Crystal Rock Candy?
After 5-7 days, large sugar crystals (1-3 cm) should form along the string, creating rock candy. Crystals are monoclinic with flat, angled faces. The result is edible.