Alum Crystal Growing

Creating large transparent alum crystals

Crystallization Beginner (Middle School) 30 min ~$6.00

Objective

Grow large, transparent octahedral potassium alum crystals from a supersaturated solution.

Background

Potassium alum (KAl(SO₄)₂·12H₂O) crystallizes in a beautiful octahedral habit. It is non-toxic, inexpensive, and easy to grow into large crystals. The octahedral shape reflects the cubic crystal system of alum.

Safety Warnings

  • Use care with hot water
  • Alum is generally safe but avoid ingestion of large amounts

Required PPE

goggles

Materials

  • Potassium alum (100 g)
    Food-grade available
  • Distilled water (300 mL)
    Hot
  • Thread or monofilament (30 cm)

Equipment

500 mL beaker Hot plate Stirring rod Pencil Filter paper

Procedure

1

Heat 300 mL of distilled water to near boiling. Slowly dissolve as much alum as possible (about 80-100 g) while stirring.

10 min Handle hot liquids carefully
2

Filter the hot solution to remove any undissolved particles.

3 min
3

Pour a small amount into a dish and let it cool to form seed crystals overnight.

2 min
4

Select the best octahedral seed crystal. Tie it to thread and suspend in the main solution.

5 min
5

Cover loosely and store in a stable, cool location. Check daily.

2 min
6

After 1-2 weeks, remove the crystal, dry with a paper towel, and photograph the octahedral faces.

5 min

Expected Results

Large, transparent, octahedral crystals up to 2-4 cm across after 1-2 weeks. The faces should be smooth and the crystal should be nearly colorless and transparent.

Cleanup

Alum solution can be saved for future crystal growing or disposed down the drain. Rinse beaker.

Details

Category
Crystallization
Difficulty
Beginner (Middle School)
Duration
30 min
Est. Cost
$6.00
Steps
6
Materials
3

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Frequently Asked Questions

What is the objective of Alum Crystal Growing?
Grow large, transparent octahedral potassium alum crystals from a supersaturated solution.
How difficult is Alum Crystal Growing?
This experiment is rated as Beginner (Middle School). It takes approximately 30 minutes to complete.
What safety precautions are needed for Alum Crystal Growing?
Key safety precautions include: Use care with hot water; Alum is generally safe but avoid ingestion of large amounts.
What materials are needed for Alum Crystal Growing?
The main materials required are: Potassium alum, Distilled water, Thread or monofilament.
What results should I expect from Alum Crystal Growing?
Large, transparent, octahedral crystals up to 2-4 cm across after 1-2 weeks. The faces should be smooth and the crystal should be nearly colorless and transparent.