Grow Crystals
Create beautiful crystals from a supersaturated solution and learn about crystallisation, molecular structure, and geology.
Adult supervision recommended for younger children.
This experiment requires handling very hot water. An adult should prepare the hot solution. The crystals may take 24-48 hours to grow — patience is part of the science!
Safety First!
- • Boiling water is dangerous — an adult must handle the kettle and hot solution.
- • If using alum or borax, do not eat the crystals or the solution. Wash hands after handling.
- • Sugar crystals are safe to eat but let them dry fully first.
- • Place jars where they will not be knocked over during the growing period.
You Will Need
Which substance should you use?
| Substance | Crystal Quality | Time to Grow | Notes |
|---|---|---|---|
| Alum | Excellent — large, clear | 12-24 hours | Best results. Found in spice aisle. |
| Borax | Very good | 12-24 hours | Great with pipe cleaners. Found in laundry aisle. |
| Sugar | Good — edible! | 3-7 days | Slower but you can eat the results (rock candy). |
Step-by-Step Instructions
Make the Supersaturated Solution (Adult Step)
Have an adult heat about 2 cups of water until it is very hot (near boiling). Slowly stir in your chosen substance (alum, borax, or sugar) one tablespoon at a time. Keep adding and stirring until no more will dissolve and you can see a small amount of undissolved substance at the bottom. This is a supersaturated solution.
Approximate amounts: ~75g alum per cup of water, ~3 tablespoons borax per cup, or ~3 cups sugar per 1 cup water.
Add Colour (Optional)
If you want coloured crystals, add a few drops of food colouring and stir. Alum and sugar absorb colour well. Note: the crystals will be lighter in colour than the solution.
Prepare the Growing Surface
Tie a piece of string (or a pipe cleaner bent into a fun shape) to the middle of a pencil. The string should hang down into the jar when the pencil rests across the top. The string should not touch the bottom or sides of the jar — crystals need to grow freely.
Pour and Position
Carefully pour the hot solution into the clean jar (leave any undissolved substance behind). Lower the string or pipe cleaner into the solution and rest the pencil across the top of the jar. The string should be fully submerged but not touching the bottom.
Wait Patiently!
Place the jar in a spot where it will not be disturbed — a warm shelf is ideal. Cover loosely with a paper towel or coffee filter to keep out dust. Now wait. Check every few hours. With alum or borax, you should see crystals forming within 12-24 hours. Sugar takes 3-7 days.
Harvest Your Crystals
When your crystals are a good size, carefully remove the string and let the crystals dry on a paper towel. Examine them closely — notice the geometric shapes, flat faces, and sharp edges. Each substance forms its own unique crystal shape.
Extension: Compare Crystal Structures
Grow crystals from two or three different substances (e.g. alum AND sugar AND salt). Compare their shapes under a magnifying glass. Research the molecular structure of each substance. Can you see the connection between the molecular arrangement and the crystal shape?
What Happened?
Beautiful crystals grew on the string! They formed because the hot water could hold more dissolved substance than cool water can. As the solution slowly cooled down, it became supersaturated — holding more dissolved substance than it normally could.
The excess substance had to go somewhere, so it began to come out of the solution and attach to the string. The molecules arranged themselves in an orderly, repeating pattern, building up layer by layer to form solid crystals with geometric shapes.
The string provided a nucleation site — a rough surface where the first crystals could begin to form. Once a tiny crystal started, more molecules were attracted to it, and it grew larger and larger. Slow cooling produces larger, more perfect crystals. Fast cooling produces many small crystals.
The Science Behind It
Crystallisation is the process by which atoms, molecules, or ions arrange themselves in a highly ordered, repeating three-dimensional pattern called a crystal lattice. This is why crystals have flat faces, straight edges, and geometric shapes.
Solubility describes how much of a substance can dissolve in a liquid at a given temperature. For most solids, solubility increases with temperature — hot water dissolves more than cold water. A supersaturated solution contains more dissolved substance than it normally could at that temperature.
When a supersaturated solution cools, the extra dissolved substance precipitates (comes out of solution) as solid crystals. The molecules arrange themselves in the most stable, lowest-energy configuration — which is the crystal lattice.
Different substances form different crystal shapes because their molecules are different sizes and shapes:
- • Salt (NaCl): cubic crystals (the Na+ and Cl- ions form a cubic lattice)
- • Sugar (sucrose): monoclinic crystals (tilted rectangular shapes)
- • Alum: octahedral crystals (eight-faced diamond shapes)
Connection to geology: This same process happens in nature! When magma cools slowly underground, minerals form large crystals (like granite). When it cools quickly at the surface (like obsidian), the crystals are too small to see or do not form at all. Geodes — those beautiful hollow rocks lined with crystals — form when mineral-rich water seeps into cavities and slowly evaporates over thousands of years.
Think About It
Why do you think slow cooling produces larger crystals than fast cooling? Think about what the molecules need to do to form an orderly pattern.
Diamonds are crystals of pure carbon formed under extreme pressure and temperature deep in the Earth. What does this tell you about the conditions needed to crystallise different substances?
Snowflakes are ice crystals that form in clouds. Why does every snowflake have six-fold symmetry, yet no two are exactly alike?
Many medicines and electronic components use highly pure crystals. How might the process of crystallisation be used to purify substances in industry?
Knowledge Check
Test what you have learned! Select the correct answer for each question.
Question 1
What is a "supersaturated solution"?
Question 2
Why do crystals have geometric shapes with flat faces and straight edges?
Question 3
For most solid substances, what happens to solubility as temperature increases?
Question 4
What is a "nucleation site"?
Question 5
In geology, granite contains large visible crystals while obsidian (volcanic glass) does not. Why?
Key Concepts Summary
- ● Crystallisation occurs when molecules arrange themselves in a repeating 3D pattern called a crystal lattice.
- ● A supersaturated solution contains more dissolved substance than normal, causing the excess to crystallise out.
- ● For most solids, solubility increases with temperature — hot water dissolves more than cold water.
- ● Slow cooling produces larger, more perfect crystals. Fast cooling produces smaller crystals.
- ● The same process occurs in nature: minerals crystallise from magma (forming rocks) and from mineral-rich water (forming geodes and cave formations).