Why Do Alum Crystals Grow Into Such Beautiful Shapes?
― Why Taking Your Time Matters
In a science experiment, you dissolve alum in water and leave it for a few days to a few weeks. Without anyone touching it, a jewel-like crystal with flat faces and sharp corners may have grown. The alum started as a powder. How does it end up in such a neat geometric shape?
Dissolve plenty of alum in hot water and let it cool slowly. Clear crystals slowly grow around a small seed crystal, either on the bottom or hanging from a thread. The finished crystal has a regular shape with flat faces and clean corners, even though nobody carved it.
Powdered alum has no such neat shape. As it dissolves and then turns solid again, what is actually going on?
Inside an alum crystal, atoms and ions are thought to line up in a fixed pattern that repeats regularly.
When crystals form slowly, atoms and ions have time to settle into the right positions, and the crystal grows neatly.
Let's look at this "regular arrangement" and the meaning of "growing slowly" in turn.
Why Crystals Have Angular Shapes: A Regular Arrangement of Atoms
When alum is dissolved in water, its atoms and ions move around freely and independently. But the amount beyond what the water can hold (the excess) starts to appear as a solid, a crystal. Atoms and ions join together and line up in a regular, repeating pattern. This regular structure is thought to be what produces the flat faces and the fixed angles of the corners.
"Solubility": The Limit of What Water Can Dissolve
The amount of alum that water can dissolve has a limit that depends on temperature. This limit is called "solubility." In general, the hotter the water, the more alum it can dissolve. So when you cool hot water full of dissolved alum, the part that can no longer stay dissolved at the lower temperature appears as crystals.
Atoms and ions simply line up by themselves, following fixed rules.
Why Slow Cooling Grows Beautiful Crystals
If you cool the water quickly, small crystals start to form everywhere at once. Each small crystal grows in a hurry, facing a different direction, so many small crystals with untidy shapes are thought to result. If you cool slowly instead, fewer new crystals are born. Around the crystal that is already there (the seed crystal), atoms and ions pile up over time, choosing the right positions. This "room to take time" is said to be the key condition for growing large, beautiful crystals.
Table salt crystals are close to cubes, and snow crystals are based on a hexagon. Both are thought to come from the regular arrangement of atoms and ions that is particular to each substance. A different arrangement gives a different crystal shape.
Something You Can Test Yourself
- Dissolve as much alum as you can in hot water, until no more will dissolve.
- Pick one small crystal (a seed crystal) from the bottom and hang it from a thread.
- Cover the container to keep dust out, and put it somewhere cool with as little temperature change as possible.
- Over a few days to a few weeks, watch crystals slowly grow around the seed crystal.
It is also worth comparing how the size and neatness of the crystals differ between a solution cooled in a hurry and one given plenty of time.
Summary
Alum crystals grow into neat shapes because of a property of crystals themselves: atoms and ions line up in a regular, repeating pattern. When the part that no longer stays dissolved appears as crystal, the more slowly it happens, the more time atoms and ions have to settle into the right positions, and the crystal is thought to grow large and well formed.
The beautiful faces and corners of a crystal are not anyone's design. They show the regularity that was already built into the world of atoms.
The way something dissolved turns back into a solid when conditions change can also be seen in nature. For how stalactites form in limestone caves, see this article.
For those who want to know more ― Terms, numbers and links to textbooksFrom middle-school science to current research, each topic is labelled with its level
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school "Chemistry," or advanced or sidebar content in textbooks
- UniversityUniversity-level specialist content (crystallography), not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled even at university
Middle schoolTerms: Words Around Alum Crystals
- Crystal: A solid made of atoms and ions arranged in a regular pattern.
- Solubility: The limit of how much of a substance can dissolve in a fixed amount of water at a given temperature.
- Seed crystal: A small crystal that acts as the core when growing a new crystal.
High schoolChecking with a Formula: How Much Crystal Appears on Cooling
We use the difference in solubility at a high and a low temperature to calculate how much crystal appears when the solution cools.
| In symbols | M ＝ （S1 − S2） × （V ÷ 100） |
| In words | Mass of crystal that appears ＝ (solubility at high temperature − solubility at low temperature) × amount of water ÷ 100 |
| Where the formula comes from | It comes from the definition of solubility and conservation of mass. When the solution cools, the limit of what can stay dissolved drops, and only the overflow comes out as crystals. M is the mass of crystals (in grams), S1 and S2 are the solubilities per 100 mL of water (in grams), and V is the amount of water (in millilitres). |
Crystal that appears (g) ＝ solubility at high temperature (g) − solubility at low temperature (g)
| Solubility at high temperature | The amount dissolved per 100 mL of water |
| Solubility at low temperature | The amount that can stay dissolved per 100 mL of water after cooling |
| Crystal that appears (g) | 60 − 10 ＝ 50 |
| Result | About 50 g of crystals appear as it cools |
| Crystal that appears (g) | 90 − 10 ＝ 80 |
| Difference from ② (g) | 80 − 50 ＝ 30 |
| Result | About 80 g of crystals appear, about 30 g more than in ② |
The bigger the difference in solubility between high and low temperatures, the more crystal appears on cooling. Alum is said to be one substance with a fairly large difference, which is one reason it is often used in crystal-growing experiments.
* The solubility figures here are rough values to help you understand the mechanism. Actual solubility is said to vary with how temperature is measured and with other conditions.
High school+"Supersaturation" and Nucleation
A state in which more substance than the solubility limit is still dissolved in water, without having crystallised, is called "supersaturation." From a supersaturated state, the formation of the first tiny seeds of crystal (nuclei) is called "nucleation." When the solution is cooled quickly, nucleation tends to happen in many places at once, and as a result many small crystals tend to form.
UniversityHow Crystallography Explains Which Faces Appear
Crystallography studies which faces develop well as a crystal grows, and which grow little. Some faces are easier for atoms and ions to pile onto, and others are harder. This difference is thought to be one factor that decides the final shape of the crystal.
📖 Derivations and further reading: Crystal growth (Japanese Wikipedia) ／ Solubility (Japanese Wikipedia)
ResearchWhat Is Still Unclear
- Which crystal faces develop well, and under what conditions, is a topic where a detailed theoretical explanation is still being worked on in crystallography.
- In making semiconductors and medicines, research and development continues on techniques for efficiently growing high-quality crystals with few impurities.
- Experiments are also carried out on growing crystals of proteins and other substances where gravity has little effect, such as on a space station. Researchers are studying whether these can yield better-quality crystals than on the ground.
Even one small alum crystal holds a rich topic where chemistry meets crystallography, and where research still goes on.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: aqueous solutions and recrystallisation | Basic terms: crystal, solubility, seed crystal |
| High school | Basic Chemistry: solubility | Calculating how much crystal appears on cooling |
| High school+ | Chemistry: properties of crystals (advanced) | Supersaturation and nucleation |
| University | Crystallography | Differences in how easily crystal faces develop |
| Research | Crystallography and materials engineering (ongoing research) | Understanding crystal face selectivity, industrial uses, growing crystals in microgravity |
- Explanations of crystals, solubility and recrystallisation in chemistry textbooks.
- Explanations of crystal growth and nucleation in crystallography materials.
- A research review of single-crystal growth techniques in materials engineering.
- Explanations of protein crystal-growth experiments in microgravity, from space science materials.
- Explanations of alum crystal-growth experiments, from chemistry education materials.
* The solubility figures are commonly cited rough values. Actual values are said to vary with conditions and measurement methods.
* This article is a general-audience science explainer. If you try growing alum crystals, be careful handling hot water, and do not put the finished crystals or the solution in your mouth.