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Grow a salt crystal on a thread: why does it come out as a neat cube?
Crystal Growth lab · Last updated: 16 October 2026
You grow a salt crystal on a thread and watch it build up layer by layer. Then they find out why it comes out as a neat cube instead of a random lump. The answer lies in how the tiny particles of salt fit together, and it is the same for snowflakes, sugar and gemstones.
What you'll discover
- Salt dissolves in water, and when water evaporates from a saturated solution, the salt comes back out as crystals.
- Salt is made of sodium and chloride particles that stack in a repeating cube-shaped pattern.
- Each new layer copies that pattern, so the whole crystal grows as a larger cube.
The science in plain words
Common salt is sodium chloride. It is made of two kinds of charged particles, called ions: positive sodium ions and negative chloride ions. When salt dissolves, water molecules pull these ions apart and they spread through the water. Water can only hold so much salt. A solution holding as much as it can at that temperature is called saturated.
If water evaporates from a saturated solution, or a hot saturated solution cools, there is no longer room for all the salt to stay dissolved. The extra ions come out of solution and attach to a surface, such as a rough thread or a tiny seed crystal. This process is called crystallisation.
In a salt crystal, each sodium ion is surrounded by six chloride ions, and each chloride ion by six sodium ions, alternating in every direction at right angles. This arrangement repeats millions of times, making a cube-shaped lattice. As the crystal grows, new ions settle into the same pattern on each face, so the crystal keeps the cube shape on a larger and larger scale. Slow growth gives large, clear crystals, while fast evaporation gives many small ones. Other substances stack in other patterns, which is why sugar, alum and snowflakes have their own typical shapes.
Where it fits
Common questions
Why are salt crystals cube shaped?
Salt is made of sodium and chloride ions that stack in alternating rows at right angles, forming a cube-shaped pattern. As a crystal grows, new ions join in exactly the same pattern, so the crystal keeps its cube shape no matter how large it gets.
How do you grow salt crystals at home?
Stir salt into hot water until no more will dissolve, making a saturated solution. Pour it into a clean jar, hang a thread in it and leave it somewhere still. As the water slowly cools and evaporates, salt crystals form on the thread over several days.
What is a saturated solution?
A saturated solution holds as much of a substance dissolved in it as it can at that temperature. Adding more simply leaves undissolved solid at the bottom. Warmer water can usually dissolve more of a solid, so a hot saturated solution forms crystals as it cools.
Related labs
- Solutions: How much can you dissolve in a glass of water, and does warming it help?
- States of Matter: Solid, liquid, gas: the same molecules, just jiggling harder.
- Molecular Bonding: What holds two atoms together, and how hard do you have to shake them to break it?
See all chemistry labs or every Taz science lab.
Try it yourself
The Crystal Growth lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.