Key Concepts
Crystals
Saturation
Evaporation
 
Introduction
Did you know you can grow your own crystals at home? You can—and it's easy! Crystals have a definite geometric pattern and if all goes well, the crystals you grow will be sharply defined, with crisp right angles and smooth faces that vary in size.
 
Background
Table salt is made of many tiny crystals. When you mix these salt crystals with water, they dissolve, losing their crystalline form. When the water evaporates, the salt crystals form once again. The science of crystals, or crystallography, calls crystals shaped like these “cubic.” This shape is determined by the way the individual atoms in salt pack together, much as the shape of a pile of oranges would be determined by the way they stack together. Your finished salt crystals should be strikingly beautiful. The key to growing these stunning crystals is quick evaporation—and some string.
 
Materials

  • One-quarter cup of table salt
  • One cup of water
  • About three feet of cotton string
  • A saucepan (Enlist an adult's help when working with hot objects and using the stove.)
  • Spoon to stir a hot solution
  • Cup or small jar (It should have sides at least five inches high and able to hold boiling water.)
  • Food coloring (optional)
  • A tray or plate to hold the cup
  • Newspapers or towel to put under the object (optional)

Preparation
  • Cut the string into six or seven pieces, each about five inches long, and tie them together at one end so they look like a string bouquet.
  • Place the cup or jar on a safe surface where it can remain undisturbed for a few days. Use newspapers or a towel underneath the container to protect the surface if necessary.
 
Procedure
  • To prepare the salt solution bring the water to a rolling boil in the saucepan.
  • Add about a quarter of the salt (optional: a couple of drops of food coloring). Mix with a spoon to help dissolve. What happens to the boiling water’s appearance?
  • Continue adding and mixing in salt until no more will dissolve into the water. How much salt were you able to add before it stopped dissolving? You should now have a “supersaturated” solution. What does the solution look like now? You might notice a film of salt crystals forming in a layer on the surface of your solution.
  • Turn off the heat and carefully pour the solution into your cup.
  • Carefully submerge the knotted end of your strings into the solution and arrange strings evenly so that the ends dangle over and around the rim of your cup. What purpose do you think the strings will serve? Did you notice the strings begin to soak up the saltwater solution?
  • Leave the container someplace where it won’t be disturbed.
  • Wait and occasionally check in on your crystals. What do you see happening after one hour? One day? Two days?
  • Extra: Once the salt solution evaporates enough that it is no longer covering the knot of the strings, you can repeat the above steps and make a new batch of salt solution to add. What happens over the course of the next day or two?
  • Extra: Repeat the activity, but this time use different lengths of string, hanging over different objects—at different angles. How does this change the way your salt strings grow?
  • Extra: Try to make your crystals different colors by adding food coloring. Make a first batch of salt solution with one color. Wait until the crystals have grown and the solution has evaporated. Now make a second batch with a different color. Add this to the same container without removing the strings. What happens as this second color evaporates and forms crystals?


Observations and results
When you add salt to water, the crystals dissolve and the salt goes into solution. But you can't dissolve an infinite amount of salt into a fixed volume of water. When as much salt has been dissolved into a solution as possible, the solution is “saturated.”

This saturation point is different at different temperatures; the higher the temperature, the more salt that can be held in the solution. When you boil a batch of saltwater, you cook the salt and water to an extremely high temperature, so the excess salt remains in the solution. But when the saltwater begins to cool, there is more salt in solution than is normally possible. The solution is therefore “supersaturated” with salt.

Supersaturation is an unstable state, and the salt molecules will begin to crystallize back into a solid. This begins at a "crystallization nucleation" site—such as the fuzzy end of the string. Stirring or jostling of any kind can also cause the supersaturated salt to begin crystallizing.

In a couple of days you should be able to see that your strings grew fatter from the crystallizing salt. If you continued adding salt solution when it's evaporated below the knot, you should be able to grow long salt crystal stalactites.

More to explore
What Are Crystals? From Science Kids at Home
Crystal Creations, from Exploratorium
How to Make Rock Candy, from WikiHow
Quick Crystal Cup, from Home Science Tools

This activity brought to you in partnership with Exploratorium

Exploratorium