Kids, this is a great way to quickly make a large amount of crystal needles!

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

In a cup or a small, deep bowl, mix 1/2 cup of Epsom salts (magnesium sulfate) with 1/2 cup of hot tap water. Stir for about one minute to dissolve the Epsom salts. There will still be some undissolved crystals at the bottom. You have created a "saturated" solution, which means the liquid has dissolved all of the salts that it can hold.

Place the cup or bowl into a refrigerator. The cup or bowl will fill with needle-like crystals within three hours.

If you would like to grow colored crystals, you can color the hot water first with a drop of food coloring.

Tips:

• Epsom salts can be purchased in drugstores.
• Don't use boiling water to prepare your solution. Just use water that is as hot as you can make it from a tap. If you use boiling water you will still get crystals, but they will be more threadlike and less interesting. The temperature of the water helps control the concentration of the solution.
• You can place a small object at the bottom of the cup or bowl to make it easier to remove your crystals, such as a quarter or plastic bottle cap. Otherwise, carefully scoop the crystal needles from the solution if you wish to examine them or save them.
• As always with science experiments, it's best to wear gloves and eye protection. Don't drink the crystal liquid. It's not dangerous but it's not good for you either. 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
October 2013

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References:

Anne Marie Helmenstine at Chemistry.About.com,
http://chemistry.about.com/od/crystalrecipes/ht/cupofcrystals.htm