Growing Crystals from Solution

One way that crystals form is from chemicals dissolved in water. If the water gets cooler, dissolves other chemicals, or evaporates, some of the dissolved chemicals can be deposited as crystals. Often, growing crystals can be a fairly long, involved process, but with this activity, we will grow some nice crystals quickly and easily.

To try this, you will need:

  • a disposable plastic or paper cup
  • epsom salts
  • water
  • a refrigerator
  • paper towels
  • a magnifying glass

Place 1/2 cup of epsom salts into the cup. Then add 1/2 cup of HOT water. You don't want the water to be boiling, but you want it to be hot enough that it is uncomfortable to put your finger in. Our hot water heater is set pretty high, and that worked well, straight from the faucet. If your hot water is set lower, heat it in the microwave for about 30 seconds or so. Don't heat the water in the plastic cup, as the cup will melt and make a mess. (That is the voice of experience talking!)

Stir the hot water and epsom salts well, to allow most of the salt to dissolve. It should not all dissolve. If it does, add a little more. When it is well stirred, place it in the refrigerator, with a small sign saying "Danger! Science experiment!" or something similar, to be sure that no one drinks it.

After three or four hours, examine the cup. You should find that the bottom of the cup has a beautiful cluster of needle-shaped crystals. At that point, carefully pour the water into the sink. Then cut the side of the cup, to allow you to lift the crystals out of the cup. Place them on a couple of folded paper towels to let them dry. If they fall apart, or if you are not pleased with the result, put them into another cup of hot water, and try again. You can regrow the crystals over and over.

OK, so what is happening? Solubility (the amount of something that will dissolve) is tied to temperature. The hotter the water is; the better it is at dissolving the epsom salts. If you had a way to see the atoms along the edge of the crystals, you would notice that some of magnesium sulfate (epsom salts) was constantly leaving the crystal to dissolve in the water, while other bits of magnesium sulfate constantly leaving the water to join the crystal. If everything balances, then the crystals stay the same size.

However, if the water is hot, the extra energy lets more of the salt leave, throwing off the balance. That means that the solid epsom salts will dissolve, until it reaches a new balance. At that point, the salts stop dissolving.

When you put the cup in the refrigerator, heat energy from the cup moves to the surrounding area. With less heat energy in the water, the balance shifts again. Now, you have more epsom salts joining the crystals, so the crystals grow. This will continue until things reach a new balance. At that point, the crystals stop growing.

Use the magnifying glass to look closely at your crystals. Notice that while some are larger than others, they all have the same shape. How many sides does each crystal have? Do they end in a point, or a flat face?

If you want to experiment further, you can continue to play with that balance point. Once your crystals have grown, pour off the water. In another cup, mix another batch of hot water and epsom salts. This time, wait until the water cools almost to room temperature, and then carefully pour it into the cup with the crystals from your first experiment. This time, do not stir! Put it in the refrigerator.

If you get the balance right, then your original crystals will not redissolve completely. As the solution cools, more magnesium sulfate will be added to the original crystals, causing them to grow larger. You can try repeating this several times. If your balance is off, and the crystals dissolve, then just heat the water again, and start over. When you are done, you can let the crystals dry and put them on your shelf. If they get broken or dusty, just dissolve them, and grow them again.