Intermediate-Level Science Projects

What Materials Make the Best Crystals?

Growing crystals isn't difficult, and it makes a great science fair project. You can grow crystals on a sheet of black construction paper on a sunny day. You also can make them on a paper clip that's tied to a piece of yarn and suspended in a supersaturated liquid solution.

If you're going to use the construction paper method, you'll need to cut a piece of black paper so it fits into the bottom of a glass or metal pie plate. For the paper clip method, you can tie a paper clip to a piece of yarn, and then pull the paper clip and part of the thread through a piece of cardboard in which a small hole has been cut.

Standard Procedure

We talk about “growing” crystals in this section, but you're not growing them in the strict sense of the word. You're causing them to form by supersaturating a solution of water and either salt or sugar.

The key to either method of growing crystals is to make a supersaturated solution of water and salt or sugar. In this experiment, water is called the solvent, and the salt or sugar is called the solute. You can use regular old sugar, and either rock salt or Epsom salt. You probably have at least one of those materials available in your home. If not, you can buy any of them in the grocery store. Designate one solute as your control and the other two as variables.

Don't forget to state your problem, formulate a hypothesis, and follow the other steps of the scientific method.

To make a supersaturated solution, you simply need to add salt or sugar to hot water until the water is incapable of dissolving any more of the solute. That happens when the molecules of the solvent get so crowded together that there's no more room between them for solute molecules.

Start with a large glass jar or metal pot filled with two liters of very hot water. Working with one tablespoon (15 ml) at a time, add either the sugar or the salt to the water, stirring the solution well after each addition. Eventually, you'll see undissolved solute starting to collect at the bottom of the container. When this happens, your solution is supersaturated.

If you're using the construction paper method, pour enough of the supersaturated solution into the pie pan so that the paper is just covered. Set the pan in the sun and allow the water to evaporate. When it does, you should be able to see crystals formed in the bottom of the pan.

If you're using the paper clip method, fill a clean jar with the supersaturated solution. Then, place the cardboard over the mouth of the jar and secure the yarn to the cardboard so that the paper clip hangs in the middle of the jar. The bottom of the paper clip should be about five centimeters from the bottom of the jar.

As the hot solution cools and the water evaporates, the solute molecules will begin to cling to the yarn and crystallize.

You'll need to repeat the experiment three times, using all three solutes in order to see which one makes the best crystals. Photographing your results would be a good idea. And be sure to record your observations.

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Excerpted from The Complete Idiot's Guide to Science Fair Projects © 2003 by Nancy K. O'Leary and Susan Shelly. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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