Experiment: Growing Crystals

When substances change from a liquid or gas to a solid, they can form crystals. In a crystal the molecules of the material lock together in a regular and repeating pattern. If a crystal is allowed to grow undisturbed, it will form regular shapes such as cubes, or hexagon columns. The type of substance and how its molecules interlock determine the shape of the crystal.

You can grow your own crystals at home. For this experiment you will need a plastic or paper cup, sugar or salt (about a cup of either), and a pan for heating water. If you want to make both sugar and salt crystals you will need two cups.

Note: The following experiment requires the heating of water with a flame and should ONLY be done under adult supervision.

STEP 1 - Put a half a cup of tap water into the pan.

STEP 2 - Heat the water over a flame. If you are going to make salt crystals you need only heat the water to around 140 degrees. If you are going to make sugar crystals, bring the water up to boiling.

STEP 3 - Pour the sugar or salt into the water and stir. Keep adding more until it will no longer dissolve. Then add just a little more water and continue to heat and stir until all the material disappears.

STEP 4 - Turn off the burner and let the water cool to room temperature.

STEP 5 - Pour the contents into the paper or plastic cups.

STEP 6 - Put the cup somewhere where it will not be disturbed until the liquid evaporates. This could take several weeks. When you check the cup after all the liquid is gone you should see crystals coating the sides and bottoms of the cups.

If you made both sugar (above right) and salt crystals (right), notice they form different shapes.

A variation to the above experiment will give you larger and more regularly-shaped crystals This will require a pencil, string, and a weight (like a small metal nut or washer). After STEP 6:

STEP 7 - Tie the string to the middle of the pencil and tie the weight to the other end of the string. Make sure the distance between the weight and the pencil is less than the depth of the cup.

STEP 8 - Put the pencil across the top of the cup so that the string and weight hang in the cup. The crystals will form on the string as the liquid evaporates.