Kartchner Caverns Microbial Observatory
National Science Foundation
photo of caverns


 

Grades K-3:cavern photo

Growing Crystals.

There is nothing more appealing than candy.  This experiment will allow young scientists to make their own candy in a jar while providing an opportunity to learn about crystal growth.  Many cave formations are actually mineral crystals that form from surrounding mineral-laden water at a site of crystal nucleation and continue to grow for many, many years.  The size of the crystals is determined by 1) the amount of dissolved minerals in the surrounding water, and 2) the amount of time in which the growing crystal is undisturbed.  If disturbed, crystal growth may cease or at least be slowed, which highlights how stable cave environments must be in order for beautiful crystal formations to develop.  Making rock candy can simulate the process of cave crystal formation.  This experiment will give students the opportunity to watch crystal growth over 2-3 weeks and to gain an appreciation for the time necessary for crystals to grow as well as the importance of keeping the environment undisturbed.  And in the end, they can eat their experiment!! 

Experiment 1, Figure 1
What you will need:

1) 2 cups of water
2) 4 cups of sugar
3) a small pan
4) a wooden spoon
5) a small, clean glass jar (baby food jar or 1 pint Mason)
6) a cotton string with a small galvanized washer tied to the end
7) a pencil to suspend the string in the jar
8) magnifying glass

 

 

The experiment:
First, heat the water in the saucepan over medium heat until it begins to boil.  Next, add the sugar to the water and stir continuously with the wooden spoon until the sugar completely dissolves and the solution becomes clear.  The water should be held at a slow rolling boil.  Once dissolved, pour the sugar solution carefully into the jar.  Tie the string with the washer to the middle of the pencil (Top Figure) (the tied string should reach about 2/3 to the bottom of the jar). (Bottom Figure)

Experiment 1, Figure 2

Experiment 1, Figure 3

Dip the string into the sugar solution, remove it, and lay it on a piece of waxed paper in a straight line, and let it dry for a couple of days.  The string will act as the “site of crystal nucleation” for this experiment.  Cover the jar with another small piece of waxed paper.
             
Now comes the fun part.  In a very quiet, still place, out of the way of household traffic, place the jar on a shelf where the water can cool to the point where normally sugar could not be dissolved.  The sugar remains dissolved, however, resulting in an unstable state.  The sugar wants to precipitate out of solution and crystallize, but it can't.  It has no site of crystal nucleation.

Then you add the treated string.  This gives the excess sugar a place it can "grab onto".  Crystals begin to grow as the sugar finds its way onto the "seed crystals" the treated string provides. Over the next weeks, you can watch as the crystals grow larger and larger on the string looking more and more like rock candy .  It’s tempting, but don’t touch the jar until the experiment is finished or else the young crystals may break off and have to start re-growing on the string.  After 2-3 weeks, the crystals will be quite large and strong, and the string can be pulled out and laid again on waxed paper for it to dry.  After the string is dry, look at it using a magnifying glass and observe their very regular crystals that have formed along it full length.  To contrast the formation of large crystals, you can prepare a second jar of dissolved sugar solution, place next to the first jar, and as the crystals are forming you can bump or swirl the solution once a day to disrupt the crystal formation process.  Notice the difference in crystal size and shape between crystals in the undisturbed jar and the jar that has been disturbed periodically.

Once the crystals on the string are dried, you can further enjoy the experiment by sucking on the delicious sugar crystals you have grown!  Remember, don’t eat the string!  You can also add a few drops of food coloring to the sugar solution before cooling to create crystals of different colors, which are even more fun to eat!! 


 

 

 

updated 7/2009

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