Experiment 4: Evaporation from a String - Stalagmites and Stalactites
Crystals Experiments Index
1. Simple Crystals on a String. 2. Growing Large Crystals with Seed Crystals. 3. Magic Salt Crystal Garden. 4. Evaporation from a String - Stalagmites and Stalactites. 5. Microcrystals Under the Microscope. 6. Supercooled Chemical Melt.
Discussion.
Ground water saturated with a calcium carbonate (or, less commonly, some other mineral) moves through the walls of a cave. Evaporation of this mineral-rich water where it drips down from the ceiling forms icicle-like deposits called stalactites. Water dripping on the cave floor underneath stalactites builts up similar deposits called stalagmites. This experiment mimics this natural process through the evaporation of salt-laden water from a string.
What you need.
- Two clean pint jars or 600ml beakers
- A water-soluble salt such Magnesium sulfate (Epsom salt) or Sodium Carbonate (washing soda).
- Water.
- Spoon.
- 18 inch long strip of cotton (e.g. torn from an old towel), about 1 inch wide. Wool or cotton string can also be used.
- Two washers - one on each end of the towel strip to act as weights.
- Tray or cookie sheet to catch the drippings.
What to do.
Fill each jar about 3/4 full of hot water. Place jars on tray. Add chemical while stirring until no more can be dissolved in the hot water. Put each end of the towel strip with washer attached in a jar. Position the beakers so the towel hangs about one or two inches from the tray. The towel will begin to drip. Set the experiment in a safe place where it will be undisturbed for one week. Your stalactites and stalagmites will begin to grow, and if you are patient, they will meet and form a column.
What can be learned.
This experiment models how crystal deposits form in caves. Water containing a dissolved salt moves through a porous string. As the water evaporates, crystals are deposited. The formation grows from the top down and from the bottom up, as the water movement and evaporation process continues.
