Drip-drop Rocks
Table of Contents
Stalactites are icicle-shaped columns of stone that hang from the ceiling of limestone caves. Grow your own stalactites and find out how these unusual rocks form. Remember: just as they take a long time to grow in nature, laboratory stalactites may seem to take forever to grow. Be patient.
What You Need
- 2 glass jars
- 1 small plate
- 2 strands of twine (jute or cotton)
- 1 spoon
- Baking soda
- Hot water
What To Do
- Fill the two jars with hot tap water.
- Spoon some baking soda into each jar and stir. When the baking soda dissolves, add more. Stir.
- Place the two jars in a warm spot. Set the plate between them.
- Twist the strands of twine together to make one thick string. Wet the string.
- Dip one end of the string into each jar, so that the middle of the string hangs low over the plate.
- Let the jars sit for a week or more. Don’t touch! Look at the string.
How It Works
Baking soda dissolved in water makes an acidic solution. This liquid is a lot like the mineral-filled water that slowly drips from the walls and roofs of caves. Eventually, the water evaporates — it warms up, gives off carbon dioxide gas, and turns to vapor — leaving behind hardened calcium carbonate minerals. Over time, sometimes hundreds of thousands of years, these minerals form spectacular columns.
In this activity, a “stalactite” hangs down from the string. If the temperature is just right, you may also observe a “stalagmite” growing up from the plate. A “column” can form when a stalactite and stalagmite join together.
