Grow your own Stalactites at Home

Drip-drop Rocks

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

  1. Fill the two jars with hot tap water.
  2. Spoon some baking soda into each jar and stir. When the baking soda dissolves, add more. Stir.
  3. Place the two jars in a warm spot. Set the plate between them.
  4. Twist the strands of twine together to make one thick string. Wet the string.
  5. Dip one end of the string into each jar, so that the middle of the string hangs low over the plate.
  6. 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.

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