Many areas of the Earth’s surface were once under the oceans. Large
coral reefs covered many of these areas. For example, Texas and New Mexico
were living coral reefs at one time, larger than the present Great Barrier
Reefs in Australia. These great reefs became rock as the sea retreated from
the land. This rock consists of the remains of shelled animals, which are
composed mainly of calcium carbonate (calcite). This type of calcium
carbonate rock is called limestone. Limestone will dissolve if acid is put
on it. Geologists usually use a 10% HCl solution for this "fizz"
Groundwater percolates through soil and rock within the earth.
Groundwater is naturally acidic. When groundwater percolates through
limestone, it dissolves it. This is how caves form.
The dissolved limestone is transported in solution by groundwater.
Stalactites and stalagmites form as groundwater drips from the ceiling of a
cave. Some of the water evaporates, and new limestone precipitates from the
remaining saturated solution. This builds hanging pillars of limestone
called stalactites, and stalagmites, limestone pillars that rise from the
floor of the cave. You can remember that stalactites hang from the top, by
remembering the "t" in "tite," stands for the
"t" in "top." Limestone formed by this type of inorganic
precipitation is called travertine.
- Before lab, gather the materials needed for each student group. You
may wish to run the experiment once before class to get a feel for how
quickly the "stalactites" form.
- Illustrate how stalactites form with the following experiment. Make a
solution of warm to hot water and dissolve baking soda in it until it
forms a saturated solution. It is easier to boil water and stir in as
much baking soda as will dissolve.
- Fill two small containers half full with the baking soda solution.
Place the containers about 20 cm apart, and put a small saucer or lid
between them. Saturate the mop thread with the solution. Put an end of
the mop thread into each container. Suspend the thread between the two
containers. Crystals of baking soda (or tiny stalactites) will slowly
form in the saddle area of the thread. Use the picture below as a guide.