Precipitates, Solution and Limestone Marl Deposition
Activity
By Kimberly Mullen, CPG
Introduction
Limestone can be found in many areas of Ohio, sometimes found beneath thick
layers of glacial deposits (up to 150 feet thick in places), or located at surface
level. Consisting primarily of calcium carbonate (CaCO3), limestone is a chemical
sedimentary rock that formed in solution. It can go into solution in contact with
acidic water. Limestone can contain caverns and cavities in which the ground
water can flow. The caverns of Ohio Caverns are formed within the Columbus
Limestone Formation. Sinkholes and karst topography can often be observed in
areas with limestone bedrock.
Other evidence of limestone bedrock exists in the formation of marl and a white
chalky residue found where springs discharge ground water to the surface in
streams. Marl can indicate the presence of cooler ground water with CaCO3 in it
mixing with the warmer surface water. The CaCO3 precipitates out of solution
and deposits on the bottom of the stream. Marl is white, gray, or buff (depending
on the minerals present).
Precipitate activity
Materials
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Alum
Household ammonia
Warm water
Mason jars (the 1-pint size works best).
Instructions
1. Divide students into teams.
2. Each team must collect 1 Mason jar, 1 cup of warm water, 1 teaspoon of alum,
and 2 teaspoons of ammonia.
3. Pour the 1 cup of warm water into the Mason jar.
4. Stir in the 1 teaspoon of alum until it completely dissolves.
5. Add the 2 teaspoons of ammonia and observe what happens.
6. Make sure that students understand that the white residue at the bottom of the jar
is a precipitate. It comes out of solution just like marl does as ground water comes
to the surface at a spring.