Leaching
When extracting some minerals, the mined ore must be soaked in a solution to remove the desired minerals. This process, called leaching, is done with copper, uranium, and other metals. A simple way to see how a solution can leach out a mineral is to use water to remove some salt from a mixture of pebbles and rock salt. (Part A) Using a known form of a crystal is one way of determining an unknown substance. Part B uses this process. Two different minerals are dissolved, evaporated, and described based on known forms of crystal structures. In an extension of this concept, we can remove calcium from a piece marble, limestone, or a sea shell with vinegar, a safe acid. (Part C)
MATERIALS
Part A
•Prepare a mixture of rock salt and aquarium pebbles. Any mixture will do.
Part B
• Unknown mixtures: Create a mixture of sand and table salt, sand and epsom salts, or sand and borax.
• Epsom salts
• Salt
• Borax (if desired)
• Water
• A shallow pan or dish, preferably glass or plastic
Part C
• LIMESTONE. These are common rocks found all over the world. Often they can be found as railroad fill rock or gravel for driveways. Chips can be found at garden supply stores. Sea shells can also work for this experiment.
• VINEGAR. This is best found as a cleaning aid because it has a stronger content of acetic acid but any vinegar will work.
• A shallow pan or dish, preferably glass or plastic
EXPLORATIONS
Part A (A basic exploration)
1. Use a prepared mix of gravel and rock salt.
2. Add water.
3. Pour the water in a pan and mix in order to dissolve any mineral.
4. Examine the rocks that remain. What disappeared?
5. Pour the water with dissolved minerals in a shallow dish.
6. Allow the water to evaporate. Observe and describe the crystals that remain in the STEM Notebook. Also note the time that has elapsed for the water to evaporate. Draw a conclusion about what has occurred.
Part B (A bit more elaborate)
1. When water dissolves a solid material, it forms a solution. The solution is a stable arrangement of the atoms of the chemical held up by the molecules of water. Most solutions are transparent and many are clear. We don't know that anything is in the water until the water is evaporated and we discover the residue. To observe the difference in how these residues appear after the water has evaporated, make at least two solutions--one of salt and water and one of epsom salts and water. You can do this simply by adding the salts to a small container of water and stirring.
2. From each container pour a small sample onto a piece of aluminum foil or wax paper so it make a puddle. Alternatively, a shallow dish can be used. In the example below watch plates are used.
3. Allow these puddles of liquid to evaporate. You can accelerate the evaporation by placing the samples on a hot plate. (A watch glass or aluminum foil will be required.)
4. Examine the crystal residue that is formed when the water is removed. Use a hand lens to examine the crystal shape and then determine the difference.
5. By examining the crystals and comparing to knowns, the unknown crystals from the solution can be identified.
6. Use the STEM Notebook to record the procedure. Note the time that has elapsed for the water to evaporate. Draw a conclusion about what has occurred and the identification of the unknown crystals. Describe the thinking that took place to do the identification.
Part C
1. A small piece of limestone or sea shell can be used to extract calcium.
2. Place a piece of limestone in a shallow dish.
3. Pour vinegar in the dish so that it almost covers the piece of limestone.
4. Allow the stone to sit overnight. Observe the formation of crystals:
Calcium acetate from this reaction can form crystals. The vinegar(acetic acid) leaches the calcium from the limestone, places it in solution, then, as the water evaporates, crystals of calcium acetate form on the rock surface in much the same way crystals form inside caves and rocks.
•Observe and describe the crystals that remain in the STEM Notebook. Also note the time that has elapsed for the water to evaporate. Draw a conclusion about what has occurred.