Even after miners have removed ore from the ground, it is usually still not ready to be used. In most cases it is mixed with or combined with other materials, which frequently are of no use. We separate the resource from the waste by taking advantage of differences in the characteristics of the two.

In this exercise, students will recieve samples of resources mixed with waste and asked to separate them. In doing so, they will get the experience of using differences of the properties of materials.

Needed materials one for each group of students:

• salt
• sand
• shortning
• containers
• water
• sieve
• access to microwave

Estimated Time: one hour: Each separation takes only about 5 minutes to perform. I can't estimate how long it would take the students to figure out what to do.

First Separation: salt and sand

Have the students mix the salt and the sand. Tell them that the salt is the resource you are mining.

Give your groups of students some time to work out a solution and try to do it. Then get back together to discuss what they did.

Discussion: Given water, heat and a sieve to separate salt from sand, most students will quickly see that water is the easiest way. Just putting the salt/sand mixture in a container with water will separate the two. Heating the water will separate the two more effectively because salt dissolves better in hot water. Also, although the sieve was useless when the salt was solid, it can be used effectively after the salt is dissolved. Finally, discuss how to get the salt out of the salt water. (To get the salt out will take more time that you have in class, but the water could be evaporated away over a few days)

This is very close to the method actually used by miners to separate saltpeter from the rock in which it was found. Although saltpeter is found in both limestone and sandstone the most productive mines are in sandstone caves where the saltpeter is embedded in the stone. Miners extracted it by crushing the sandstone and letting water percolate through it. The water was then filtered to get rid of bits of rock and evaporated to get the saltpeter crystals.

Second Separation

Have the students mix the sand and the shortning and ask the to develop a method of separating them. Tell them that the sand is the resource we want to purify

There are several ways of doing this.

• You could push the mixture through a seive. If the seive was the right size, you could end up with pure shortning, but you would not end up with pure sand. (separation by differences in particle size)
• If you heat the mixture, you could get it through the seive easier, but you still wouldn't end up with pure sand. (separation by differences in melting point and particle size).
• If you heat the mixture in water, the shortning will melt and rise to the surface, and the sand will sink to the bottom. This procedure will give you pure sand (differences in specific gravity, melting point, and particle size.)

Ask your students how they would get the sand out. While it is possible to decant the fat off the top, probably the easiest method is to leave it overnight (or put it in the refrigerator) until the shortning solidifies.

Both zinc and copper have separation by differences in specific gravity as part of their processes of purification.

Third Separation

Have students mix the salt and the shortning. Have the students touch the mixture and taste it. It should taste salty. After having done the first two, the students should realize that the only way to separate these two is to heat them in water.

Once the fat has melted and risen to the surface, have the students touch the surface of the fat and taste it. It should not taste salty.

I learned this procedure from a book on the history of soap making. Soap is made by mixing melted fat with a lye solution. Fat was a very precious resource on the frontier and pioneers sometimes had to make do with bacon drippings or other scrap fat. Before it could be used, the salt had to be removed. (NOTE: Pioneers made lye from fireplace ashes using the techniques of the first separation. Lye was put in a container with small holes in the bottom and water was dripped throuh it. The water was then evaporated to leave lye.)