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TESTING OF MIXTURES
This project will demonstrate how chemical tests can be performed on known substances, then on unknown substances, to look for similarities. Certain kinds of tests can determine the known materials that are in unknown mixtures.
We have five envelopes numbered 1 through 5. Each envelope contains 5 teaspoonfuls of substances named in the "Teacher's Version" of this project (see T111 09). To the students, they will be unknown.
We have put 1 teaspoonful of each substance from envelopes 1, 2, and 3 onto a piece of waxed paper. We can examine them, using our senses of touch, smell, and sight (NO TASTING), to determine what they are. (BE CAREFUL WHEN SNIFFING, SO THAT NONE OF THE SUBSTANCE IS INHALED.) We'll make some notes of our observations, and our opinions of what they are.
It is very difficult to make accurate determinations, isn't it, by the use of our senses only. We now see the need for more sophisticated testing methods.
Your teacher will tell you what these three substances are.
There are still two envelopes that we haven't examined, tested, or discussed. They each contain a mixture of two of the known materials. Our task is to test them with in a more scientific manner, to see what we can learn about the ingredients of the mixtures.
Before we can understand the results of these tests when performed on the unknown mixtures, we have to perform the same tests on the known substances. That way, we will have a valid point of comparison.
For our first test, we will see how our known substances behave when mixed with water. We know that some kinds of solid materials will dissolve in water.
Using three test tubes, we'll put one level teaspoonful of each of the known substances into a test tube half-filled with water. Shake the tubes to mix them, let stand a few minutes, and record our observations.
Next, we'll do the same with the two unknown mixtures and record our observations of those tests.
Our second test will use heat. We know that heat melts or burns some solid materials sooner than others. We'll put one level teaspoonful of each of the known materials into dry test tubes. Using a test tube holder, we'll hold each tube over a flame for a few minutes. We need to make sure that the mouth of the tube is pointed away from people. Move the tube in a small circular motion to distribute the heat evenly. We'll record our observations, and then test the unknown materials, and record our observations of those tests.
The next test will use iodine, to see if it causes any kind of reaction. We will use it in our tests because it indicates the presence of starch by turning a blue-black color.
[Iodine solution is available at drugstores. It is poisonous, and stains are difficult to remove. Wash your hands after using it. A dilute solution of sodium thiosulfate (also available in drugstores), in water can be used to remove stains.]
Using another set of five dry test tubes, we'll put one level teaspoonful of each known substance into a tube. We'll put a few drops of iodine solution into each tube (with an eyedropper), and observe the reactions, if any. Record our observations, and perform the same test on the unknown substances. These observations must be recorded, as well.
[For added enjoyment and edification, we can use iodine solution to test for starch in various other materials, such as slices of apple, bread, candy, potato, banana, butter, and cheese.]
test can be performed exactly as the last one, using vinegar instead of
iodine. Both white vinegar and red vinegar, being acetic acids, indicate
the presence of a
Make the vinegar test on all known substances, record the observations, then test the unknown materials, and record those observations.
[For more fun, we can use vinegar to test slices of apple, flour, ground chalk, limestone rock, marble rock, bones (we won't ask for personal contributions!), egg shells, or sea shells.]
The drugstore has Benedict's solution and Fehling's solution, both of which indicate the presence of simple sugars (sucrose or dextrose). They do not react to complex sugars such as table sugar.
We'll put one level teaspoonful of each known substance into the three test tubes (of another set of five dry test tubes). Add 1/2 tube of water and 10 drops of Benedict's (or Fehling's) solution to each.
Using a test tube holder, and pointing the mouth of the tube away from people, heat the tubes gently. The color will change from blue to green, to yellow, to orange, to brick read, depending upon the presence, or concentration of sugar in the materials. Record the results of this test, and of the same test performed on the unknown mixtures.
Now, using the information we have, and your noted observations, we can attempt to identify the ingredients of envelopes numbered 4 and 5.
There are other materials we can test similarly: powdered plaster of paris; granulated sugar; biscuit, pancake, or cake mix; powdered milk; flour; or powdered potatoes.
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