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SEMI-SOPHISTICATED TESTING OF MIXTURES-TEACHERS VERSION
Various tests can be performed on known materials to see what reactions take place.
Unknown materials can then be compared to them for similarities.
Such tests serve as indicators that show the presence of known materials.
To introduce students to some simple indicators, prepare five numbered envelopes for each student or for small groups of students.
In the envelopes, place 5 teaspoonfuls (five 5-ml spoonfuls) each of the following materials:
1. Granular dextrose or glucose (available at drugstores).
(Note: Ordinary sugar cannot be used. It contains a small amount of starch to keep it from caking. It will not produce satisfactory results in the tests described here.)
2. Powdered starch (available at grocery stores).
3. Baking soda (available at grocery stores).
4. An equal mixture of dextrose (or glucose) and starch.
5. An equal mixture of dextrose (or glucose) and baking soda.
Have students place 1 teaspoonful (one 5-ml spoonful) each from the numbered envelopes 1, 2, and 3 on a piece of waxed paper, plastic, or aluminum foil.
Let them describe the materials without naming them -- by sight, touch, smell, but not by taste.
(Caution: Tasting unknown materials can be dangerous.)
Observations can be recorded on a table similar to that shown.
Have students discuss limitations of the senses in trying to identify materials.
When finished, the materials can be named.
Now give students the envelopes numbered 4 and 5.
Tell them they are mysteries to be solved.
Each contains a mixture of two of the known materials.
Students can then perform the following tests and record their observations.
When finished, they can deduce what materials make up the mystery mixtures.
Test 1 -- Water as an indicator.
Water dissolves some solid materials.
can half-fill three test tubes with water, put 1 level teaspoonful (one
5-ml spoonful) of the known materials in them, shake the tubes to mix
Students need not agree in descriptive words. It is best to let each record in his or her own terms.
Next, test the mystery mixtures and record findings.
Test 2 -- Heat as an indicator.
Heat melts or burns some solid materials sooner than others.
Students can put 1 level teaspoonful (one 5-ml spoonful) of each of the known materials into three dry test tubes.
Holding the test tubes with a test tube holder, heat over a flame by moving the tube in a small circular motion to distribute the heat more evenly.
(Caution: Keep the open end of the test tube pointed away from faces and other people.)
Record observations, then repeat the test using the mixtures.
Test 3 -- Iodine as an indicator.
Iodine, available at drugstores, indicates the presence of starch by turning a blue-black color.
Students can put 1 level teaspoonful (one 5-ml spoonful) of each known material into dry test tubes or place materials in three piles on a piece of waxed paper, plastic, or aluminum foil.
Using an eyedropper, put a few drops of iodine solution on each material.
Record observations, then test the mystery mixtures.
(Caution: Iodine solution is poisonous, and stains are difficult to remove. Have students wash their hands after using it. A dilute solution of sodium thiosulfate, available in drugstores, in water can be used to remove stains.)
Students can use the iodine to test for the presence of starch in various other materials, such as slices of apple, bread, candy, potato, banana, butter, and cheese.
Test 4 -- Vinegar as an indicator.
Vinegar (white or red) indicates the presence of a carbonate by bubbling and fizzing.
can put 1 level teaspoonful (one 5-ml spoonful) of each of the known materials
into three dry test tubes or in three piles on waxed paper, plastic, or
Add a few drops of vinegar to each with an eyedropper.
Record observations, then test the mystery materials.
Vinegar is acetic acid and reacts in the presence of a carbonate by releasing carbon dioxide gas.
Students can use it to test various other materials, such as slices of apple, flour, ground chalk, limestone rock, marble rock, bones, egg shells, or sea shells.
Test 5 -- Benedict's solution as an indicator.
Benedict's solution (or Fehling's solution, both available at drugstores) indicates the presence of simple sugars (sucrose or dextrose).
(Note: It does not react to complex sugars such as table sugar.)
Demonstrate the use of Benedict's solution by placing 1 level teaspoonful (one 5-ml spoonful) of each known material into dry test tubes.Add 1/2 tube of water and 10 drops of the Benedict's solution.
Heat gently, holding the mouth of the tube away from yourself and others.
Students will see the color change from blue to green to yellow to orange to brick red, depending on the concentration of sugar present.
Next, test the mystery mixtures for the presence of sugar.
Using their own version of the table as a guide, students can now deduce the materials that make up substances numbered 4 and 5.
Following this experience, other materials can be tested in similar ways: try powdered plaster of paris; granulated sugar; biscuit, pancake, or cake mix; powdered milk; flour; powdered potatoes.
After characteristics are identified, mixtures can provide new mysteries to solve.
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