Procedure Dfiltration

1. Review a textbook section on filtration.

2. To demonstrate filtration, follow these steps:

a. Place a glass funnel in the ring of a ring stand over an empty beaker. Fold a piece of filter paper in half and then in half again. Open one thickness of the filter paper to form a cone, wet the cone, and place it in the funnel. The filter paper is used to demonstrate how movement across membranes is limited by the size of the molecules, but it does not represent a working model of biological membranes.

b. Prepare a mixture of 5 cc (approximately 1 teaspoon) powdered charcoal and equal amounts of 1% glucose solution and 1% starch solution in a beaker. Pour some of the mixture into the funnel until it nearly reaches the top of the filter paper cone. Collect the filtrate in the beaker below the funnel (fig. 5.3).

c. Test some of the filtrate in the beaker for the presence of glucose. To do this, place 1 mL of filtrate in a clean test tube and add 1 mL of Benedict's solution. Place the test tube in a water bath of boiling water for 2 minutes and then allow the liquid to cool slowly. If the color of the solution changes to green, yellow, or red, glucose is present (fig. 5.4).

d. Test some of the filtrate in the beaker for the presence of starch. To do this, place a few drops of filtrate in a test tube and add a few drops of iodine-potassium-iodide solution. If the color of the solution changes to blue-black, starch is present.

e. Observe any charcoal in the filtrate.

3. Complete Part D of the laboratory report.

Figure 5.3 Apparatus used to illustrate filtration.

Benedicts Blue Solution
Figure 5.4 Heat the filtrate and Benedict's solution in a boiling water bath for 2 minutes.
Starch Anatomy

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