Biology Diffusion Lab

Biology Diffusion Lab Name:____________________________ Period: _____ Date:_________________ Introduction: In this lab, you will observe the diffusion of a substance across a semipermeable membrane. Iodine is a known indicator for starch. An indicator is a substance that changes color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch. Prelab Observations: In the space below, describe what happens when iodine comes into contact with starch. It changes its color to a brown, blue-black or purple. Procedure: 1. And a teaspoon of cornstarch to a plastic baggie, then add 20 mL of water. Close the baggie with a rubber band. 2. Rinse any spilled cornstarch off of the bag. 3. Fill a plastic cup halfway with water. 4. Add enough iodine so that the water is approximately the color of apple juice. 5. Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture. Do not fully submerge the baggie or allow its open end to go below the water line 6. Wait twenty minutes and record your observations in the data table. While you are waiting, answer the questions below. 7. Your instructor may have to place your baggie aside to look at again during the next class period. Pre-Lab Questions: 1. Define diffusion. The movement of molecules from a high concentration to a low concentration. Biology Diffusion Lab Name:____________________________ Period: _____ Date:_________________ 2. Why is iodine called an “indicator”? Through diffusion it changes color when mixed with starch. 3. What does “selectively permeable” mean? It only allows specific molecules to pass through it. 4. Molecules diffuse from areas of ___high____ concentration to areas of ___low_____ concentration. What's in the Bag? Consider the concentrations of the substances used. The more concentrated the mixture is, the larger amount of a substance it contains. 1. Which has a higher concentration of starch, the plastic cup or the baggie? _________Baggie________ 2. Which has a higher concentration of iodine, the plastic cup or the baggie? ________Plastic Cup_________ Pre-Lab Predictions 1. If the bag is permeable to starch, which way would the starch move, into the bag or out of the bag? ___Out___ 2. If the bag is permeable to iodine, which way would the iodine move, into the bag or out of the bag? __In___ 3. If the bag is permeable to iodine… a. what color would you expect to see in the baggie? _____Black and blue______ b. what color would you expect to see in the plastic cup ____Apple Juice color___ 4. If the bag is permeable to starch… a. what color would you expect to see in the baggie? ___White____ b. what color would you expect to see in the plastic cup _____Clearer Apple juice color____ 5. Predict the color you expect to see in the baggie: ____Blue/Black______ 6. Predict the color you expect to see in the plastic cup: ___Clearer Apple Juice ___ Observations Write your observations in the table below: Biology Diffusion Lab Solution in Plastic Cup Solution in Bag Name:____________________________ Period: _____ Date:_________________ Starting Color Final Color Apple Juice Color Apple Juice White Blue and Black Post Lab Analysis 1. Based on your observations, which substance moved, the iodine or the starch? _Iodine__ 2. How could you tell this was the case? The iodine turned the starch to a bluish-black color. 3. Was the plastic baggie permeable to starch, iodine, or both? The bag is permeable to both changing the color of both 4. Was the baggie selectively permeable? Why or why not? The bag is selectively permeable because it allows for iodine to pass through. 5. On the sketch of the cup and baggie in the space below, draw and label arrows to how diffusion occurred in this lab. 6. Imagine if you repeated the experiment, except this time you put the iodine solution in the baggie and the starch solution was in the plastic cup. What colors would you expect to observe in the baggie and in the cup. Be detailed in your description. The baggie would be apple juice color and the cup would be a bluish color. Biology Diffusion Lab Name:____________________________ Period: _____ Date:_________________ 7. Based on what you observed in this experiment, why do you think it might not be a good idea to store iodine in a plastic bag? If iodine is stored in a plastic bag it would be able to leak through and contaminate other substances.