Part 1: Starch-iodine complex

As we observed in Lab 2, iodine binds with starch to form a deep blue-black complex:

iodine + starch ⟷ starch-iodine complex

â

Place 50 mL of water into one of your beakers (tap water is fine). Add a pinch of cornstarch or a few drops of liquid laundry starch and stir to dissolve. Add 3 drops of iodine tincture and swirl. You should see the blue-black color of the starch-iodine complex. Fill two test tubes with the solution. Place one test tube in a coffee mug filled with ice water and the other test tube in a coffee mug filled with hot water. Wait 5 minutes and then observe the two test tubes.

Part 2: Butterfly pea flowers

Place your butterfly pea flowers into one of your beakers. Fill the beaker with hot water and let the flowers steep for 5 minutes. Pour the liquid into three test tubes, leaving a little bit of room in each test tube (you can use your other beaker as a test tube holder). To one of the three test tubes, add 1 mL of vinegar. To another test tube, add a scoop of baking soda. Record your observations.

All reagents can be safely poured down the drain.

---

ANALYSIS

Answer the following questions in detail:

1) Which way (left or right) did the starch- iodine reaction shift when the solution was cooled in ice water?

2) Which way (left or right) did the starch-iodine reaction shift when the solution was heated in hot water?

3) Based on your answers to questions 1 and 2, is the formation of the starch-iodine complex endothermic or exothermic?

4) Do a little bit of research to understand where the color in butterfly pea flowers comes from.

5) Why does the color of the butterfly pea liquid change when vinegar is added? When baking soda is added?