For a handy experiment you can show your young children or let your teens do with your supervision, two well-known experiments exist that demonstrate chemical reactions with iodine and cornstarch. Iodine is a common element found in many medicine cabinets. One of the properties of iodine is that it turns purple in the presence of starch, which is a common staple of most kitchens in the form of cornstarch. You can use this property to look at how starch reacts with different chemical and enzymes. The objective of the first experiment is to show how enzymes in saliva begin to digest the starch in the iodine and starch solution. Hypothesize with your audience how the starch and iodine solution changes when starch is digested. When you add saliva to the iodine and starch solution, the enzyme amylase breaks down starch in saliva to begin digestion, and the solution becomes clear while the control solution that has no saliva remains purple. The objective of the second experiment is to show how much vitamin C is in each juice. Vitamin C buffers the reaction between the iodine and starch and makes the purple color disappear. This experiment hypothesizes that the juice with the highest level of vitamin C will require the fewest drops to clear the purple color from the solution. Orange juice, with the highest vitamin C content will require the fewest drops to stop the reaction while cherry juice will require the most.
Saliva and Starch Digestion
Pour a teaspoon of water into one of the test tubes. Mark this "Tube A" with a piece of masking tape.
Spit into the teaspoon until it is full. Pour the saliva into the second test tube. Mark this "Tube B" with a piece of masking tape.
Measure 1/4 teaspoon of cornstarch and place in each test tube. Shake each tube to dissolve the starch.
Put on the safety glasses. Fill the eye dropper with iodine.
Place four drops of iodine into each test tube. Watch as the fluid in both tubes turns a deep blue color.
Place the tubes in the holder and leave them undisturbed for 30 minutes.
Check the color after 30 minutes. The test tube filled with water and cornstarch will still be purple. But the test tube with saliva will have lightened or even become clear. This is because the enzymes in saliva break down starch. This shows the first steps in digestion.
Exploring Vitamin C Content in Juice
Pour a cup of water into a bowl. Add 2 tablespoons of cornstarch and mix with the fork until the starch is completely dissolved.
Put on the safety glasses. Fill the eyedropper with iodine. Add the iodine to the cornstarch mixture one drop at a time until the entire mixture is a deep blue color. Empty the rest of the eyedropper. Rinse out the dropper with water.
Pour 2 tablespoons of the iodine and cornstarch mixture into four test tubes and place them in the rack. With masking tape and a pen, label each tube for Orange, Lemon, Apple, or Cherry juice.
Fill the eyedropper with orange juice. Put two drops into the first test tube. Swirl the tube to mix the solution. Continue to add juice and swirl until the solution is clear. Record the number of drops needed to make the solution clear.
Repeat with the other three juices, recorded the number of drops for each juice. Because ascorbic acid, or vitamin C, stops the reaction between cornstarch and iodine, the juice with the highest level of vitamin C will require the fewest drops to clear the solution. Juices that contain less vitamin C will require more drops of juice to clear the solution.