Prepare a Vitamin C test solution by stirring 1/4-teaspoon of cornstarch into four cups of distilled water. Break the class into four groups, and have each group fill four glasses with 1/4-cup of the water solution. Have students use an eye dropper to add single drops of iodine to each glass until the water turns blue. Give each group four small cups of different juices: orange, apple, grapefruit and pear. Then, have them add juice into the cups, one drop at a time until the water is no longer blue. Have them record their results. The blue goes away because the Vitamin C breaks apart the iodine from the water solution: the higher the Vitamin C content, the fewer drops of juice it will take.
For this activity you'll need two distilled water glasses, a hot plate, a small pot, a carrot, an eye dropper, a cup, and a batch of Vitamin C solution. First, add drops of test solution to 1/4 cup of distilled water. The water will immediately turn blue, which means that no Vitamin C is present. Next, place distilled water and the carrot into a pot, and boil it on the hot plate. Let the water cool, and then pour 1/4 cup of the water from the pot into the other glass. Add drops of iodine, and count how many it takes before the water turns blue. If it takes more drops than the first sample, some Vitamin C was cooked out of the carrot and into the water.
Groups of students will use the test solution to determine whether the ripeness of citrus fruits affects how much Vitamin C they contain. Give each group a manual citrus juicer and three oranges: fully ripe, half-ripe and not ripe at all. Have each group juice their oranges and collect the juice into individual, labeled glasses. They should wash the juicer between oranges. Then, have them empty out any extra juice so that the amounts in each glass are equal. Students then add drops of test solution until each sample starts to turn blue. Have them record their results. The oranges that turned blue the fastest had the lowest amount of Vitamin C.
This activity will show students that Vitamin C is a powerful antioxidant, which means that it can be used to slow the oxidation -- or in this case, the spoiling -- of apple slices. Split the class into four groups, and give each group two small plates, a few apple slices, a small bottle of liquid Vitamin C and a paint brush. Have each group label their plates "Control" and "Vitamin C," and then brush liquid Vitamin C onto each apple slice on their "Vitamin C" plates. Let the plates sit overnight. The control slices will brown faster than the Vitamin C slices.