When you mix ascorbic acid (Vitamin C) and iodine, something cool happens: the solution turns blue for a second or two, then returns to the color of the juice. If you keep adding the iodine, the mixture will turn a very inky dark blue color. Why is this? Here’s a kid friendly explanation.
Iodine and vitamin C like each other, and when they are put in the same container, they will combine. When you are adding the iodine to the juice and the starch mixture, as long as there is still vitamin C that has not been combined with the iodine, the color will stay the color of the juice, but after you reach the equivalence point- when you have added as much iodine as there is vitamin C – then the iodine starts combining with the cornstarch. When iodine combines with cornstarch it turns blue.
And if you’re doing our award winning experiment Vitamin “C”itrus, here’s a hint: the more iodine it takes, the more vitamin C there is.