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Saturday Science: Incredible Iodine
Back a few decades ago, before hydrogen peroxide was the go-to treatment for minor scrapes and cuts, adults would make kids sit through the agony of putting iodine on their injuries. Peroxide stings, but there are no words for how iodine hurts when you put it on a big scrape. A mixture of a small amount of iodine into water is an excellent (if painful) antiseptic, but it can do other things, too. It’s also a really good chemical indicator. Wonder what that means? Well, that’s what today’s experiment is all about. We’re going to do what’s called a starch test, to see how much starch is in a few different food items.
- A bottle of iodine (you can still get it at the pharmacy)
- Rice (Minute Rice works best)
- Apple slices
- Potato slices
- A slice of bread
- A leaf of lettuce
- An eye dropper
- A plate
- Paper towels
- First you need to set up your experimental area. Iodine stains all sorts of stuff, so we want to stay clean. First, lay down a couple of layers of paper towel, then put your plate down on top.
- Suck some iodine up into your eyedropper. Most iodine solutions you can buy are a deep orangey-yellow, and that’s important here.
- Cook your rice (it needs to be cooked for the experiment to work) and then lay out all of the other foods you’ll be testing for starch.
- Your test is simple: squeeze a few drops of the iodine onto the food. What happens? You should notice a remarkable color change in some of the food items. That change means that there’s starch present in that food. The darker the resulting color, the more starch there is. Which foods had a lot of starch, and which didn’t?
- Brainstorm other foods (and even drinks!) to try. Milk? Soda? Salt? Sugar? Onion? A chicken nugget? Just make sure you don’t eat or drink anything after you test it for starch!
- Clean up very carefully! You don’t want to get any iodine on your clothes or your carpet!
As you probably noticed, when you dropped some iodine on the potato, the rice, and the bread, it went from orangey-yellow to dark blue or black. That’s how you know there’s starch in that food. Iodine indicates the presence of the chemical starch, which is why it’s called a chemical indicator.
Starch is a major component of plants and is also a type of nutrient for animals called a carbohydrate. It’s made of up chains of a substance called glucose, a sugar that’s an important fuel for almost all life forms. Even though it’s important to plants, different plants, and different parts of those plants, tend to have more starch in them than others.
A potato is the root of the potato plant, while lettuce, even though it’s a vegetable, is a leaf. Leaves don’t contain as much starch as a nice thick root like a potato. An apple, on the other hand, is a fruit. It has some starch before it’s ripe, but sweet ripe fruits contain a sugar called fructose and very little glucose, so they don’t cause the iodine to react.
The reason iodine changes color when it contacts starch is still kind of a mystery. Scientists think that the reason has to do with the shape of some of the starch molecules. Some of them are long branching chains, and some are straight chains of round coils. These ones are called amylose. What seems to be happening is that the iodine molecules fit perfectly inside the coils of amylose, which changes how the amylose absorbs and reflects light, giving it a blue or black color instead of whatever color it was before. It’s kind of crazy that something you can do so easily in your kitchen is still kind of a mystery, but if there weren’t any mysteries left then we wouldn’t need science anymore!