3% hydrogen peroxide (from the grocery or pharmacy)
your hand
a potato

Hydrogen peroxide is one of those magical cures that many adults remember from their childhood, and we pass it along to our kids. If you get a cut or scratch, you put some peroxide on it. You can tell if the cut has germs, because then it will bubble, right? Well, that is what we are told, but as we will see, things are not always as they seem.

Lets start out with your skin. Are there any germs on your skin? Look closely. Do you see them? No. Are they there? Yes, they are. No matter how much you wash your hands, no matter how much you scrub, you have bacteria and other things living on your skin. Since there are germs on your skin, it should bubble if you put some hydrogen peroxide on it, right? Try it and see.

Did you get any bubbles? Probably not, unless you have a cut or scratch. What makes the bubbles is not the presence of germs. Instead, it is the presence of a substance called catalase. This is a chemical that is found in your blood and in the cells of your body. To see this, we don't have to use your blood. Luckily, it is also found in potatoes. If you cut a slice of potato and put some peroxide on it, it will bubble. When a cut bubbles, it does not mean that it is infected. It just means that it has either some blood or some damaged cells. This brings the hydrogen peroxide in contact with catalase and you get bubbles.

OK, then what causes the bubbles? Hydrogen peroxide is chemically very similar to water. Where the chemical formula for water is H2O, hydrogen peroxide is H2O2. The H stands for an atom of hydrogen and the O stands for an atom of oxygen. A molecule of water contains two atoms of hydrogen and one atom of oxygen. The only difference between hydrogen peroxide and water is a little oxygen. That may not sound like much of a difference, but it is. In chemistry, things can change a lot when you change the formula. Adding oxygen to iron gives you rust. The soft, reddish rust is very different from a piece of iron metal. An extra atom of oxygen also makes hydrogen peroxide a very different chemical, which can be dangerous if it is concentrated. That is why the stuff you get at the grocery is only 3%. The rest is plain water and it is diluted enough to make it safe for household use.

Left to itself, hydrogen peroxide will slowly lose the extra oxygen and change into water. Certain chemicals will speed up the reaction, causing the extra oxygen to be freed very quickly. Catalase is one of those chemicals. But why would that be a good thing? In high concentrations, oxygen is poisonous to some germs. The idea is that you get a lot of oxygen into the cut, to kill these germs. Then a scab forms, keeping any more germs from getting into the cut. Looking online, there seems to be a lot of debate over how effective hydrogen peroxide is for treating cuts. Some sources say that it is very useful. Others say that it does not do more than soap and water, and some say that it actually causes more harm than good. The majority say that it is at least as useful as soap and water, so I will continue to use it. Besides, it is still amazing to see a cut bubble and foam as you clean it.