The classic brown hydrogen peroxide bottles that you’ve seen in medicine cabinets your whole life may seem old-fashioned, but there’s a reason why they have that distinctive look.

Hydrogen peroxide molecules have the chemical formula of H2O2, which is pretty close to water. And if the label says it’s 3%, that means 3% hydrogen peroxide and 97% water. And it doesn’t take a whole lot to break the bond that holds hydrogen peroxide molecules together.

Heat and light can break the bond quickly and that’s why hydrogen peroxide is stored in opaque brown bottles so no light can penetrate the bottles.

And if you have moved your hydrogen peroxide to a clear container or spray bottle, you can tell if it will still be effective to use if there’s foam when you apply it to a surface.

No foam = no good anymore.

And now you know!