Hydrogen peroxide is sold in a brown bottle because it is sensitive to light. When exposed to light, hydrogen peroxide can decompose, producing oxygen and water. This decomposition can cause the bottle to burst or the hydrogen peroxide to become less effective. The brown bottle helps to block out light and protect the hydrogen peroxide from decomposing.

In addition to protecting hydrogen peroxide from light, the brown bottle also helps to prevent it from evaporating. Hydrogen peroxide is a volatile liquid, meaning that it evaporates easily. The brown bottle helps to slow down the evaporation process, keeping the hydrogen peroxide from losing its potency.