Date chemicals when they enter the laboratory.

Chemical Shelf Life All chemicals, but particularly reactive materials, have a shelf life after which the unused portion of the chemical must be disposed. Unstable materials may decompose over time. Some chemicals (such as ethers) can form shock-sensitive peroxide compounds after exposure to air, which, if lodged in the cap of the container, can initiate an explosion when opening the container. Reactive materials that have remained in the lab beyond their shelf lives have caused many accidents. Write the date when a reactive chemical (or one that may decompose or form peroxides) is received on its label. Never keep highly reactive materials in the laboratory for more than six months. Always minimize the amount of flammable and reactive substances present in your laboratory.