After years of exposure to light or oxygen, compounds in them break down and turn yellow or even brown.
暴露在光照或氧气中多年后,它们中的化合物分解成黄色甚至棕色。
英语百科
Compounds of oxygen
(重定向自Oxygen compounds)
The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides. Compounds containing oxygen in other oxidation states are very uncommon: −⁄2 (superoxides), −⁄3 (ozonides), 0 (elemental, hypofluorous acid), +⁄2 (dioxygenyl), +1 (dioxygen difluoride), and +2 (oxygen difluoride).