Why is chloroform kept in dark-coloured bottles?

Chloroform is kept in dark-coloured bottles to prevent its reaction with sunlight and oxygen, which can produce a harmful chemical called phosgene gas. Dark bottles reduce exposure to light and help maintain the stability of chloroform during storage.

When chloroform (CHCl₃) is exposed to air and sunlight for a long time, it undergoes oxidation and forms phosgene (COCl₂), a highly poisonous gas. To prevent this reaction, chloroform is stored in amber or dark-coloured bottles that block light.

A small amount of ethanol is also commonly added to chloroform because it reacts with any formed phosgene and converts it into a less harmful compound. Proper storage of chloroform is important in chemistry laboratories to ensure safety and prevent the formation of toxic substances.