Silver articles turn black when kept in the open for longer time due to the formation of _________.

The correct answer is ​Ag2S.

• When silver articles are kept in open or direct contact with air, then a layer of silver oxide is formed on the surface of the articles.

Key Points

• Due to the reaction of silver with the oxygen present in the atmosphere Silver oxide turns black in colour.
• After an interval of time, it will form silver sulphide after coming in contact with oxygen and Hydrogen sulphide present in the atmospheric air.
• The phenomenon is known as corrosion. It is called silver tarnishing, particularly for silver. The black substance obtained is known as the silver sulphide.
• Silver sulphide is also black in colour.
• The reaction that happens is:
  • 4Ag + O2 + 2H2S → 2Ag2S + 2H2O
• A dense black solid is the only sulfide of silver. It is useful as a photosensitizer in photography. It constitutes the tarnish that forms over time on silverware and other silver objects. 
• The size of silver sulfide particles in deposited powders and colloidal solutions has been estimated with the use of X-ray diffraction, dynamic light scattering, transmission electron microscopy, and Brunauer–Emmett–Teller techniques.
• The average size of silver sulfide Ag2S nanoparticles in stable colloidal solutions is equal to 10–20 nm. Special attention has been devoted to the investigation of the stability of synthesized colloidal solutions on the duration of their storage.