Growth is fast at first, then slows: once a continuous film covers the metal, silver and sulfide ions must diffuse through it, so thickness rises roughly with the square root of time.
Silver does not rust, but it does react with sulfur-bearing gases. The black layer is silver sulfide, Ag₂S.
The reaction as it actually proceeds in air - oxygen is required:
The simplified form quoted in most Class 10 textbooks:
Cleaning by electrochemical reduction - the aluminium-and-baking-soda method:
A new tarnish film is thinner than a wavelength of light. Light reflecting off the top of the film interferes with light reflecting off the silver underneath, so the colour you see depends on how thick the film is - not on any change in the compound.
| Thickness | Appearance |
|---|
Past about 250 nm the film scatters rather than interferes, and it turns the familiar matte brown-black. That is why heavily tarnished silver looks dull, while lightly tarnished silver looks golden.
Silver tarnish is not oxidation by air. It is a reaction with traces of hydrogen sulfide, which produce a thin black film of silver sulfide on the surface. That is why silver tarnishes faster near eggs, onions, rubber bands and wool, and why it can be cleaned chemically rather than by polishing: wrapping the item in aluminium foil in hot salt solution reverses the reaction and gives the silver back.
4Ag + 2H2S + O2 → 2Ag2S + 2H2O
Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.