Why silver turns black
Tarnishing of silver · Redox · Corrosion
Silver doesn't rust, but a whiff of sulfur from a boiled egg is enough to start turning a spoon black.
Reactants approach
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ΔH = -596 kJ · exothermic, releases heat
ΔH per 4 mol Ag.
Meet the molecules
- Reactantsolid
Ag
silver
- Reactantgas
H2S
hydrogen sulfide
- Reactantgas
O2
oxygen
- Productsolid
Ag2S
silver sulfide
- Productliquid
H2O
water
What you notice
Silver jewellery, cutlery and coins slowly darken from bright white to yellow, brown and finally black. The coating is silver sulfide, Ag2S, and it forms even though air contains only traces of hydrogen sulfide, measured in parts per billion. Eggs, onions, mustard, wool and rubber bands all give off sulfur compounds, which is why an egg spoon tarnishes quickly. At first the layer is only nanometres thick, and the early colours come from light interfering in the thin film. Silver polish rubs the sulfide off, taking a little silver with it each time.
Try it
Line a bowl with aluminium foil, add hot water and a tablespoon of baking soda, and lay a tarnished silver spoon on the foil; the black fades as aluminium reduces the Ag+ back to silver metal.
Safety: The foil cleaning method releases a little hydrogen sulfide, so do it in a ventilated room.
Step by step
- 1
Sulfur reaches the surface
H2S molecules from the air stick to the silver surface.
- 2
Silver is oxidized
Each silver atom loses one electron to become Ag+, which pairs with sulfide from the H2S to form Ag2S.
- 3
Oxygen takes the electrons
O2 accepts the electrons and combines with the hydrogen atoms released from H2S to make water.
- 4
The film grows
Ag2S absorbs visible light strongly, so once the film is a few tens of nanometres thick it looks black.