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Silver on being exposed to air, forms a black film. Now my question is whether the film is silver sulphide (due to reaction with hydrogen sulphide in air) or silver oxide.

Both compounds are black.

In my view it would be silver oxide because oxygen is more abundant and, intuitively, silver seems react to oxygen faster than hydrogen sulphide- oxygen is more reactive than hydrogen sulphide.

But a little google search loosely hints it is silver sulphide. Any suggestions on this too?

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    $\begingroup$ I really like this question. I ran a cursory Google search too, and apparently the tarnishing is due to the trace quantities of hydrogen sulfide in the air (silver oxide constitutes a really tiny fraction of the black film). One way we could confirm this is to scrape off some of the film (you'll need a decent amount of it for this to work) and treat it with an easily available mineral acid (like HCl). If it is (largely) silver sulfide, then the reaction that ensues will liberate H2S gas which has a characteristic rotten-egg smell. [I don't have any tarnished silver-ware :( ] $\endgroup$ – paracetamol May 5 '17 at 17:05
  • $\begingroup$ It will help me if you list some sites you encountered which are in support of H2S @paracetamol. And isn't this counter intuitive, that it forms silver sulphide and lesser silver oxide? inspite of reactivity and abundance? $\endgroup$ – alto santa May 5 '17 at 17:23
  • $\begingroup$ Wikipedia (under the "Properties section") en.wikipedia.org/wiki/Silver_sulfide and this: silvermagpies.com/2011/10/19/silver-tarnish and this scifun.chem.wisc.edu/HomeExpts/tarnish.html ...none very informative though :/ $\endgroup$ – paracetamol May 5 '17 at 17:25
  • $\begingroup$ From above links, I went furthur to study about tarnishing, there is a link in H2S page in same section. It says tarnishing happens not only due to oxygen but at times due to sulphide. When and why, i can't figure out. en.wikipedia.org/wiki/Tarnish $\endgroup$ – alto santa May 5 '17 at 17:41
  • $\begingroup$ Some excepts from theassayoffice.co.uk/tarnish_labexpert "The tarnishing agent is most likely to be free sulphur or a volatile sulphur-liberating compound" and "Moisture may also be a vital ingredient. Paper/paperboard containing 0.5 or 2.5 per cent sodium sulphide tarnishes silver only in the presence of moisture. Similarly, various types of paper containing sulphur compounds which, on alkaline extraction and acidification, liberate hydrogen sulphide do not blacken silver unless they are moist." It also says 0.006 per cent of free and dry sulphur causes tarnish stains. conclusions? $\endgroup$ – alto santa May 5 '17 at 17:47
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There is no such thing as "being reactive". Say, you have A and B; some compounds tend to react with A and not with B, and some would react with B and not with A. Which is more reactive? You can't tell; you can't even ask this question, for it's meaningless.

Black tarnish on silver is mostly silver sulphide, as confirmed experimentally. I'd even say that silver does not form a black film on being exposed to air, unless there is some source of hydrogen sulphide nearby (raw eggs, sewage pools, or something else).

It is just that $\ce{Ag}$ tends to prefer $\ce{S}$ over $\ce{O}$. Many other heavier metals are like that, and many lighter metals have different preferences. This is explained (kind of) by the hard/soft acids/bases concept.

So it goes.

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  • $\begingroup$ I don't think it is necessary to have a hydrogen sulfide source. Silver gets tarnished pretty much everywhere. $\endgroup$ – Nilay Ghosh Jul 16 '17 at 11:08

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