This was news to me, and also (I believe) to everyone else involved in the quiz I took part in. As someone who did Chemistry 'A' Level (albeit 40–odd years ago), I was under the impression that it was the result of oxidation, and would have expected the answer to be "oxygen".
Hydrogen sulphide is present in air in minute quantities (less than methane, which is approximately one part in 5,600). It's produced by volcanoes, and by the microbial breakdown of organic matter in the absence of oxygen gas, such as in swamps and sewers – as well as by industrial pollution.
This is one of those quiz questions that's clearly been bouncing around the Internet for years, despite the fact that no one really seems to know, or to be able to explain, why the answer given is the correct one. Try googling "brass discoloured hydrogen sulphide" and you'll see what I mean. It's invariably a multiple choice question, and it is:
Brass gets discoloured in air because of the presence of which of the following gases in air?
A. Oxygen
B. Hydrogen sulphide
C. Carbon dioxide
D. Nitrogen
It's always the same four options; the correct answer is always "B, Hydrogen Sulphide".
I could find no confirmation of this, other than the proliferation of examples of this quiz question.
Many of the places where the question appears are educational websites – often forums where anyone can provide an explanation. The explanations are invariably gobbledegook. For example: "Copper reacts with sulphide ion and forms cupric sulphide which is black in color where as zinc reacts with hydrogen and forms zinc hydroxide which is faint red in color. Hence brass gets discolored when it exposed to air."
Not everyone agrees. According to a contribution to Yahoo Answers, "Oxidation is the process which creates a patina on coppe[r], and brass. Hydrogen sulfide is corrosive but the actual discoloration comes from oxidation, and thus, oxygen."
This is not entirely convincing either. There does seem to be some science behind the assertion that hydrogen sulphide is responsible, so I'm prepared to accept that it's probably true. But having failed to find a convincing explanation on the Internet, I remain sceptical; and I think that as a quiz question this leaves a lot to be desired.
© Macclesfield Quiz League 2018