We shorten chemistry to chem, just like we shortened mathematics to math because we are just taking the first few letters of the word.
"Mathematics is a field of study..."
https://en.wikipedia.org/wiki/Mathematics
Is A field of study. Mathematics isn't a "plural" even though it has an s at the end.
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From the very article you linked:
> In English, the noun mathematics takes a singular verb. It is often shortened to maths or, in North America, math.
