el arma vs las armas
Why is singular "el arma" masculine, but plural "las armas" is feminine ?
The gender does not change. The article changes from "la" to "el" to emphasize that the next word begins with "a," which would otherwise be difficult to hear. This occurs for [almost] any feminine singular noun that starts with a stressed "a:"
But: la abogada (because the initial "a" is not stressed)
In the plural, using the masculine article is not necessary because the feminine one ends in "s" anyway, so it's going to be obvious that the noun starts with an "a:"
If you do have to use a masculine article with a feminine noun, the gender of the noun doesn't change. For example, you would still say "el águila dorada" or "el arma es tuya."
This is really similar to what happens with the indefinite article in English. We say "a cherry pie" but "an apple pie." And it happens a few other places in Spanish too, with the conjunctions "y" and "o," and with the indefinite article "una," which becomes "un" under the same conditions that "la" becomes "el."
Note: The only feminine noun for which this does not occur is the word "a" when referring to the letter A. For example, you could say: "La a está chueca."
edit: nvm, you're right. applies to agua as well ^^
that's not what i meant. the gender is still unpredictable. if, for some reason, the word is feminine and happens to start with an "a", i believe that the "la" becomes "el", for the sake of being expressible. it does not mean that all words that start with an "a" and happen to have "el" in the singular automatically turn into "las ..." in the plural. hope that helps clearing that up a little bit
to clarify: "el arma" is technically feminine, it just uses the masculine article