Quote Originally Posted by Carcul View Post
I was browsing викисловарь when in the entry about the word "мышьяк" (which means the element arsenic or a general pharmaceutical preparation having arsenic or its compounds as ingredients) I found that it has a Partitive Case form in the singular. I thought it was logical that other russian words for chemical elements should also have such form, but that's not what I found. Why is that so? Is it just because such form coincides with other? Later I also found such form for the word "мех" (which also have a Locative case form). Some questions:

1) What is the meaning and usage of this case in Russian?
2) Are there more words which admit a Partitive Case form?
3) Can someone give several examples of usage of this case?

Thanks in advance.
The partitive is the form "of sth" in phrases like "a cup of tea".

In Russian, it's the same as the Genitive except in a few cases like стакан чаю.

It's worth noting the following distinction: "дай хлеб" give me (the) bread (accusative case, referring to some specific loaf of bread).
"дай хлеба" give me some bread (partitive genitive)

This site has a good explanation:
http://russianmentor.net/gram/mailbag/topics/gen2.htm