[susan53]

Hi Beatrix,

separation is one of many words which can be used either with or without the article. Eg in the sentence : Separation is often prelude to divorce, the indefinite article could be used with any of the three nouns. In general, the more generic the sentence is the more likely the article is to be omitted, though this is a tendency rather than a rule - your sentence is generic (all dorms, everywhere in China) but the article could have been used. If you're talking about one specific event (eg They agreed on a one-month separation), then the article is necessary.

As far as the "bottle of medicine" text is concerned, if this is a written text then it's definitely a mistake - probably a typing error. There's no way bottle can be used in that sentence without "a".