I do not have the reputation to make tags, so my original question had a pretty random tag. Originally, I wanted to use the tags "distillation" and "vacuum", neither of which exists. Another user with the requisite rep added the tag "distillation" since, as he said, 44 different topics have "distillation" in their titles alone, not to mention questions which deal with distillation but do not have that word in their title. Later, some other user deleted the "distillation" tag, apparently on the grounds it is too specific a tag and all such questions should be tagged "purification" and nothing more specific than that. I guess by that logic there should be no purification-related tags (extraction, centrifuging, filtration, evaporation, crystallization, affinity-purification etc).
Personally I think there should not only be tags for "vacuum" and "distillation", I think there should also be tags "fractional-distillation" and "vacuum-distillation", "steam-distillation", and "simple-distillation." So, I am putting these potential tags out there for discussion.
Just as an example of mis-tagged questions, consider this one on steam distillation of Acai powder. I would have expected that to be tagged "steam-distillation." Currently its only tag is "reaction". Simple search shows there to be dozens of questions with similar issues.