when is it appropriate to use gravity filtration and when to use vaccume filtration

Answer 1

Vacuum (note the correct spelling) is used when time is important OR when gravity filtration is far too slow and complete separation is not necessary. However, for most quantitative work, vacuum filtration is appropriate only when using a filter integrated with the holder; i.e., a Buckner type funnel and filter paper allows too many of the solid particles to pass. Integrated filters, such as Selas crucibles, in which the correct porosity filter is sealed into the porcelain crucible can be used for quantitative work with BaSO4, AgCl, and other ppts. Older type vacuum filters that were made with macerated filter paper vacuumed onto perforated metal back plates can be used also but those have been almost completely supplanted with the more modern porcelain type.