vacuum filtration
Summary
Vacuum (suction) filtration separates an undissolved solid from a liquid by drawing the mixture through filter paper in a Büchner funnel, with lower pressure in the flask below. It is faster than gravity filtration, and it is the usual way to collect crystals after recrystallization.
Detail
Meaning: Vacuum filtration separates an undissolved solid from a liquid. The mixture is poured onto filter paper in a Büchner funnel seated on a side-arm flask connected to an aspirator or pump. Pressure in the flask is lower than the atmospheric pressure above the funnel, so the higher outside pressure drives the liquid (the filtrate) through the paper quickly while the solid stays behind. The solid is usually rinsed with a little ice-cold solvent and dried by continued suction.
On the MCAT: Passages describe a recrystallization or a precipitation in which the product forms crystals, and vacuum filtration collects them. Questions ask where the product ends up (the solid on the paper), why the rinse solvent is cold (less product redissolves), or why vacuum beats gravity filtration (faster, drier solid). Hot gravity filtration is the contrast case: it removes insoluble impurities from a hot solution, so the product passes into the filtrate.
Common confusion: The product is not always the solid. Which fraction you keep depends on the design, and in hot filtration of impurities the filtrate holds the product. Also, filtration cannot separate a dissolved solute from its solvent. It only separates a solid phase from a liquid phase, so dissolved impurities stay in the cold filtrate (the mother liquor).
Sources
- Introduction to Organic Laboratory Techniques
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.