The side-arm flask looks like an Erlenmeyer flask with a short arm coming out of it. The arm is designed to connect the flask to a vacuum source. When sealed on the top with a stopper or a Buchner funnel, the vacuum flask will hold a reduced pressure. Note the heavy walls of this flask - they are designed to withstand reduced pressure without imploding. This design feature is the reason for the high cost of this flask. Side-arm flasks may also be called vacuum flasks or filtering flasks.
Vacuum flasks are used either for filtering solutions or for removing solvent under reduced pressure. The larger side-arm flasks (125 mL or 250 mL) work best for vacuum filtering, and the smaller ones (25 mL or 50 mL) work best for removing solvent under reduced pressure.
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