Polytetrafluoroethylene stoppers will not freeze or bind, chemically or mechanically. Needs no lubrication. Snug fit and easy removal. Use with flasks, separatory funnels and mixing cylinders that require flask-length stoppers. Chemically inert and unbreakable. Hollow stopper reduces expansion and contraction when exposed to heat or cold, making it superior to solid stoppers. Weight of top keeps hollow tip up, preventing contamination of wetted surface when placed on lab bench. Inverted stopper has a hollow cup that can be used as a mini-beaker for microanalytical work. Size refers to standard flask opening.