Round-bottom flasks (also calledround-bottomed flasks orRB flasks) are types offlasks having spherical bottoms used aslaboratory glassware, mostly forchemical orbiochemical work.^[1] They are typically made ofglass for chemicalinertness; and in modern days, they are usually made of heat-resistantborosilicate glass. There is at least one tubular section known as theneck with an opening at the tip. Two- or three-necked flasks are common as well. Round bottom flasks come in many sizes, from 5 mL to 20 L, with the sizes usually inscribed on the glass. In pilot plants even larger flasks are encountered.

The ends of the necks are usually conicalground glass joints. These arestandardized, and can accept any similarly-sized tapered (male) fittings.24/40 is common for 250 mL or larger flasks, while smaller sizes such as 14/20 or 19/22 are used for smaller flasks.

Because of the round bottom, cork rings are needed to keep the round bottom flasks upright. When in use, round-bottom flasks are commonly held at the neck by clamps on a stand.

A round-bottom flask is featured prominently on the logo of theOPCW, the implementing body for theChemical Weapons Convention.^[2]

• Heating and/or boiling of liquid.
• Distillation.
• Contain chemical reactions.
• Distilling flask inRotary evaporators.
• Storage of the culture media.
• Preparation of gas-phase standards for flasks fitted with septa (requires volumetric calibration)

The round bottoms on these types of flasks allow more uniform heating and/or boiling of liquid. Thus, round-bottom flasks are used in a variety of applications where the contents are heated or boiled. Round-bottom flasks are used indistillation by chemists as distilling flasks and receiving flasks for the distillate (see distillation diagram). One-neck round-bottom flasks are used as the distilling flasks inrotary evaporators.

This flask shape is also more resistant to fracturing under vacuum, as a sphere more evenly distributes stress across its surface.

Round-bottom flasks are often used to containchemical reactions run by chemists, especially forreflux set-ups andlaboratory-scalesynthesis.^[3]Boiling chips are added in distilling flasks for distillations or boiling chemical reactions to allow a nucleation site for gradual boiling. This nucleation avoids a sudden boiling surge where the contents may overflow from the boiling flask.Stirring bars or other stirring devices suited for round-bottom flasks are sometimes used.^[4] Round bottom flasks suffer from poor stirring when compared withErlenmeyer flasks, as they can't accept large stir bars and material can become trapped at the base. For a reflux set-up, acondenser is typically attached to the middle or only neck of the flask being used. Additional necks on a flask could allow athermometer or a mechanical stirrer to be inserted into the flask contents. The additional necks can also allow adropping funnel to be attached to letreactants slowly drip in.^[5]

Special electrically poweredheating mantles are available in various sizes into which the bottoms of round-bottom flasks can fit so that the contents of a flask can be heated for distillation, chemical reactions, boiling, etc. Heating can also be accomplished by submerging the bottom of the flask into aheat bath,water bath, orsand bath. Similarly cooling can be accomplished by partial submerging into acooling bath, filled with e.g. coldwater,ice,eutectic mixtures,dry ice/solvent mixtures, orliquid nitrogen. For gas preparation where heating is required. Since the flask is round bottomed heat is uniformly distributed throughout on heating.

• Flat-bottomed flask: A flask with similar uses as the round-bottom flask, but with a flat bottom that allows it to stand on a level surface.
• Florence flask: A flask similar to the flat-bottomed flask that has round bodies and either a round bottom or a flat bottom so that one can stand the flask on a level surface. Florence flasks typically have one neck which is longer and may be somewhat wider than the usual neck of a round bottom flask. The necks of traditional Florence flasks often do not have a ground glass joint like modern round bottom flasks do. Round-bottom flasks are used more commonly by professionalchemists than Florence flasks.
• Retort: A spherical vessel with a long downward-pointing neck, specially used for distillation ordry distillation of substances.
• Schlenk flask: A round-bottom flask with a built-in plug valve orstopcock.

In the United States, it is illegal for an individual to possess, manufacture, buy, or sell a three neck round bottom flask while knowing, suspecting or intending the flask's purpose to be the manufacture of illegal or controlled substances. It isnot illegal, however to possess, manufacture, buy or sell a three neck round bottom flask for legal activities.^[6]