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Round-bottom flasks and cork rings

One-neck Round-Bottom Flask on the left and three-neck Round-Bottom Flask on the right with all three necks parallel and pointing straight up

Laboratory distillation set-up using a Liebig condenser, without a fractionating column. Both distilling (left) and receiving (right) flasks are round-bottom flasks.

Round-bottom flasks (also called round-bottomed flasks and Erlenmeyer Bulbs) are types of flasks having spherical bottoms used as laboratory glassware, mostly for chemical or biochemical work. They are typically made of glass for chemical inertness; and in modern days, they are usually made of heat-resistant glass called Pyrex. There is at least one opening known as the neck. Two or three-necked flasks are common as well. Round bottom flasks come in many sizes, from 5 mL to 5 L, with the sizes usually inscribed on the glass. In pilot plants even larger flasks are encountered.

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

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

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 usually used in distillation 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 in rotary evaporators.

Round-bottom flasks are often used to contain chemical reactions run by chemists, especially for reflux set-ups and laboratory-scale synthesis. Boiling chips are often 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. For a reflux set-up, a condenser is typically attached to the middle or only neck of the flask being used. Additional necks on a flask could allow a thermometer, a mechanical stirrer, to be inserted into the flask contents. The additional necks can also allow a dropping funnel to be attached to let reactants slowly drip in.

Special electrically powered heating mantles are available in various sizes into which the bottoms of round-bottom flasks can fit into 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 a heat bath, water bath, or sand bath. Similarly cooling can be accomplished by partial submerging into a cooling bath, filled with e.g. cold water, ice, eutectic ice/salt mixtures, dry ice/solvent mixtures, or liquid nitrogen

Main article: Laboratory flasks

• Flat-bottomed flask A flask with similar uses as the round-bottom flask, but the flat bottom allows it to stand on a level surface.
• Florence flasks are similar flasks that have 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 don't have a ground glass joint like modern round bottom flasks do. Round-bottom flasks are used more commonly by professional chemists than Florence flasks.
• Retort a spherical vessel with a long downward-pointing neck, specially used for distillation or dry distillation of substances.