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A round-bottom flask is a laboratory flask with a spherical bottom and one or more necks, used mostly for chemical and biochemical work. It is the usual vessel for heating or boiling liquids, distillation, reflux and rotary evaporation. Who uses it: chemists and lab staff running synthesis and distillation setups. This page shows how to read a flask’s size and joint markings and which limits apply to heating and vacuum work. The figures come from ISO 4797 (the standard for boiling flasks with ground joints) and from the US National Academies’ Prudent Practices in the Laboratory.
What a round-bottom flask is
It is also called a round-bottomed flask or an RB flask. ISO 4797 groups round-bottom flasks with other glassware under the heading “boiling flasks with conical ground joints” and lists three types: conical flasks, flat-bottom flasks and round-bottom flasks.
Other names for a round-bottom flask
A flask with a round bottom is normally called a round-bottom flask. A flask with a round body but a flat base is a different item, the Florence flask (see the table below).
Round bottom flask function and uses
The spherical bottom allows more uniform heating or boiling of the contents, so the flask is used where contents are heated or boiled: heating and boiling liquids, distillation (as the distilling flask and the receiving flask), containing chemical reactions, and as the distilling flask on a rotary evaporator. Prudent Practices notes that heating mantles are commonly used to heat round-bottom flasks. See also the guides to the rotary evaporator and fractional distillation.
How it is built
- Material. ISO 4797:2004 requires borosilicate glass 3.3 (per ISO 3585), free from visible defects and from internal stress that would impair performance.
- Necks and ground joints. Each neck ends in a conical ground joint that complies with ISO 383 (k6 series).
- Marking. Four permanent inscriptions are required: the nominal volume, the joint size (for example “29/32”), the maker’s name or mark, and an area suitable for pencil marking. “ISO 4797” is recommended as an additional inscription.
One, two or three necks
A round-bottom flask has at least one neck, and two- and three-neck versions are common. In a reflux setup a condenser is typically attached to the middle or only neck. Extra necks can take a thermometer, a mechanical stirrer, or a dropping funnel that adds reactants slowly (Wikipedia, “Round-bottom flask”, checked 2026-11-07).
Types and variants: round-bottom versus other flasks
| Type | Typical use | Key difference | Source |
|---|---|---|---|
| Round-bottom flask (ISO 4797 type c) | Heating, boiling, distillation, reactions, rotary evaporator flask | Spherical bottom heats more uniformly; needs a cork ring or a neck clamp to stay upright | ISO 4797; Wikipedia |
| Flat-bottom flask (ISO 4797 type b) | Boiling flask with ground joints (ISO 4797 type b) | Flat base stands on a level surface | ISO 4797; Wikipedia |
| Conical flask (ISO 4797 type a) | Boiling flask with ground joints (ISO 4797 type a) | Accepts larger stir bars than a round-bottom flask | ISO 4797; Wikipedia |
| Florence flask | Similar to a flat-bottom flask | Round body with a round or flat bottom; one longer neck that often has no ground joint | Wikipedia |
| Schlenk flask | A flask that can be closed with its own valve | A round-bottom flask with a built-in plug valve or stopcock | Wikipedia |
| One-mark volumetric flask (ISO 1042) | Holding a calibrated volume | Contains its nominal volume at a 20 °C reference temperature; accuracy Class A or B | ISO 1042:1998 |
For the neighbouring types, see the CASRAI guides to the Erlenmeyer flask and the volumetric flask.
Round-bottom flask versus volumetric flask: what the two standards say
A round-bottom flask is a boiling flask for heating, reactions and distillation; a volumetric flask is a calibrated vessel for one defined volume. ISO 1042 sets a 20 °C reference temperature and maximum permitted volume errors: for a 100 ml one-mark flask ±0.100 ml (Class A) and ±0.200 ml (Class B), for 250 ml ±0.150 ml and ±0.300 ml. ISO 4797 gives a nominal volume and an overall height with a tolerance in millimetres, and no permitted volume error. Do not use the markings on a round-bottom flask to measure volume.
Sizes and specifications that matter
ISO 4797:2004 (second edition, 2004-07-01) specifies two series for each flask type. The series differ in height and in the joint sizes offered, and the standard recommends that national standards choose one. The table gives round-bottom values up to 2 l; the standard continues to 4 l, 5 l, 6 l and 10 l (the 2004 edition added the 10 ml, 25 ml, 5 l, 6 l and 10 l sizes). Where one list of joints serves several rows, the standard lists them together, and so does the table.
| Nominal volume | Series 1: height (mm) | Series 1: joints | Series 2: nominal height (mm) | Series 2: joints |
|---|---|---|---|---|
| 10 ml | 70 ± 3 | 14/23 | not listed | not listed |
| 25 ml | 85 ± 3 | 14/23 | not listed | not listed |
| 50 ml | 90 ± 6 | 14/23, 19/26, 24/29, 29/32 (50 and 100 ml) | 90 | 14/23, 19/26, 24/29, 29/32, 34/35 (50 and 100 ml) |
| 100 ml | 105 ± 6 | as above | 110 | as above |
| 250 ml | 140 ± 6 | 19/26, 24/29, 29/32 | 135 | 19/26, 24/29, 29/32, 34/35 |
| 500 ml | 163 ± 6 | 24/29, 29/32 (500 ml to 2 l) | 155 | 19/26, 24/29, 29/32, 34/35, 45/40 (500 ml to 2 l) |
| 1 000 ml | 200 ± 7 | as above | 185 | as above |
| 2 000 ml | 240 ± 7 | as above | 220 | as above |
Wikipedia quotes 24/40 as a common joint on flasks of 250 mL or larger. That designation is not in the ISO table, so read the marking printed on the flask and on the mating glassware instead of assuming a joint size from the volume.
How to choose
- Match the joint. Joints are standardised and accept any similarly sized tapered fitting, and ISO 4797 requires the joint size to be marked on the flask. Compare it with the condenser, adapter or stopper you plan to fit.
- Count the necks you need. A condenser goes on the middle or only neck; further necks hold a thermometer, a mechanical stirrer or a dropping funnel.
- Check the heating source fits. Heating mantles are made in various sizes into which the bottom of a round-bottom flask fits; heating can also be done in a heat, water or sand bath.
- Check the vacuum limit. Prudent Practices says to use only round-bottom or thick-walled vessels designed for reduced pressure, and its rule reads: “Never evacuate thin-walled, Erlenmeyer, or round-bottom flasks larger than 1 L.”
- Consider stirring. A round-bottom flask stirs poorly compared with an Erlenmeyer flask because it cannot take large stir bars and material can collect at the base.
Safe use and handling basics
- Inspect the glass. Before every vacuum use, check for flaws such as star cracks, scratches and etching marks by holding the vessel up to a light. Repaired glassware must be annealed and inspected with a cross-polarizer before vacuum or thermal stress is applied.
- Shield evacuated glass. Evacuated glass vessels may collapse violently, spontaneously from strain or from a blow, so use adequate shielding.
- Heating mantles. Use a mantle with a variable autotransformer and never plug it directly into a 110 V line. Do not exceed the input voltage the manufacturer recommends: higher voltage overheats the mantle and melts the fibreglass insulation. A mantle poses minimal shock hazard only while its fibreglass coating is unbroken and no water or chemicals have spilled into it.
- Rotary evaporators. Use Pyrex or similar glass, fully enclosed in a shield, and increase rotation speed and vacuum gradually.
- Distilling flammable solvents. Work in a chemical hood, keep stills attended at all times, and fit an automatic high-temperature shutoff.
- Support. Support heavy apparatus from below as well as by the neck, and assemble it so the joints can move without passing strain to the necks of the flasks.
More on inspection and disposal is in the glassware safety guide; the wider glassware families are covered in types of laboratory glassware.
Common mistakes
- Evacuating a flask the rule excludes: “thin-walled, Erlenmeyer, or round-bottom flasks larger than 1 L” (Prudent Practices 7.E.3.1).
- Reading the printed volume as a measuring accuracy: ISO 4797 specifies a nominal volume, not a permitted volume error.
- Leaving the flask unsupported: a round bottom needs a cork ring or a clamp at the neck to stay upright.
- Omitting a boiling chip in a distilling flask or boiling reaction. Boiling chips give a nucleation site for gradual boiling and avoid a sudden surge that can make the contents overflow.
FAQ
What is a round flask called?
A round-bottom flask, also written round-bottomed flask or RB flask.
Can we heat a conical flask?
ISO 4797 groups conical flasks with ground joints together with round-bottom and flat-bottom flasks as “boiling flasks” made from borosilicate glass 3.3. The standard gives no temperature limit or heating method, so follow the maker’s instructions and your lab’s rules. Conical flasks are not for vacuum work: Prudent Practices lists Erlenmeyer flasks among those never to be evacuated. See the Erlenmeyer flask guide.
What is the difference between a round bottom flask and a volumetric flask?
A round-bottom flask is a heating and reaction vessel with only a nominal volume; a volumetric flask is calibrated at 20 °C with Class A or B permitted errors (see above).
What are the three types of flasks?
ISO 4797 specifies three types of boiling flasks with conical ground joints: conical, flat-bottom and round-bottom. One-mark volumetric flasks are specified separately, in ISO 1042.
Sources
- ISO 4797:2004, Laboratory glassware — Boiling flasks with conical ground joints (second edition, 2004-07-01), full-text preview at standards.iteh.ai, checked 2026-11-07.
- ISO 1042:1998, Laboratory glassware — One-mark volumetric flasks (fourth edition), preview at cdn.standards.iteh.ai, checked 2026-11-07.
- National Research Council (2011), Prudent Practices in the Laboratory: Handling and Management of Chemical Hazards, Updated Version, chapter 7 (sections 7.C.5.3, 7.C.6, 7.E.3.1, 7.E.3.4, 7.E.3.5), National Academies Press, nap.nationalacademies.org, doi 10.17226/12654, checked 2026-11-07.
- Wikipedia, “Round-bottom flask”, en.wikipedia.org, checked 2026-11-07.
