Flask, Boiling, Two/Three Neck (Round/Flat Bottom)

Quick Answer: A two neck or three neck flask is a boiling flask with extra side necks around its main neck, so several pieces of apparatus can be fitted at once, such as a condenser, a thermometer and a dropping funnel. This listing covers two-neck and three-neck versions with round or flat bottoms.

One Flask, Several Openings

A single-neck flask allows one fitting at a time: a condenser, a thermometer or an addition funnel. Many preparations need more. A reaction heated under reflux while a reagent is added dropwise and the temperature is watched needs three openings, and a three-neck flask provides them: a central neck, usually for the condenser or a stirrer, and two side necks for a dropping funnel, a thermometer or a gas inlet.

A two-neck flask is the simpler option when only two fittings are needed, such as a condenser plus a thermometer or a gas inlet. Side necks may be straight or angled away from the centre, and angled necks give more room between fittings. The listing does not state the neck arrangement, joint sizes, capacity or glass type, and it covers both round and flat bottomed forms, so specify all of these when ordering.

Round bottomed versions are the usual choice for heating in a mantle and for reduced-pressure work, while flat bottomed versions stand on a bench or hot plate and suit gentle warming with magnetic stirring. Either way, every fitting in a multi-neck set-up should be clamped or clipped, because the extra weight makes the assembly top-heavy.

Applications

• Reactions heated under reflux while a reagent is added dropwise
• Preparations that need the temperature of the reaction mixture monitored throughout
• Reactions under an inert gas, with the gas inlet and outlet on separate necks
• Mechanically stirred reactions with the stirrer passing through the central neck

Specifications

Care & Handling

• Close any unused neck with a matching stopper so vapour cannot escape and dirt cannot enter.
• Clamp the flask at the central neck and support heavy side fittings separately.
• Make sure the thermometer bulb dips into the liquid if the reaction temperature, rather than the vapour temperature, is to be measured.
• Clean each neck and joint surface individually; residues trapped where the necks meet the body are easy to miss.

Why Choose LabEquip

Organic synthesis courses and research laboratories use multi-neck flasks when a preparation needs more than one fitting. LabEquip lists them in the chemistry lab equipment range next to the single-neck short neck boiling flask; send the neck count, joint sizes and base shape you need through the contact page.

Frequently Asked Questions

When do I need three necks instead of two?

Choose three necks when three fittings are needed at once, for example a condenser, a dropping funnel and a thermometer. A two-neck flask is enough for two fittings, such as a condenser and a gas inlet.

What goes in the central neck?

The central neck usually carries the reflux condenser or, in stirred reactions, the stirrer shaft and its guide, because it sits directly above the centre of the flask. Side necks take the addition funnel, thermometer or gas inlet.

Should I choose a round or flat bottom?

Round bottoms heat more evenly in a mantle and are suitable for reduced pressure. Flat bottoms stand on a bench and sit well on a hot plate with a magnetic stirrer, but should not be used under vacuum.

Are angled side necks better than straight ones?

Angled necks point fittings away from the centre, leaving more room between the condenser, funnel and thermometer. Straight necks keep the assembly narrower. Either works if the fittings can be clamped without strain.

How do I add a reagent dropwise to a multi-neck flask?

Fit a pressure-equalising dropping funnel to a side neck, fill it with the reagent and open the tap slowly while the mixture is stirred. Watch the temperature and slow the addition if the reaction becomes too vigorous.

What should be done with unused necks?

Close them with ground-glass or PTFE stoppers of the right size. An open neck lets solvent vapour escape and allows moisture or air into reactions that must be kept dry.

Last Updated: September 2026