Stoppers, Glass

Quick Answer: Glass stoppers are ground-glass plugs that close the necks of flasks, the unused openings of multi-neck apparatus and the mouths of reagent bottles. The ground surface seals against a matching socket, giving a closure that resists most solvents and reagents.

Which Kind of Stopper

Two families share the name. Interchangeable stoppers have a cone ground to a standard joint size, so they fit any socket of that size: flask necks, side necks of multi-neck flasks and adapters, and the tops of separating and dropping funnels. Bottle stoppers are ground to fit one particular bottle neck and may not seal in another bottle of the same nominal size. The title does not say which kind, which head style (flat, hexagonal or pennyhead) or which sizes are supplied.

Glass is chosen where a plastic or rubber stopper would be attacked or would contaminate the contents: concentrated acids, halogenated solvents, bromine and iodine solutions and most organic liquids. Hexagonal or flat heads stop a stopper rolling off the bench. The limitation is alkali: sodium and potassium hydroxide solutions attack the ground surfaces and can cement a glass stopper into its neck, which is why alkali bottles are closed with plastic stoppers or rubber bungs.

A hollow stopper puts less load on a thin neck, while a solid one survives knocks better. For ground-joint apparatus, choose the stopper by joint size exactly as for any other fitting.

Care & Handling

• Store stoppers in a separate compartment or tied to their bottle, as ground surfaces scratch against each other.
• Wipe the ground zone clean before inserting; a grain of solid on the joint stops it seating and can crack the neck when pushed.
• Keep a stopper with the bottle it was ground to, and mark matched pairs with a glass marker.

Specifications

Applications

• Closing unused necks on multi-neck flasks during reactions
• Stoppering ground-necked conical and round-bottomed flasks during mixing or storage
• Closing reagent bottles that hold acids, solvents or halogen solutions
• Closing the top of a dropping or separating funnel

Why Choose LabEquip

Chemistry stores and teaching labs replace lost or broken stoppers regularly, and ordering by joint size keeps flasks and funnels in service. LabEquip lists glass stoppers in its chemistry lab equipment range; state sizes and quantities on the contact page.

Frequently Asked Questions

Will an interchangeable glass stopper fit any flask?

Only flasks whose neck carries the same joint size as the stopper. A stopper one size away will not seal, however close it looks.

Why do glass stoppers stick in alkali bottles?

Alkaline solutions dissolve a little of the glass surface and deposit silicate in the ground joint, which cements the stopper. Use rubber or plastic closures for sodium or potassium hydroxide.

How do I free a stuck glass stopper?

Tap around the stopper head lightly with a wooden block, then twist. If that fails, warm the neck briefly under hot water so it expands, wearing gloves and eye protection; never use a flame on a bottle of flammable liquid.

Should glass stoppers be greased?

Not usually on bottles, since grease can contaminate reagents. For vacuum work or long reactions on jointed apparatus, a smear of grease on the upper cone, or a PTFE sleeve, tightens the seal and stops the stopper freezing in place.

What is the advantage of a hollow stopper?

It weighs less and puts less strain on thin necks and small flasks. Solid stoppers are heavier but less likely to break if dropped.

Can I stopper a flask while it is still hot?

It is better not to. As the contents cool the pressure inside drops, pulling the stopper in so tightly that it can lock in place. Let the flask cool first, or rest the stopper loosely.

Last Updated: September 2026