Cork Stopper, Rubber, Solid

Quick Answer: The Cork Stopper, Rubber, Solid is a tapered rubber bung with no hole, used to close the neck of a flask, test tube or bottle completely. A solid rubber stopper keeps contents from spilling or evaporating and lets a closed vessel be shaken or inverted.

Closing a Vessel Completely

Without a bore, this stopper does one job: it shuts the neck. That matters when a solution must be mixed by inversion, when a test tube of reagent is shaken to dissolve a solid, or when a prepared sample is set aside and must not lose water to evaporation or pick up dust. The rubber compresses as it is pressed in and pushes back against the glass, giving a tighter closure than a loose cap.

A sealed vessel must never be heated. Warming the air inside raises its pressure, and the stopper will either shoot out or, if held firmly, let the glass crack. For anything on a burner, switch to a one hole stopper with an open outlet. Gases that form slowly, such as carbon dioxide from a fermenting mixture, create the same problem at room temperature.

Rubber is a sound general-purpose closure for aqueous solutions, dilute acids and alkalis, but it is not universal. Hydrocarbon and chlorinated solvents swell it, and halogens such as bromine and iodine attack it. Glass stoppers or PTFE-lined caps are preferred for those, and for long-term storage of reagents a screw-cap bottle is often more practical.

Specifications

Care & Handling

• Press the stopper in with a twist rather than hammering it; a firm hand fit is enough.
• Wash with warm water and mild detergent, rinse, and let it dry before storing.
• Store stoppers in a closed box sorted by size, away from ozone sources such as motors and out of sunlight.
• Discard any stopper that has become sticky, hard or cracked, as it will no longer seal.

Applications

• Stoppering a conical flask so that a standard solution can be mixed by inverting it
• Closing test tubes of reagent that students shake to dissolve a solid
• Covering prepared samples between lessons to stop evaporation and dust
• Keeping a flask of indicator or titrant closed during a long practical session

Why Choose LabEquip

Most solid stoppers are bought for general classroom glassware, so schools often order them together with conical flasks or test tubes. LabEquip lists them under General Lab Products; tell us which glassware you need to close on the contact page.

Frequently Asked Questions

What is the difference between a solid and a bored stopper?

A solid stopper has no hole and closes the vessel completely. A bored stopper has one or two holes so that a thermometer, tube or funnel can pass through. Choose solid only when nothing needs to enter or leave the vessel.

Why did the stopper shoot out of my flask?

Pressure built up inside. Warm contents, a gas-producing reaction or even a flask carried from a cold room into a warm one can push a stopper out. Release the pressure from time to time, or use a bored stopper with an outlet.

How far should the stopper go into the neck?

Roughly a third to a half of the stopper should sit inside. If nearly all of it disappears into the neck, choose the next size up; if only the tip enters, it is too large and will not seal.

Can a rubber stopper be autoclaved?

Many rubber stoppers tolerate autoclaving, but repeated cycles harden and crack general-purpose rubber. Check the compound’s rating first; for frequent sterilisation, silicone stoppers are usually the more durable choice.

How do I remove a stopper that is stuck?

Twist it rather than pulling straight up. If it will not turn, warm the neck gently in warm water or slide a thin spatula around the edge to break the grip. Never lever it out with a knife against the glass rim, which can chip.

Does a rubber stopper contaminate solutions?

For routine school work, no. In trace analysis, rubber can release small amounts of additives such as sulfur compounds and zinc, so analysts use glass or PTFE closures instead.

Last Updated: September 2026