Cork Stopper, Rubber, One Hole

Quick Answer: The Cork Stopper, Rubber, One Hole is a tapered rubber bung with a single bore through its centre. A one hole rubber stopper closes a flask, boiling tube or bottle while letting one thermometer, glass tube or funnel stem pass through it.

One Bore, One Passage Through the Seal

Despite the word cork in its name, this stopper is moulded rubber rather than cork bark. Rubber grips glass more tightly than cork, springs back after compression and does not crumble into the vessel, which is why it has largely replaced natural cork for school apparatus. The single bore turns a closed vessel into one with a controlled opening: whatever passes through the hole becomes the only route in or out.

Typical set-ups use that route for a thermometer in a boiling tube when plotting a cooling curve, a bent delivery tube carrying gas from a heated test tube to a trough, or the stem of a funnel feeding a flask. Pair it with glass delivery tubes for gas collection; where a second tube is needed, such as a thistle funnel plus an outlet, the two hole stopper is the right choice.

Fit is judged by taper. A correctly sized stopper enters the neck by roughly one third to one half of its length and is held around its middle. One that sits almost entirely inside is too small and may be pushed or drawn in, while one that barely enters will pop out under slight pressure.

Specifications

Applications

• Holding a thermometer in a boiling tube for melting-point or cooling-curve practicals
• Carrying a delivery tube from a heated test tube to limewater or a gas jar
• Fitting a thermometer into the neck of a distillation flask with its bulb level with the side arm
• Joining a short glass tube to rubber tubing where a flask or bottle must connect to other apparatus

Care & Handling

• Wet the glass and the bore with water or a drop of glycerol before insertion, hold the tube close to the stopper through a cloth, and twist rather than push.
• Never force a tube into a bore that is too small; the glass can snap and drive into the hand.
• Take glass tubes out after the practical while the rubber is still supple, since a tube left in for weeks can bond to the bore.
• Keep stoppers out of direct sunlight and away from heaters, which harden and crack rubber over time.

Why Choose LabEquip

School chemistry and physics prep rooms usually keep one hole stoppers in several neck sizes alongside the solid and two hole types. LabEquip supplies them within the General Lab Products range; send the neck diameters or glassware list you need to fit through the contact page.

Frequently Asked Questions

Why is a rubber stopper called a cork?

Stoppers were once cut from cork bark, and the name stayed in common use after rubber replaced it. Rubber seals better, is less porous and does not shed crumbs, so a rubber cork is simply a rubber stopper.

How do I push a thermometer through the hole safely?

Lubricate the thermometer stem and the bore with water or glycerol, protect your hand with a cloth, grip the stem close to the stopper and ease it in with a gentle twisting motion. Never grip the bulb end or push from far up the stem.

Can I heat a flask closed with this stopper?

Yes, provided the bore is open to the air or connected to an open outlet, so pressure cannot build up. Keep the rubber out of the flame and away from very hot glass surfaces, which soften and char it.

How do I choose the right size?

Measure the inside diameter of the neck. Choose a stopper whose narrow end is smaller and wide end larger than that figure. The bore must also suit the outside diameter of the tube or thermometer you plan to fit.

Will organic solvents damage the stopper?

Many can. Natural and general-purpose rubbers swell or soften in hydrocarbons and chlorinated solvents such as toluene or chloroform. For solvent work, use ground-glass joints or a stopper made from a resistant material.

How do I get a stuck glass tube out of the stopper?

Soak the stopper in warm water to soften it, then work a little glycerol into the bore and twist the tube gently. If it still will not move, cut the rubber away with a knife rather than forcing the glass, because a snapped tube can cause a deep cut.

Last Updated: September 2026