Rubber stoppers are one of those unglamorous but essential pieces of laboratory labware that show up in nearly every setting, from high school chemistry classrooms to industrial research labs. They’re inexpensive, reusable, and remarkably versatile, and if you browse any range of rubber stoppers you’ll notice they come in solid, one-hole, and two-hole varieties, with the type you reach for depending entirely on what you need the stopper to do beyond just plugging a container.
In this post, we’ll break down the three most common types (solid, one-hole, and two-hole stoppers) and look at what each one is used for.
Solid Stoppers
A solid stopper has no holes at all. It’s just a tapered plug made of rubber or silicone. Its main jobs are:
- Sealing containers airtight: keeping test tubes, flasks, and bottles closed to prevent contamination, evaporation, or exposure to air and moisture.
- Storing chemicals: especially reactive or volatile substances that need to be isolated from atmospheric oxygen or humidity.
- Creating a vacuum or pressure seal: solid stoppers can withstand pressure differences, making them useful in vacuum flasks or during filtration setups (e.g., with a side-arm flask) where an airtight fit matters.
- Protecting samples: solid stoppers keep out dust, insects, and contaminants during storage.
Because there’s no opening, solid stoppers are the go-to choice whenever total isolation from the outside environment is the goal.
One-Hole Stoppers
A one-hole stopper introduces a single passage through the plug, usually fitted with glass or plastic tubing. This turns a simple seal into a functional interface between the inside and outside of a vessel. Common uses include:
- Gas generation and collection: a classic setup is a flask with a one-hole stopper and delivery tube that channels gas (like CO₂ or H₂ produced in a reaction) into a collection vessel or gas jar.
- Thermometers or temperature probes: the hole lets a thermometer pass into the vessel while keeping the rest of the system sealed, useful in distillation or reflux setups.
- Dropping funnels or single-line delivery: allowing controlled addition of a liquid reagent into a sealed reaction vessel.
- Aspirator or vacuum connections: connecting a flask to a vacuum line while keeping the rest of the container sealed.
Essentially, a one-hole stopper is used whenever you need one controlled point of access (one tube in or out) while maintaining a seal everywhere else.
Two-Hole Stoppers
Two-hole stoppers allow two separate passages, which opens up more complex setups where gas or liquid needs to move in and out simultaneously, or where two instruments need to enter the same vessel. Typical applications:
- Gas washing or scrubbing bottles: one tube delivers gas into the liquid (often extending below the liquid surface), while the second tube lets the washed or dried gas exit near the top.
- Distillation and reflux apparatus: one hole might hold a thermometer, while the other connects to condenser tubing or an inlet/outlet line.
- Kipp’s generators and gas-producing setups: one hole for gas outlet, another for adding reagents or acid safely without disturbing the seal.
- Dual-function reactions: for example, one tube supplying an inert gas (like nitrogen) to prevent oxidation, while the other vents excess gas or carries product out.
Two-hole stoppers are the standard choice any time a closed system needs two independent connections, such as simultaneous gas inlet and outlet, or a thermometer alongside a delivery tube.
Quick Comparison
| Type | Openings | Typical Use |
| Solid | 0 | Airtight sealing, storage, vacuum sealing |
| 1-hole | 1 | Single tube/thermometer access, gas collection |
| 2-hole | 2 | Simultaneous gas in/out, dual-instrument setups |
Material Considerations
Most stoppers are made from natural rubber or silicone, though neoprene and butyl rubber versions exist for specific chemical resistance needs. Silicone stoppers are popular for high-temperature work (like autoclaving) since they tolerate heat better than natural rubber, while natural rubber is often preferred for general chemical resistance at a lower cost.
Key Takeaways
- Solid stoppers: total isolation. Airtight storage, vacuum seals, contamination-free containers.
- One-hole stoppers: one controlled access point. Thermometers, delivery tubes, gas collection.
- Two-hole stoppers: two independent connections. Gas in/out, dual-instrument setups, reflux and distillation work.
Choosing the right stopper ultimately comes down to how many points of access your setup needs. It’s a small detail, but getting it right keeps your experiments safe, contained, and running smoothly.
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