What is the purpose of the stopper in a flask?

Answer 1

Discounting any ridiculous side-usage for rubber stoppers that may exist, they are simply used to seal glasswear in a temporary manner.

For example, if you want to shake your solution but don't want to get it all over yourself. Or if you want to transport it and minimize risk of spillage.

We generally don't use rubber stoppers much anymore, though. Glass stoppers are considered safer since they will pop out more easily if pressure builds inside the glasswear (which can be VERY dangerous). I would never stopper anything that I am applying heat to or that I expect gas production from, so be careful in your usage.

Glass stoppers are also a little better because you know if your solution isn't reacting with your glasswear, it won't react with the stopper....whereas it may still react with a rubber stopper.

Another advantage is that glass stoppers are easier to remove (try greasing them if they leak air or don't work smoothly). A good example is if you are using a separating funnel you will usually want to shake the funnel and then relieve the pressure by removing the stopper (at very regular intervals). This can be a pain with a rubber stopper as they require more force, which can also lead to accidents. Again, glass stoppers make this much easier.

Cork stoppers are used similarly.

Answer 2

The easy answer is, "to cap test tubes and flasks." The real answer is, rubber stoppers are used in the lab because the rubber in them is unaffected by most chemicals - and unlike the glass stoppers that used to be standard in laboratories, they don't get stuck in the container.

Answer 3

To keep things in flasks. Glass is less reactive than rubber, and some corrosive chemicals will eat right through the rubber!

Answer 4

Stoppers placed in the mouths of flasks and test tubes prevent spillage and evaporation.

Answer 5

Either to prevent things from getting in OR getting out (or both).