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I placed a glass stopper in a $\pu{100 mL}$ round bottom flask while my solution was still warm and let it cool overnight. Now I think I created a vacuum seal and I can’t force the stopper out to filter my product. I’ve tried heating it up again but to no avail. I don’t want to slip oil in there and risk getting it in.

Anyone have a good idea?

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    $\begingroup$ Sometimes tapping the joint lightly can help let some air in. $\endgroup$ – Zhe Oct 4 '18 at 19:04
  • $\begingroup$ Cooling the stopper helps, LN2, dry ice. $\endgroup$ – Karl Oct 4 '18 at 21:06
  • $\begingroup$ Have you access to an ultrasound bath? $\endgroup$ – Waylander Oct 4 '18 at 22:03
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If all else fails (unlikely unless you had a strong base in your flask, but who knows), use a glass cutter and snap off the neck of the flask.

Heating the neck with a heat gun should however be sufficient. Don't heat for too long, so the stopper is still cold. The expansion coefficient of borosilicate is small, you want to use the full effect.

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I found gently taping a glass stopper with another glass stopper sometimes loosens it(maybe the shock is allowing air to pass through), but you could always put ice on the stopper and surround the top of the flask with warm water and wait a minute or two. This allows the top of the flask to expand while the stopper shrinks just a bit. Then, just take the stopper out. If you lose all hope (I hope that doesn't happen) and don't care about about your solution I would just use WD-40.

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I've had success using a heat gun (or hair dryer) to heat up the joint and then using clamps to turn the stopper while pulling it out.

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