Question

Why should the stopper of a chemical reagent container be handled carefully?

Answer 1

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The person before you may have been careless and didn't put it securely back in. There may well be some reagent dripping down the side of the bottle as well...

Either way, you don't want to spill it by lifting the bottle by the stopper...

It is in fact poor laboratory practice to leave a stopper loose in a reagent bottle. Never do that! If it houses concentrated acid, that is even worse, as then one has to waste neutralizer (e.g. sodium bicarbonate) to clean it up.