Camphor is reduced to isoborneol by sodium borohydride in ethanol. the instructions for safe disposal of the chemical waste generated by this reaction state that the lids should not be screwed back onto the waste containers. why should the lids not be put back on the waste containers?

The lids should not be put back on the waste containers because the  reaction waste is still generating hydrogen gas that would get trapped in the container.

Explanation:

Camphor is reduced to isoborneol by dissolving camphor in ethanol in erlenmeyer flask . This reaction is followed by  addition of  excess sodium borohydride in small portions . The reaction mixture is left for 10 minutes  and heat is then applied to the reacting mixture. Therefore for safety purpose, during heating, the lids should not be put back on the waste containers because the  reaction waste is still generating hydrogen gas that would get trapped in the container.

The reacting mixture is then poured into a beaker of ice water  afterwards and Vacuum filtration and blotting dry is used to collect the precipitate instead.