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PCAT Chemical Processes – Excess Reagents

A chemistry student is carrying out several reactions in the lab. At the end of his work, he has excess of three reagents: X, Y, and Z. The student examines the beakers with each reagent. X is a clear liquid and looks to be fine, so he pours X back into the stock bottle in the storage room. There is very little Y left, so he pours it down the drain with lots of water. Finally, there is a significant amount of Z left, but the student knows that Z is a volatile liquid, so he leaves it on the lab bench overnight, knowing that it will have evaporated by the time he returns in the morning. Which of the following correctly characterizes the student’s choices:


A) The student properly dealt with reagent X and Y but not Z. Open containers of excess reagents should never be left sitting out on the lab bench overnight.
B) The student properly dealt with reagent Y but not X nor Z. To avoid contamination, reagents should never be returned to the original stock bottle, and leaving a beaker unattended is a safety risk.
C) The student properly dealt with reagent X but not Y nor Z. Reagents should not be flushed down the sink and they should not be left unattended overnight.
D) The student did not properly deal with any of the excess reagents.


Waste materials in the lab should be disposed of in properly marked waste containers. The student did not deal correctly with any of the reagents. Returning it to the stock bottle risks contamination, pouring it down the drain risks damage to the plumbing or (eventually) the environment, and leaving it sitting out on the lab bench is all sorts of dangerous. Therefore, the answer is (D).

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