Final answer:
Removing the tube from the suction flask before turning off the vacuum pump prevents backflow contamination and filter paper tearing, ensuring that the separated maleic acid is not compromised.
Step-by-step explanation:
In the laboratory experiment you described, where maleic acid was separated by suction filtration, it is crucial to remove the tube from the suction flask before turning off the vacuum pump. This is because if the vacuum is maintained after filtration and the pump turned off while the flask is still sealed, it could possibly result in the reverse flow of water or other solvents from the water trap of the vacuum pump back into the filtered sample or the flask.
This backflow could contaminate your purified product, leading to ruined results and potentially requiring a repetition of the entire procedure. Additionally, the sudden change in pressure could cause the filter paper to tear, thus mixing the precipitate that you've carefully separated back into the filtrate.