Paramedics who had been called to the scene then recommended that the building be evacuated. Several other students began reporting similar symptoms as they were evacuating - light-headedness, nausea, dizziness, and difficulty breathing, university officials said.
By the end of the day, 45 people had been treated on-scene by paramedics. Of those, 29 were taken to hospitals for evaluation, but were released.
Meanwhile, hazmat crews inspected the building and the classroom where, university officials said, students had been conducting "a commonly performed organic synthesis using propionic acid and alcohol to create a compound." But the crews found no evidence of a chemical spill or leak, officials said.
The experiment "has been run the same way numerous times before without incident," said Jon Gust, the university's director of media relations.
Radnor Fire Chief Dan Kincade said the student who initially fell ill has asthma and could have had a reaction to a vapor released during the experiment.
"It is conclusive that it had to be something to do with vapors from the experiment," he said.
Other students may have seen her symptoms - which included a nosebleed - and become distressed, although that is just "one angle that could be looked at," Kincade said.
"I have a feeling that somebody saw somebody getting sick and got a little wheezy themselves," Kincade said.
Kincade said hazmat crews found no vapors in the lab, but said Mendel Hall has a "fantastic" ventilating system that would have cleared the air by the time crews reached the classroom.
Mendel was open for normal operation by Wednesday morning, officials said. In the meantime, the chemistry department is conducting a review of safety procedures, the university said.
Contact Aubrey Whelan at 610-313-8112, at email@example.com or on Twitter at @aubreyjwhelan.