This trophy is earned for finding optional photo #9. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional photos through
collectibles mode. The following collectible is described and can be seen below:
[
5:57] - Optional Photo #9
Inside the school, go to the chemistry room, talk to Warren, rewind, talk the to teacher and ask about the experiment. Return to Warren and recommend Chlorine. take a picture of the successful experiment afterwards.
Collecting all 10 photos will also reward you with the following achievement:
https://www.trueachievements.com/a197682/lab-master-ach....htm