Thermos Experiment

I made my thermos by wrapping tin foil around a styrofoam cup numerous times. One mistake, though, was not making it a close system, my thermos had no lid. Everyone should have started at 353.15 K and from there recorded 30 minutes of decreasing temperature, ours went down 27 K within 30 minutes. If it was a closed system maybe the thermos would have decreased slower.