Science Buddies: "Ask an Expert"

When I put UV detecting beads in different temperatures of water, the beads that were in water temperatures above 60 degrees C did not change color in the sun, but the beads in water temperatures from 0 to 50 degrees C did. Why did the beads in the hotter water not change color?

brendypendy - I'm not an expert on UV beads, but was curious when I saw your post so I did a little research. The fundamental action is that when the colorless beads are subjected to UV light source, certainly like the sun, or perhaps a flourescent light, the pigments will be activated and turn colored. When the beads are returned to a non-UV environment, they will revert to the colorless state. In reading one of the bead suppliers descriptions it states:

"Although UV light is needed to excite the molecule to form the high-energy planar structure, heat from the surroundings provides the activation energy to change the molecule back to its colorless structure. If colored beads are placed in liquid nitrogen, they will not have enough activation energy to return to the colorless form".

That suggests that there is a thermal aspect to how these beads act and reverse their state. Extreme cold prevents them for reverting back to colorless. Higher temperatures may have the reverse effect, preventing the beads to transition to a colored state. Perhaps the higher temperatures you used caused the beads to be temporarily insensitive to the UV radiation. If you cool the beads, do they then change color when exposed to UV light? In the experiments I looked at on the WEB, they had not demonstrated the effect of the warmer beads to be insensitive, or the need to keep the beads cooler.

Rick Marz

Thanks, that's sort of what I was thinking. Can't find an exact answer, but this does make sense. After being cooled, the beads did turn colour when placed in the sun.