Subject: What caused BTB to turn green in the refrigerator?

Students were studying the color change of BTB (bromothymol blue) and 
through their own inquiry experimental designs, some students thought the 
heat in their own breath made it change. From this hypothesis they 
developed and experiement where they put one cup of water with BTB in the 
microwave and one cup of BTB in the fridge.  The cup in the micorwave 
became a lighter blue, and the cup in fridge turned green.  Can you help me 
explain this? The students have learned since that time it is the Co2 in 
our breath that changes BTB (Bromothymol blue)color.  Can you tell us what 
happened in the micorwave and frige?

Re: What caused BTB to turn green in the refrigerator?