If I left a solution of 2C-D in dH2O (1mg/1mL) exposed to the air inside my house for a few days, would the water evaporate, leaving the 2C-D behind? If so, would it be as potent as before?
Ya; should be fine; 2C-D is a phenethylamine which are very stable.
It shouldn't take a few days though; put it somewhere like an open window for a breeze or put it in front of a fan. Air softly blowing over the surface of the water will speed evaporation up quite a bit.