rastvoriti

1. to dissolve
   Rastvori tabletu u čaši vode. — Dissolve the tablet in a glass of water.
2. (rastvoriti se) to dissolve
   Šećer se brzo rastvorio u toplom čaju. — The sugar quickly dissolved in the hot tea.
3. formal (of doors, windows) to open wide; to fling open
   Rastvorila je prozor i pustila svež vazduh. — She flung open the window and let in fresh air.

The imperfective is rastvarati; for 'to dissolve' on its own it takes se.