Flat Bottom Flask usually used for storage, transferring, mixing, warming and rarely boiling of liquids. There are multiple names and types of Flat Bottom Flasks based on its usage. There are few issues with boiling in flat bottom flask even though they perfectly fit onto hotplate without additional support. First issue is uniformity is not longer exist due to increase thickness of the glass on the corners where round becomes flat. Based on my experience I can heat up Flat Bottom Flask to around +120C, however the most important issue is when and if liquid gets trapped between hotplate and the bottom of the flask, the impact of liquid evaporation maybe that high that flask will jump and crack on landing. That is why I would never boil in a flat bottom flask. Mixing in the flask is perfect due to flat bottom there is ability to use larger stir bar at warm around +100C temperatures.