* Evaporation is a surface phenomenon where molecules at the surface of a liquid gain enough energy to escape into the gas phase. This happens at all temperatures, but the rate increases with temperature.
* Boiling is a bulk phenomenon where the liquid reaches a temperature called the boiling point, where its vapor pressure equals the surrounding atmospheric pressure and the liquid turns into gas throughout its entire volume.
Bromine has a relatively low boiling point (59°C or 138°F). While it doesn't fully boil at room temperature, its vapor pressure is high enough for a significant amount of bromine molecules to escape into the gas phase, creating a reddish-brown vapor that we can see.
This is why you often see bromine stored in sealed containers. The vapor is toxic and corrosive, so it's important to handle bromine carefully.
