chemicals become gas at specific temperatures, although it's also a function of the pressure at which the substance finds itself. At ground level we have a pressure of "1 atmosphere" or approx. 1 bar. Water boils at 100° C at 1 bar. Some evaporation takes place at lower temperatures, but at the boiling point, you have inevitable, rapid evaporation.
The temperature is different for different substances. Impure substances might not evaporate cleanly, and might leave residues, or maybe they don't even reach boiling point, just melting point, thus "fucking up" your equipment.