From Longman Dictionary of Contemporary Englishboil away phrasal verbDFCHif a liquid boils away, it disappears because it has been heated too much The soup’s almost boiled away. → boil→ See Verb table
Examples from the Corpus
boil away• At constant pressure, the boiling point remains constant until all the liquid has boiled away.• Primitive sensations skittered here and there, triggering twinges of irritation that carelessly boiled away.• Check every 20 minutes or so that the water has not boiled away and, if so, replenish with boiling water.• Large generators can overcharge batteries, ultimately destroying them as the electrolyte boils away and the plates heat up and buckle.• It was bubbling and boiling away like mad inside your head.• This might connect with experiences of seeing liquids boiling away on a cooker. 