godan verb (-ku), intransitive verb
boil; come to a boil; grow hot
Intransitive: the liquid or bath water itself becomes hot or boils. The transitive counterpart is 沸かす.
お湯が沸いたら、コーヒーを入れよう。
When the water boils, let's make coffee.
もうすぐお風呂が沸く。
The bath will be ready soon.
godan verb (-ku), intransitive verb
get excited; erupt in cheers; be lively
Metaphorical use, often with a crowd, venue, town, or event becoming excited or animated, especially with applause, cheers, or festival energy.
決勝点が入ると、スタジアム全体が沸いた。
When the winning goal was scored, the whole stadium erupted.
町は夏祭りで沸いている。
The town is buzzing with excitement over the summer festival.
godan verb (-ku), intransitive verb
Specialized use for fermentation becoming active, especially in brewing or food contexts; everyday Japanese often uses 発酵する instead.
蔵では、酒母が勢いよく沸いている。
At the brewery, the starter mash is fermenting vigorously.
godan verb (-ku), intransitive verb
melt; become molten
Technical or limited use for metal becoming molten under high heat; in ordinary speech, 溶ける is much more common for 'melt'.
鍛冶の文脈では、「鉄が沸く」は鉄が高温で溶ける状態をいう。
In a blacksmithing context, 鉄が沸く refers to iron melting under high heat.