沸かす
わかす
wakasu
= to boil; to heat (a liquid)
Whether it’s for a cup of green tea, a pot of ramen broth, or a relaxing bath at the end of the day, 沸かす (wakasu) is the verb Japanese speakers reach for whenever they bring a liquid to a boil. It sits at the heart of daily domestic life in Japan, where heating water is a ritual as much as a task.
沸かす is a transitive verb meaning to boil or to heat a liquid until it reaches boiling point. Because it is transitive, it always takes an object — the liquid being heated. Common objects include お湯 (oyu, hot water), お茶 (ocha, tea), お風呂 (ofuro, bath). The intransitive counterpart 沸く (waku) describes the liquid itself boiling: お湯が沸く means ‘the water is boiling (on its own),’ whereas お湯を沸かす means ‘to boil the water (intentionally).’ The related noun 沸騰 (futtō) refers to the state of boiling and is used in more formal or scientific contexts. In everyday speech, 沸かす covers everything from quickly heating water in an electric kettle to slowly bringing a large pot of dashi stock to temperature.
Learners often mix up 沸かす, 煮る (niru), and 温める (atatameru). 沸かす specifically means bringing a liquid to boiling point — it is used almost exclusively with water, tea, and bath water. 煮る means to cook something by simmering or boiling it, so the focus is on cooking an ingredient in liquid (野菜を煮る, to simmer vegetables), not just heating the liquid itself. 温める means to warm something up to a comfortable or moderate temperature without necessarily boiling it — you 温める leftover soup in the microwave but 沸かす water for tea. A quick test: if the liquid must reach 100 °C, 沸かす is usually the right choice.
The character 沸 combines the water radical 氵 (sanzui) on the left with 弗 (futsu) on the right. 弗 originally suggested something bubbling or overflowing, and paired with the water radical it vividly captures the image of water bubbling up and boiling over. The kun’yomi reading わ(かす) is used in 沸かす and 沸く, while the on’yomi フツ appears in compounds such as 沸騰 (futtō, boiling) and 沸点 (futten, boiling point).
Everyday use
パスタを茹でる前に、大きな鍋でお湯を沸かしてください。
Pasuta wo yuderu mae ni, ōkina nabe de oyu wo wakashite kudasai.
Please boil water in a large pot before cooking the pasta.
Casual / Social Media
寒いから帰ったらすぐお茶沸かした。体が温まるよ。
Samui kara kaettara sugu ocha wakashita. Karada ga atatamaru yo.
It was cold, so I boiled some tea as soon as I got home. Really warms you up.
Formal / Cultural context
昔の銭湯では、薪を使って大きな浴槽のお湯を沸かしていた。
Mukashi no sentō de wa, maki wo tsukatte ōkina yokusō no oyu wo wakashite ita.
In the old public bathhouses, they used to heat the large tubs with firewood.
In Japan, boiling water for tea is not simply a step before drinking — it is a moment of pause built into the rhythm of the day. At home, an electric kettle (電気ケトル, denki ketoru) sits permanently on the kitchen counter, ready to 沸かす water for green tea, instant miso soup, or cup noodles within minutes. The act is so routine that phrases like ‘お茶沸かすね’ (‘I’ll put the kettle on’) serve as a quiet gesture of hospitality when a guest arrives, signaling care without fanfare.
The phrase お風呂を沸かす (ofuro wo wakasu) reveals another distinctly Japanese use of this verb. Traditional Japanese bathtubs are designed for soaking rather than washing, so the water is heated to a specific temperature — typically around 41–42 °C — and kept hot for the whole family to use in sequence. Many households also practice 追い焚き (oitaki), reheating the same bath water between users using a built-in heater. Modern smart-home systems even allow you to start 沸かす from your phone so the bath is ready the moment you walk in the door, blending a centuries-old ritual with contemporary convenience.
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