I start by saying that I'm a beginner in Japanese and maybe my question will seem obvious to you.

I've been told by Japanese people that there's a difference between 水 and 湯. I understand that 湯 means "hot water", but isn't it the same if I said あつい水?

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    As a native speaker, I don't feel あつい水 is immediately ungrammatical, but it can't avoid sounding like "invisible pink unicorn". – broccoli facemask - cloth Jan 30 '15 at 9:56

Although most dictionaries or word lists translate "mizu" simply as "water", the actual meaning of "mizu" is "cold/cool water" whereas "o-yu" is "warm/hot water". So to a Japanese "atsui mizu" sounds like "hot cold water". They can understand it because they know in English there is a word called "water" including both "mizu" and "o-yu", but a Japanese person would never express it that way.

The criterion basically is the human body temperature. Everything below it is "mizu", above it "o-yu". It is possible to say "tsumetai mizu" however in order to stress that the water is really very cold.

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  • Note that "mizu" can also be used to mean just water, when the temperature is not a matter at all (I believe this is why we can understand "atsui mizu"). – Yosh Mar 22 '15 at 2:17

The original difference between 水 and お湯 is that the latter was prepared, but the former was never. Hence, the honorific お is an indicator that this has been done for the listener's well being. Using temperature as a divider is for simplicity's sake.

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Adding to the other answers and regarding the phrase あつい水, I'd like note this is possible in certain contexts.

An article on Tokyo Language Center:






(summary: お湯 has got a cultural aspect to it. It sounds inappropriate in a purely scientific context.)

An article on ShareWIS press using it in a similar context:

海底にある ”特異点” ホットスポットが熱い!



A book titled 熱い水のような砂 by 桐野 夏生 (ISBN-10: 438785121X):

日本女性と冒険家の砂漠ロマンスです。ある理由があって、サハラ行きにOKしたフィルこと、イギリス貴族で冒険家のフィリップ・コンウェイ。 日本から遥々〜会いにやって来た日本女性、小津香子に次第に惹かれていくことに…

(beginning of an amazon review)

お湯 is something nice. The scorching hot sand of the Sahara is not.

A basic introduction to thermodynamics on NHK:


enter image description here





And a a short article I found talking about that temperature is not the only difference:



所長 澤 田 剛

「お湯」は、英語では「hot water」になります。では「hot water」を直訳すると、どうなるでしょう。「熱い水」ですね。つまり、英語話者にとっては「お湯」はただ「温度が上がった水」でしかないのです。しかし、日本人にとって「湯」と「水」には、単に温度の高い低いではない違いがあるようです。「お湯」は、「お風呂」や「温泉」(=いでゆ)を意味しますし、「湯ざまし」「ぬるま湯」等の言葉がある通り、一度沸かした「水」は、冷めても「お湯」です。風呂が沸いたと思い足を入れたら冷たかった。すると「なんだ、まだ水じゃないか」ということになります。


Which explains this as well, from 中学数学の基本問題 on geisya.or.jp:



※ A君がa(゜C)の冷たい水b(g)とc(゜C)の熱い水d(g)を,B君が,e(゜C)の冷たい水f(g)とg(゜C)の熱い水h(g)を持っていて,各自の水を混ぜる場合にも,上の問題と同様の取り扱いになります.


  • That's what the site self-evaluation is for: reviewing old questions ツ – blutorange Mar 20 '15 at 18:45

In general, We call what you might think of as "あつい水" -> "(お)湯". "あつい水" is not used in conversation for that meaning.

However, "あつい(お)湯" is correct and can be used.

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