Is 「湯を沸かす」 an exception or an example of a general rule?

Question

Japanese people use 湯を沸かす instead of 水を沸かす to say "to boil water", is it an exception or a general rule? Do we use the word 冷水 to say "to cool down water" for example?

Answer 1

One could safely say that it is a general rule.

We say 「湯{ゆ}を沸{わ}かす」 to mean "to boil (cold) water". The phrase does not mean "to boil water that is already hot" just because 「湯」 means "hot water" all by itself.

「湯」 is the result of boiling cold water, but saying 「水を沸かす」 is not nearly as common as saying 「湯を沸かす」.

The pattern here is:

「Finished Product + を + Verb」

even though the finished product will not exist until the action described by the verb is completed.

Other examples:

・「ローストビーフを焼{や}く」 ("to make roastbeef"). Literally, "to roast roastbeef" even though what you roast (焼く) is raw beef.　Illogical!

・「穴{あな}を掘{ほ}る」 ("to dig a hole"). Same in English, sort of. Why dig (掘る) if there is already a hole? What you are digging is the earth.

・「ツルを折{お}る」 ("to make a crane (in origami)") What you really fold (折る) is plain paper, not a crane.

・「シチューを煮{に}る」 ("to make stew") Why boil (煮る) stew? What you actually boil are the ingredients. Weird.

・「もちをつく」 ("to make mochi") What you pound (つく) is steamed mochi rice and not mochi itself! Gimme a break!

Do we use the word 冷水{れいすい} to say "to cool down water" for example?

No, 「冷水」 can only mean "cold water". "To cool down water" would be 「水{みず}を冷{ひ}やす」.