Difference between 沸騰する and 沸く


I don't really get the difference since the two of them seem to be translated
as "boiling". Do we have to use them in different situations?




**_Origin:_**

「沸{わ}く」, as I hope you could tell from its kun-yomi pronunciation, is a 100%
originally Japanese word. The word already existed when Japanese was only a
spoken language.

「沸騰{ふっとう}」, as its on-yomi reading would suggest, is a Sino-loanword.

Thus, 「沸騰する」 tends to sound more formal, academic, technical, etc. than 「沸く」
does as one could generally expect between a Japanese-origin word and its Sino
counterpart.

The fact that many "big" words come from somewhere else is a characteristic
shared between Japanese and English -- Chinese for Japanese and mostly Latin
for English. Chat vs. converse, deep vs. profound, sweat vs. perspire, etc.

**_Scope of meaning:_**

「沸騰する」 means "(of water) **to reach the temperature of 100 degrees Celsius**
".

「沸く」, while it can mean the exact same as above, it has an extra meaning that
「沸騰する」 does not. (This is where @Chocolate's comment above comes into play.)

「沸く」 has an extra meaning of "(of water) **reaching the desired temperature
for a given purpose** ". For the "hot" water in the bath tub, for instance, it
would only be around 40 degrees Celsius. It certainly would not be 100.

Thus, whenever the water is "hot enough" for a specific purpose, you can use
「沸く」 but not 「沸騰する」.

Finally, there are non-water-related meanings for both verbs, but I will not
go into that as I presume that is not what the questioner wanted to find out
about.

