Pronunciation of 不: 불 vs 부

Question

不 can be pronounced either 불 or 부 depending on the word (examples). Is there a rule or pattern when to which one or is it just a matter of memorizing word by word?

Answer 1

Found a couple of documents:

1) '미채택', '불채택', '부채택'의 차이

When 不 is pronounced as '부', it is because the following noun usually starts with 'ㄷ' or 'ㅈ'. Examples are 부도덕(immoral), 부정확(imprecise), etc.

2) '부실'과 '불실'

There are a few exceptions, such as '부실(不實)', however since the proportion of vocabulary meeting the condition is large, the rule is still valid according to National Institute of the Korean Language.