You can think of it as a in a glass of clear water, or a streak of another metal running through a bar of gold. Something small got in that does not belong there. It is not the main thing, and there is not much of it. But once it is in, the whole thing can no longer be called pure — and someone usually wants it out.
From the word’s history
The word is im- (not) plus pure plus -ity (a state, or a thing). Literally: not-pure-ness, or the thing that makes something not pure. Both of those readings survive as separate uses today.
這個字是 im-（不）加 pure（純）加 -ity（狀態，或某個東西）。字面上有兩種讀法：「不純的狀態」，以及「讓東西不純的那個東西」。這兩種讀法今天都還活著，正好就是它的兩個主要用法。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:716264747:20691), entry 'impurity': From impure + -ity. From Middle French impurité, from Latin impuritas.
登入就可以看每一支怎麼連回核心畫面。