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.
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.
Two branches
混進去的那個東西
鏡頭對準畫面裡那顆小黑點本身。看得見、數得出來、可以拿掉。
noun · B2
A small amount of an unwanted substance that is mixed into something else and makes it less pure, such as dirt in water, other metals in rock, or oil on skin. It is usually something people try to take out.
雜質
不純這個狀態
把鏡頭從那顆小黑點拉遠，看整杯水。這時說的不是雜質本身，而是「這杯水已經不純了」這件事。
noun · C1
The state of not being pure — that is, of having something mixed in that does not belong, so the material is not clean or complete on its own.
不純（的狀態）