noun
Refers to something mixed in, often unwanted. Commonly used in the negative form 混じり気のない to mean 'pure' or 'unadulterated'.
この水は混じり気がなくておいしい。
This water is delicious with no impurities.
彼の親切は混じり気のないものだ。
His kindness is unadulterated.
Alternate kanji using 交; less common.
不純物 is a more technical term for impurities, often used in scientific or industrial contexts, while 混じり気 is a more everyday word for something mixed in.
Compound of 混じり (continuative form of 混じる 'to be mixed') + 気 ('feeling, sign'). The exact historical derivation is uncertain, but the word has been used since at least the Edo period.