变质 biànzhì to degenerate
to degenerate; to deteriorate; (of food) to go bad; to go off; (geology) metamorphism
How to remember 变质: 变 change + 质 essence = to degenerate.
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How it is used
- Everyday sense: food going off - 牛奶变质了 (niúnǎi biànzhì le) the milk has gone bad.
- Figuratively, something changes for the worse in its nature - 关系变质 (guānxì biànzhì) a relationship turns sour; 变质分子 a degenerate element.
- Note the literal make-up 变+质 = its 'quality/nature' changes, which links both senses.