verb · to go bad or go off, especially referring to food or organic matter

由于天气炎热，剩菜已经变质了。

verb · to undergo a change in nature or character for the worse; to deteriorate

要警惕腐朽思想使人变质。

verb · the geological process of change in the structure of rock through pressure or heat

变质作用形成了许多独特的矿石。