The old distinction (I think it is now being challenged) between a 'chemical combination' and a 'meehanical mixture' answers fairly well to that drawn here by St Thomas between mixtio (mixis) and confusio (krasis). Oxygen was supposed to become something other than actual oxygen, when it combined with hydrogen to form water. The spiritual soul is not lost in man in the way that oxygen is lost, or was supposed to be lost, in water.

Of God and His Creatures: 2.56