Water of separation - In Numbers 8:7, the water of purification from sin is the “water of purifying.” So that which was to remedy a state of legal separation is here called “water of separation.”
Related Commentaries of Numbers 19:9
Numbers 19:9
9 And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place, and it shall be kept for the congregation of the children of Israel for a water of separation: it is a purification for sin.