Does the impurity itself disappear with water alone, and what is the meaning of its ruling remaining?

Pure water is a condition for purifying impurities according to the majority of scholars. Legal impurity is purified by water flowing over the spot, while physical impurity is removed by eliminating the substance itself. It is stipulated that the water used must be pure, unchanged, and that the taste, color, and odor of the impurity are removed. However, if the color and odor are difficult to remove, they are excused. As for the impurity of a dog, it is washed seven times, one of which is with earth or a substitute for it.