Answers: There are three kinds of water:
1. Pure and purifying water, which removes hadath and impurity.
2. Pure yet not purifying water. This means that it is not valid to use it for ablution or ghusl though it is pure. An
example of this is the water that has already been used in ablution.
3. Impure water, which a Muslim is not allowed to use for purification or removing impurity.
The first kind of water is the absolute water, that is, water which is pure in its own right and can be used for purifying
other things, such as the water of rain, ice, and hail, as well as the water of springs, wells, rivers, groundwater, and seawater.
The second kind of water, which is pure yet not purifying, is like the water which a man used in ablution or ghusl when there was not any tangible impurity on his body, because if there was any such impurity the water would lose the characteristic of being pure. This indicates that this water is still pure in itself, as it was used for ablution on a clean body, but it is not purifying, that is, it is not suitable for ablution or for any other form of ritual purification for a second time, because it has become void of the characteristic of being a ritually purifying water, for it has already been used as such and thus cannot be used again for the same purpose.
The third kind of water, which is the impure water, is the water which is afflicted by impurity when its quantity is little and this impurity changes some of its qualities such as taste, color, or smell. It is not valid to use such water for ritual purification or removing impurity in general. And, Allah knows best.