The Shari`ah (Islamic Law) has divided water into four kinds:
1. Mutlaq water,
2. Used water (for purification),
3. Water mixed with pure elements and
4. Water mixed with impure elements. We shall discuss each of them separately.
1. Mutlaq water
This kind of water is considered pure because of its inherent purity and as such, it can be used by an individual to purify him or herself. It consists of the following categories:
A. Rain water, snow, and hail
These substances are pure because Allah says so:
And sent down water from the sky upon you, that thereby He might purify you… (Al-Anfal 8:1)
In another verse Allah says:
We send down purifying water from the sky. (Al-Furqan25:48)
This is also supported by the following hadith: Abu Hurairah (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) used to be silent between the takbir (opening) of the prayer and the verbal Qur’anic recitation. Abu Hurairah asked him, “O Messenger of Allah, may my father and mother be sacrificed for you, why do you remain silent between the takbir and the recital? What do you say (silently during that time)?” He said, “I say, ‘O Allah, make a distance between me and my sins similar to the distance you have made between the East and the West. O Allah, cleanse me of my sins in the manner that a white garment (is cleansed) from dirt. O Allah, wash my sins from me with snow, water, and hail.”‘ (Agreed upon)
B: Sea water
Sea water’s purity is based on the following hadith: Abu Hurairah related that a man asked the Messenger of Allah:”O Messenger of Allah, we sail on the ocean and we carry only a little water. If we use it for ablution, we will have to go thirsty. May we use sea water for ablution?” Said the Messenger of Allah “Its (the sea) water is pure and its dead (animals) are lawful. (Al-Bukhari)
C: Water from the well of Zamzam
`Ali (may Allah be pleased with him) narrated that the Messenger of Allah called for a bucket that contained water from the well of Zamzam. He drank from the bucket, and then made ablution (with its water). (Ahmad)
This involves water whose form has been altered because of its being in a place for a long period of time, or because of the place in which it is located, or because of its being mixed with a substance that cannot be completely removed from it (i.e., water mixed with algae, tree leaves, and so on). The scholars agree that this type of water falls under the heading of mutlaq water.
The rationale is simple: everything that falls under the general term of water, without any further qualifications, is considered pure, for the Qur’an says:
…and if you find not water, then go to clean, high ground…(Al-Ma’idah 5:6)
2. Used water
This category refers to water which drips from the person after he performs ablution or ghusl (ritual bath). It is considered pure because it was pure before its use for ablution, and there is no basis to think that it has lost its purity. “The Messenger of Allah wiped his head with the extra water that was in his hand.” (Abu Dawud)
Abu Hurairah also reported that the Messenger of Allah met him alone in the streets of Madinah while he was in post-sex impurity. He therefore slipped away, made ghusl and returned. The Messenger of Allah asked him “Where have you been, Abu Hurairah?” He answered, “I was in post-sex impurity and did not want to sit with you while I was in that condition.” The Prophet replied, “Glory be to Allah. The believer does not become impure.” (Agreed upon)
This is based on the rationale that since a believer never becomes impure; the water he uses for purification also does not become impure. Thus, a pure object touching a pure object cannot result in one’s becoming impure.
3: Water mixed with pure elements
This category includes water that has been mixed with substances like soap, saffron, flowers, and so on, that is, objects considered pure by the Shari`ah. Such water is considered pure as long as it has not been so mixed with other substances that one can no longer call it water. If this is the case, the water is still considered pure, but it cannot be used for purification.
Umm `Atiyah (may Allah be pleased with her) narrated that the Messenger of Allah entered her house after the death of his daughter Zainab (may Allah be pleased with her) and said, “Wash her three or five or more times–if you see fit to do so–with water and dry tree leaves. For the final washing, use some kafur or something from kafur. When you are finished, inform me.” She did so, after which he gave the women his outer garment and told them to wrap Zainab in it. (Agreed upon)
Source: Quoted with slight modifications from Sheikh Sayed Sabiq’s Fiqh-Us-Sunnah.