Pure water is used essentially in matters of purification or wudu' (minor ablution) and ghusl (major ablution). Hence the necessity to investigate water's purity. Water has four essential attributes: smell, color, taste, and fluidity. Any pure and purifying water is judged according to whether it retains these attributes or not. As a result, water is classified into two categories: mutlaq and muqayyad water.

Mutlaq water is "natural" water, such as that which comes from rain water , snow, hail, sea water, and water from the Zamzam well.

It is subdivided as follows:

• Water that is both pure and purifying (e.g., rain water, snow, hail, sea water, and water from the Zamzam well).
• Water that drips from a person after he or she has performed the minor or major ablution, and therefore is considered "used." It is considered pure, but cannot be used for another minor and major ablution.
• Water that is both pure and purifying, but whose usage is disliked (makruh) (e.g., water left in a container after a cat, bird, or another "allowed" animal has drunk from it).
• Water mixed with impure elements. Water whose taste, color, or smell has been altered by an impure substance cannot be used for purification. However, if the liquid is still considered water, meaning that the impure substance has not altered its taste, color, or smell, it can be used for purification.
• Water that is pure but may or may not be purifying. One example of this type of water is the water that remains in a pot after a donkey or a mule has drunk from it.

Muqayyad water includes naturally muqayyad water, such as fruit juices and water that has been mixed with various substances (e.g., soap, saffron, flowers) or objects that the Shari'a considers pure. Such water is considered pure until, due to being mixed with other substances, one can no longer call it water. In this case, the water is still considered pure, but it cannot be used for purification (minor and major ablution).