When water changes

Water has three characteristics which must not change. These are

Color
Taste
Smell

If anyone of these characteristics changes such as the smell, for example, to the extent that the name of water cannot be applied without a qualification, then the water is no longer purifying.

For example, take rose water. You cannot call rose water, water. You have to qualify water by including the word rose. You have to say rose water. It's taste has changed to such an extent that it cannot simply be called water. You have to call it rose water. The unqualified noun water can no longer be applied to it.

So, when the color, taste, or smell of water changes to such an extent, after something has been mixed with it, that you cannot simply call it water, then the water is not purifying. You cannot use it for Wuduu or ghusl.

If the thing which mixed with the water was a pure substance, then the water will lose its property of being purifying but will remain pure. You will not be able to use it to attain purity for Wuduu or ghusl.

This is the Madhhab of the majority of scholars of Islaam.

The Hanafi school has opposed the majority.

The Hanafi schools says that water will not lose its ability to purify as long as it does not lose its natural disposition. According to them, its natural disposition is its being a refreshing liquid which pacifies thirst.

According to the school if the characteristics of the water change but it still remains a liquid which is refreshing to drink and pacifies thirst, then it is also purifying. You can perform Wuduu with it and make ghusl.

This is in stark opposition to what the majority of scholars of Islam adhere to.

Reference(s):
Kuwaiti Encyclopedia of Fiqh
Volume 22 Page 41