If the smell of the impurity (najasah) remains, does it matter?
Question

What is the ruling on using a hosepipe in which some dead mice were found, and the smell still remains after cleaning it?
Praise be to Allah.If the mice died in the hosepipe, and something came out of them, or any part of them disintegrated, then the hosepipe has become impure (najis). If it is washed with pure water, or liquid other than water – according to the scholarly view that liquids other than water are sufficient to remove impurity – in such a way that there is no trace of the impurity left, such as taste, colour or smell, then it becomes pure (tahir).
If it is difficult to remove the smell after washing the hosepipe thoroughly and removing the substance of the impurity, then it is deemed to be pure.
Is it necessary to remove the smell by using something other than water, such as detergents?
There is a difference of scholarly opinion regarding that.
An-Nawawi (may Allah have mercy on him) said in al-Majmu‘ (2/594): If the smell alone remains and is difficult to remove, such as the smell of wine, the urine of a person affected by birsam [a kind of tumour] and some types of excrement.
There are two views, the more sound of which is that it becomes pure…
If the colour and the smell remain, then it does not become pure, according to the correct view, but ar-Rafa‘i narrated a different view concerning it.
The author of al-Tatimmah said: If the impurity cannot be removed with water only, but it can be removed by using potash and the like, that must be done.
Moreover, whatever we deem to have been purified in this manner, even if the colour or smell remains, is pure in a real sense. This is the correct view which is held by the majority.
Another view is also mentioned in at-Tatimmah, which is that it is still impure, but it may be overlooked, and there is nothing to worry about. End quote.
Al-Hattab said in Mawahib al-Jalil (1/163), in a comment on the view of Khalil: Once the taste has gone away, it does not matter if any colour or smell remains, if it is difficult to remove them.
If the taste remains, then the impurity has not been removed, because the fact that the taste remains indicates that the impurity is still present. Therefore the taste must be removed, even if it is difficult.
In other words, the place where the impurity was becomes pure thereby, provided that the taste of the impurity is removed. It is not stipulated that the colour or smell should be removed, if that is too difficult. But if it is not too difficult to remove them, that place does not become pure if either of them remains.
If the colour remains, that is more serious than if the smell remains.
It says in al-Jawahir: If the taste remains after the substance has disappeared, as seen by the naked eye, then that place is still impure, because the fact that the taste is still there indicates that the impurity is still there.
The same ruling applies if the colour or smell remains and it can be removed with water.
But if it is difficult to remove it, it may be overlooked, and the place is deemed to be pure.
He narrated it in adh-Dhakhirah and added: The smell may also be overlooked in the case of cleaning oneself after relieving oneself (istinja’), if it is too difficult to remove it from the hand or from the place.
Note…
-2- What matters with regard to removing it is removing it with water, as may be understood from the words quoted above from al-Jawahir, and it is easy to remove it with water. What may be understood from this is that if it is possible to remove the colour or smell with something other than water, that is not obligatory, and this is indeed the case. Ibn al-‘Arabi and Ibn al-Haajib also said something similar.
If it is possible to remove the colour and smell with potash or soap, then what appears to be the case is that it is not obligatory. Ash-Shafa‘i had a different opinion regarding that.
In the hadith of Khawlah bint Yasar regarding blood that is difficult to remove it says: The Prophet (blessings and peace of Allah be upon him) said: “Water is sufficient for you, and it does not matter if its trace remains.” Narrated by Ahmad and Abu Dawud.
Smell may be regarded as being like colour, when it is difficult to remove either. End quote.
It says in Fatawa al-Lajnah ad-Da’imah (6/210): If you washed the impurity that is on your clothes or body, and removed its substance, then you prayed, your prayer is valid, and so is the prayer of those who prayed behind you. It does not matter if any of its smell remains, so long as the substance of the impurity has disappeared.
Shaykh ‘Abdullah ibn Ghadyan, Shaykh ‘Abd ar-Razzaq ‘Afifi, Shaykh ‘Abd al-‘Aziz ibn ‘Abdillah ibn Baz. End quote.
And Allah knows best.