When Does Water Become Impure?
Question

Is it permissible for me to do Wudu with water when I know that something impure fell into it, but it was a small amount of impurity, and it did not leave any trace in the water?
Praise be to Allah.If something impure falls into water , it will affect the water in one of the following three ways:

The impurity changes one of the characteristics of the water (colour, taste or smell). This water becomes impure according to scholarly consensus, regardless of whether the water was of a great or small amount.

Ibn Al-Mundhir (may Allah have mercy on him) said:
“The scholars unanimously agreed that if something impure falls into water , of a small or great amount, and the impurity changes the water in taste, colour or smell, then it is impure so long as it remains like that, and it is not acceptable for the purpose of doing  Wudu or Ghusl." (Al-Awsat  1/260)

If something impure falls into a large amount of water, but it does not change any of its characteristics (taste, colour or smell). This water is pure, according to scholarly consensus.

Ibn Al-Mundhir (may Allah have mercy on him) said:
“The scholars unanimously agreed that if something impure falls into a large volume of water, such as the Nile, or the sea and the like, but it does not change it in colour, taste or smell, then it remains as it was, and may be used for the purpose of purification." (Al-Ijma`  p. 35)

If something impure falls into a small amount of water, but it does not change any of its characteristics, such as a drop of blood or a drop of urine that falls into a vessel in which there is water, but it does not change any of its characteristics – is it to be deemed impure because of the impurity falling into it , or is it to be deemed pure because there is no change in its characteristics?

The correct scholarly view is that the water is not to be deemed impure unless it is changed by that impurity whether the water was a small or great amount.
This is the view of the Malikis, and it is one report that was narrated from Imam Ahmad. It was regarded as more likely to be correct by Shaykh Al-Islam Ibn Taymiyah and his student Ibn Al-Qayyim, and was the view favoured by many contemporary scholars such as Shaykh Ibn Baz, Ibn `Uthaymin, and the scholars of the Standing Committee for Issuing Fatwas (may Allah have mercy on them). (See: Adh-Dhakhirah by Al-Qurafi, 1/172); Al-Mughni by Ibn Qudamah, 1/39); Majmu` Al-Fatawa by Shaykh Al-Islam, 21/32); Ash-Sharh Al-Mumti`, 1/41)
They quoted as evidence for that the report narrated from Abu Sa`id Al-Khudri (may Allah be pleased with him) who said: It was said: O Messenger of Allah, can I do Wudu from the well of Buda`ah? It is a well into which are thrown menstrual rags, the flesh of dogs and other putrid things. The Messenger of Allah (blessings and peace of Allah be upon him) said: “Verily water is a means of purification and nothing makes it impure.” (Narrated by Abu Dawud, 66; At-Tirmidhi, 66; and An-Nasa’i, 326. It was classed as authentic by Imam Ahmad, Yahya ibn Na`im, At-Tirmidhi, An-Nawawi, Ibn Mulaqqin and Ibn Hajar (may Allah have mercy on them). See: Al-Majmu`, 1/82 and Al-Badr Al-Munir, 1/381)
In this Hadith we see that water is a means of purification and nothing makes it impure. The scholars unanimously agreed that it becomes impure if it is changed by something impure falling into it, but otherwise it remains as it originally was, which is pure.
Shaykh Al-Islam Ibn Taymiyah (may Allah have mercy on him) said: “This wording is general in meaning and applies whether the amount of water is small or great, and it is general in meaning and applies to all kinds of impure things.
Nonetheless, if it is changed by that impurity , then it is only prohibited to use it because the impure substance is still there, so when using the water it is like using the impure substance too. This is unlike the situation when the impure substance has been completely absorbed and disappeared, for water is a means of purification and there is no impurity remaining in that case ." (Majmu ` Al-Fatawa , 21/33)
Shaykh Ibn Baz (may Allah have mercy on him) said:
“The correct view is that anything less than two Qullahs [a Qullah is a type of vessel] does not become impure unless it is changed (by the impure substance), as is the case when the amount of water is two Qullahs or more, because the Prophet (blessings and peace of Allah be upon him) said: “Verily water is a means of purification and nothing makes it impure.” The Prophet (blessings and peace of Allah be upon him) only mentioned two Qullahs in particular so as to highlight the fact that anything less than that needs to be checked and examined with care, not that it becomes impure in all cases, because of the Hadith of Abu Sa`id (may Allah be pleased with him) mentioned above.
What we learn from that is that a very small amount of water is usually affected by impurity, so it should be poured away, disposed of and avoided." (Majmu` Fatawa Ibn Baz 10/16)
The scholars of the Standing Committee for Issuing Fatwas said:
The basic principle concerning water is that it is pure, but if its colour, taste or smell is changed by contact with something impure, then it becomes impure, whether it is a small or great amount. But if the impurity does not change it, then it is valid as a means of purification." (Fatawa Al-Lajnah Ad-Da’imah 5/84)
Whatever the case, the dispute concerning this matter is an old one, and it is a matter that is very complex, and there is a great deal of argument about it; each party has its evidence on which it based its view.
Ibn Al-Qayyim (may Allah have mercy on him) said concerning this matter:
“On this issue there have been real battles and a great deal of argument, with many different points of view – namely the issue of water when it comes into contact with an impure substance which is absorbed completely by the water and no trace of it is left at all." (Bada’i` Al-Fawa’id,  3/257)
Ash-Shawkaani (may Allah have mercy on him) said:
“The general meaning of the Hadith “nothing makes it impure” indicates that the water does not become impure by mere contact with an impure substance, and the Hadith about the two Qullahs may imply that it does become impure by mere contact with an impure substance. So some scholars said that the Hadith of the two Qullahs may be understood as meaning that water is regarded as still being pure if there is contact with an impure substance, unless it is less than two Qullahs. Others said that even if it is two Qullahs or more, the water is deemed to become impure if there are traces of the impure substance in the water.
The former view is supported by the evidence that is quoted by those who say that a small amount of water is rendered impure as soon as contact with an impure substance occurs, even if no change occurs in the water.
This issue is very confusing and no one could reach the correct view except a few." (Nayl Al-Awtar,  1/46)
It is better for a person to err on the side of caution and keep away from small amounts of water if he knows that some impure substance has fallen into the water, even if none of its characteristics were changed as a result, so as to avoid any grey area, especially if the amount of water is very small. In that case it becomes more important to be more cautious and refrain from using it (for purification).
And Allah knows best.