THE CHAPTER OF IMPURITIES (AN-NAJAASAAT):
Najaasaat (impurities) is the plural of najaasah (impurity), and it is everything which the people of sound nature hold to be dirty, and which they guard themselves from, and which they wash their clothes from if it fall upon them, such as urine and excrement.
And the basic principle for all things is permissibility (ibaahah) and that they are pure (at-Tahaarah). So whoever claims that a particular thing is impure must bring evidence. So if he produces it then that is the case; but if he is unable to do so, or he comes with something that does not establish proof, then what is obligatory upon us is to remain upon what is necessitated by the basic principle and the basic natural state.
This is because passing a judgement that something is impure is a ruling which will bring about general duty and responsibility, so it is not permissible except after proof has been established. 
So from that which has been proven to be impure are:
1 & 2: Human urine and excrement:
As for excrement (al-ghaa.it), then because of the hadeeth of Aboo Hurayrah that Allaah’s Messenger (صلّى الله عليه وسلّم ) said:
<<If one of you steps with his shoe upon something harmful (al-Adhaa), then the earth will be purification for it.>> 
And ‘al-Adhaa’ is everything which harms/offends, from impurities, filth, rocks, thorns, and other then that. 
And what is meant by it in the hadeeth is impurity (najaasah), as is clear. As for urine, then because of the hadeeth of Anas: that a bedouin man urinated in the mosque, so some people stood up to restrain him. So Allaah’s Messenger (صلّى الله عليه وسلّم ) said: <<Leave him, and do not interrupt him.>>
He said: “When he finished he (صلّى الله عليه وسلّم ) called for a bucket of water, and poured it upon it.” 
3 & 4 Pre-seminal fluid (al-madhee) and ‘al-wadee’:
As for pre-seminal fluid, then it is clear, thin, slippery fluid which discharges when there is (sexual) desire. Its discharge does not itself give a feeling of delight, nor does it shoot out all at once, nor is it followed by slackness. The person may not notice its discharge, and this happens to men and women. 
And it is impure (najas), and therefore the Prophet (صلّى الله عليه وسلّم ) ordered that the penis be washed from it.
From `Alee who said: “ I was a man who had profuse pre-seminal discharge, and I felt shy to ask the Prophet (صلّى الله عليه وسلّم ) because of my position with respect to his daughter. So I told al-Miqdaad ibn al-Aswad, and he asked him. So he said:
<<He should wash his penis and perform wudoo..>> ” 
As for ‘al-wadee’, then it is a thick white fluid which may come out after urination,  and it is impure.
From Ibn `Abbaas who said: “Semen, and ‘al-wadee’, and pre-seminal fluid (al madhee). As for semen, then it is what necessitates a bath (ghusl); and as for ‘alwadee’and ‘al-madhee’, then he said: “Wash your penis, or: the parts of your penis, and perform wudoo. in the manner in which you perform wudoo· for the Prayer.” 
5. The dung of animals whose meat may not be eaten:
From `Abdullaah who said: the Prophet (صلّى الله عليه وسلّم ) wanted to go out to the toilet, so he said: <<Bring me three stones.>> So I found two stones for him and a piece of dung from a donkey. So he took hold of the two stones, and he threw the piece of dung away, and he said: <<It is something impure.>> 
6. Menstrual blood:
From Asmaa· bint Abee Bakr who said: “A woman came to the Prophet (صلّى الله عليه وسلّم ) and said: ‘If menstrual blood falls on the garment of one of us, what should she do?’ So he said: <<She should scrape it off, then she should rub it with water, then she should wash it (all). Then she can pray whilst wearing it.>> 
7. The saliva of a dog:
From Aboo Hurayrah who said, Allaah’s Messenger (صلّى الله عليه وسلّم ) said:
<<The purification for the vessel of one of you if a dog licks from it is that he washes it seven times, the first of them being with earth.>> 
8. Animal carcasses:
Referring to animals which die without having been slaughtered in the legislated manner, because of his (صلّى الله عليه وسلّم ) saying:
<<When an animal skin is tanned then it has become pure.>> 
And an ‘animal skin’ (ihaab) is the hide of an animal which has died.
And exceptions to that  are:
(i) Dead fish and locusts, because of the hadeeth of Ibn `Umar- radiyallaahu `anhu)maa- who said: Allaah’s Messenger (صلّى الله عليه وسلّم ) said:
<<Two dead animals and forms of blood have been made lawful for us. As for the two dead animals, then they are fish and locusts; and as for the two forms of blood it is the liver and the spleen.>> 
(ii) Dead animals which do not have blood which flows, such as flies, ants, bees, and their like.
From Aboo Hurayrah -radiyallaahu `anhu- that Allaah’s Messenger (صلّى الله عليه وسلّم ) said:
<<If a fly falls into the vessel of one of you then let him immerse it in it, totally. Then let him throw it out, for indeed in one of its wings there is a poison and in the other there is a remedy.>> 
(iii) The bones of a dead animal, and its horn, claws, hair, and feathers.
All of that is pure, in keeping with the basic principle, which is that things are pure; and because of what al-Bukhaaree reported in disconnected form, saying, ‘az-Zuhree said about the bones of a dead animal, such as the elephant and its like, “I met people from the Salaf of the scholars using it as combs, and as pots for oil. They did not see any harm in that.” And Hammaad said: “There is no harm in (using) the feathers of dead birds.”
HOW IMPURITY IS TO BE PURIFIED.
You should know that the bringer of the legislation, who made us aware that a thing is impure, or causes other things to become impure, has also made us aware of how we are able to purify it. So what is obligatory upon us is to follow his saying and to comply with his command. So whatever it is reported about that it must be washed until no colour, smell, or taste remains from it, then that is its purification; and whatever it is reported about that water must be poured upon it, or sprinkled upon it, or that it should be scraped, or wiped upon the earth, or merely walking upon pure earth, then that is its purification. Furthermore you should know that water is the basic substance for purifying impurities, since the bringer of the Legislation described it with his saying: <<Allaah created water as being pure and purifying>> , so nothing else besides it is turned to, unless that is established from the bringer of the legislation; otherwise not, since it would be a case of turning aside from something known to be a purifier towards something which is not known to be a purifier, and this would be a departure from what the legislated manner of proceeding demands.
So when you know this, then here is what occurs in the legislation regarding the manner of purifying substances which are impure or which cause impurity:
1. Purifying the animal through tanning:
From Ibn `Abbaas -radiyallaahu `anhumaa- who said: I heard Allaah’s Messenger (صلّى الله عليه وسلّم ) say:
<<Whichever animal skin is tanned, then it becomes pure.>> 
2. Purifying the vessel which a dog has licked:
From Aboo Hurayrah- radiyallaahu `anhu- that Allaah’s Messenger (صلّى الله عليه وسلّم ) said: <<The purification of the vessel of one of you when a dog has licked it is that he washes it seven times, the first of them being with the earth.>> 
3. Purifying a garment which menstrual blood has fallen onto:
From Asmaa· bint Abee Bakr -radiyallaahu `anhaa- who said: A woman came to the Prophet (صلّى الله عليه وسلّم ) and said: “If menstrual blood falls onto the garment of one of us, what should she do?” So he said:
<<She should scrape it off, then she should rub it with water, then she should wash it (all). Then she can pray whilst wearing it.>> 
If a trace still remains after that, then there is no harm: From Aboo Hurayrah -radiyallaahu `anhu- that Khawlah bint Yasaar said, “O Messenger of Allaah! I have only a single garment, and I menstruate whilst wearing it.” He said:
<<When you become clean wash the place of the blood, then pray whilst wearing it.>>
She said: “O Messenger of Allaah! If its trace is not removed?” He said:
<<The water will suffice you, and its trace will not harm you.>> 
4. Purifying the trailing hem of the woman’s garment:
From a slave-girl who gave birth to a child of Ibraaheem ibn `Abdir-Rahmaan ibn `Awf, that she asked Umm Salamah -the wife of the Prophet (صلّى الله عليه وسلّم ) I am a woman who makes the hem of my garment long, then what if I walk upon a filthy place?” So Umm Salamah said: “The Prophet (صلّى الله عليه وسلّم ) said: <<What comes after it will purify it.>> 
5. Purifying the garment from the urine of a suckling baby: 
From Abus-Samh, the servant of the Prophet (صلّى الله عليه وسلّم ) who said: “The Prophet (صلّى الله عليه وسلّم ) said:
<<It should be washed from the urine of a girl, and sprinkled with water from the urine of a boy.>>
6. Purifying the garment from pre-seminal fluid (madhee):
From Sahl ibn Hunayf who said: I used to experience difficulty and trouble on account of pre-seminal fluid, and I used to frequently take a bath from it. So I mentioned that to Allaah’s Messenger (صلّى الله عليه وسلّم ) and he said:
<<The wudoo· would suffice you for that.”>>
I said: “What about that which fall upon my garment from it?” He said:
<<It will suffice you to take a handful of water, and to rub your garment with it wherever you see it has fallen upon it.>> 
7. Purifying the underneath of the shoe:
From Aboo Sa`eed -radiyallaahu `anhu- that the Prophet (صلّى الله عليه وسلّم ) said: <<When one of you comes to the mosque, then let him turn his shoes over and examine them. So if he sees some filth, then let him wipe it upon the earth, then let him pray whilst wearing them.>> 
8. Purifying the earth:
From Aboo Hurayrah -radiyallaahu `anhu- who said: “A Bedouin stood and urinated in the mosque. So the people began raising their voices against him. So the Prophet (صلّى الله عليه وسلّم ) said to them:
<<Leave him, and pour a bucket of water upon his urine; for you were sent as people who make things easy, and you were not sent as people who make things difficult.>> 
And the Prophet (صلّى الله عليه وسلّم ) commanded that in order to quickly purify the earth. So if it had been left until it became dry, and then the trace of the impurity departed, then it would have become pure, because of the hadeeth of Ibn `Umar -radiyallaahu `anhumaa- who said:
“Dogs used to urinate in the mosque, and come and go, in the time of Allaah’s Messenger (صلّى الله عليه وسلّم ) and they had not used to sprinkle anything upon it.” 
 ar-Rawdatun-Nadiyyah (1/12) .
 as-Saylul-Jarraar (1/31) of ash-Shawkaanee.
 ar-Rawdatun-Nadiyyah (1/15) of Siddeeq Hasan Khaan.
 Saheeh: Saheeh Abee Daawood (no. 834); Aboo Daawood (2/47/381).
 `Awnul-Ma`bood (Sharh Abee Daawood) (2/44).
 Agreed upon. Muslim (1/236/284), and the wording is his; and al-Bukhaaree (10/449/6025).
 Sharh Saheeh Muslim: (3/213).
Agreed upon: Muslim (1/247/303), and the wording is his; al-Bukhaaree (1/230/132) in abridged form.
 Saheeh…al-Bayhaqee (1/115).
 Saheeh: Saheehul-Jaami`: (253); Ibn Khuzaymah (1/39/70), and it is reported by others without the words ‘from a donkey’. It was reported by al-Bukhaaree (1/256/156), an-Nasaa·ee (1/39), at-Tirmidhee (1/13/17), and Ibn Maajah (1/114/314).
 Agreed upon: Muslim (1/240/291), and the wording is his; al-Bukhaaree (1/410/307).
 Saheeh: Saheehul-Jaami` (3933); Muslim (1/234/91/279).
 Saheeh: Saheehul-Jaami` (511); Muslim (1/277/366), Aboo Daawood (11/181/4105).
 i.e. to the condition of being impure (transl.)
 Saheeh: Saheehul-Jaami` (210); Ahmad (1/255/96), al-Bayhaqee (1/254)
 Saheeh: Saheehul-Jaami` (837); al-Bukhaaree (10/250/5782), Ibn Maajah (2/1159/3505)
 as-Saylul-Jarraar (1/42, 48) -with some paraphrasing. And you should know that regarding his saying “Allaah created water as being pure and purifying” al-Haafiz (Ibn Hajr) said in ‘at-Talkhees’ (1/14): “I have not found it in this form, and it has preceded as a hadeeth of Aboo Sa`eed with the wording: “Water is pure and purifying, nothing causes it to become impure.””
 Saheeh: Saheehul-Jaami` (2907); Ahmad (1/230/49), at-Tirmidhee (3/135/1728), Ibn Maajah (2/1193/3609), an-Nasaa·ee (7/173).
 Saheeh: Saheehul-Jaami` (3933), Muslim (1/234/91/279)
 Agreed upon: Muslim (1/240/291), and the wording is his; al-Bukhaaree (1/410/307).
 Saheeh: Saheeh Abee Daawood (351); Aboo Daawood (2/26/361), al-Bayhaqee (2/408)
 Saheeh:Saheeh Ibn Maajah (430), Muwatta· Imaam Maalik (27/44), Aboo Daawood (2/44/379), atTirmidhee (1/95/143), Ibn Maajah (1/177/531).
 i.e. the baby who is still breast-fed and has not been weaned onto solid food (transl.).
 Saheeh: Saheeh an-Nasaa·ee (293), Aboo Daawood (2/36/372), an Nasaa·ee (1/158).
 Hasan: Saheeh Ibn Maajah (409), Aboo Daawood (1/357/207), at-Tirmidhee (1/76/115), Ibn Maajah (1/169/506).
 Saheeh: Saheeh Abee Daawood (605); Aboo Daawood (2/353/636).
 Agreed upon: al-Irwaa. (171); al-Bukhaaree (1/323/220), an-Nasaa·ee (1/48 & 49); and it is reported in longer form by Aboo Daawood (2/39/376) & at-Tirmidhee (1/99/147).
 Saheeh: Saheeh Abee Daawood (368); al-Bukhaaree in disconnected form (1/278/174), Aboo Daawood (2/42/378).
Source : Al-Wajeez, The Book Of Purification – Abdul-`Azeem ibn Badawee al-Khalafee, Translated by Abu Talhah Dawood Burbank . An excellent book on the rules and regulations of various acts of worship, with evidences from the Book and Sunnah. This chapter deals with water and aspects of purification, wudhoo, ghusl, the toilet etc
Related Link: https://abdurrahman.org/tahara-purification/