Self-ionization of water

self-ionizationionic productself-ionization constantauto-dissociateautoionizationautoionization constant for waterautoionization of waterautoprotolysisequilibrium constant for waterionised state
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.wikipedia
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Properties of water

waterH 2 Owater molecule
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.
Related to its amphoteric character, it undergoes self-ionization.

Hydroxide

OHhydroxide ionOH −
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −. :where [H 3 O + ] is the molarity (≈molar concentration) of hydrogen or hydronium ion, and [OH − ] is the concentration of hydroxide ion.
It is an important but usually minor constituent of water.

Hydronium

hydronium ionH 3 O + hydrogen ions
The hydrogen nucleus, H +, immediately protonates another water molecule to form hydronium, H 3 O +. :where [H 3 O + ] is the molarity (≈molar concentration) of hydrogen or hydronium ion, and [OH − ] is the concentration of hydroxide ion.
The molecules in pure water auto-dissociate (i.e.: react with each other) into hydronium and hydroxide ions in the following equilibrium:

Autoprotolysis

autoproteolysis
It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.
For example, water undergoes autoprotolysis in the self-ionization of water reaction.

Acid dissociation constant

p''K'' a pKapK a
This is analogous to the notations pH and pK a for an acid dissociation constant, where the symbol p denotes a cologarithm.
The self-ionization constant of water, K w, is thus just a special case of an acid dissociation constant.

Heavy water

deuterium oxideheavy-waterD 2 O
Heavy water, D 2 O, self-ionizes less than normal water, H 2 O;
In the liquid state, a few water molecules are always in an ionised state, which means the hydrogen atoms can exchange among different oxygen atoms.

Base (chemistry)

basebasicbases
If an impurity is an acid or base, this will affect the concentrations of hydronium ion and hydroxide ion.
In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 at standard conditions.

Chemical equilibrium

equilibriumchemical equilibriaequilibria
A particular case is the self-ionization of water itself

Buffer solution

bufferbufferedbuffers
If a pH of exactly 7.0 is required, it must be maintained with an appropriate buffer solution.
K w is the constant for self-ionization of water.

Ionization

ionizedionizeionisation
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.

Aqueous solution

aqueouswater solubilityaqueous solutions
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.

Deprotonation

deprotonateddeprotonatedeprotonates
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.

Hydron (chemistry)

H + hydronproton
The hydrogen nucleus, H +, immediately protonates another water molecule to form hydronium, H 3 O +.

Protonation

protonatedprotonateprotonating
The hydrogen nucleus, H +, immediately protonates another water molecule to form hydronium, H 3 O +.

Amphoterism

amphotericamphiproticampholyte
It is an example of autoprotolysis, and exemplifies the amphoteric nature of water.

Ultrapure water

Milli-Qultra-pure waterChemically pure water
Chemically pure water has an electrical conductivity of 0.055 µS/cm.

Electricity

electricalelectricelectrically
Chemically pure water has an electrical conductivity of 0.055 µS/cm.

Conductivity (electrolytic)

conductivityelectrical conductivityelectrolytic conductivity
Chemically pure water has an electrical conductivity of 0.055 µS/cm.

Siemens (unit)

siemensmicro-siemensmho
Chemically pure water has an electrical conductivity of 0.055 µS/cm.

Svante Arrhenius

Svante August ArrheniusArrheniusArrhenius, Svante August
According to the theories of Svante Arrhenius, this must be due to the presence of ions.

Fast ion conductor

solid electrolyteionic conductorionic conductors
According to the theories of Svante Arrhenius, this must be due to the presence of ions.

Thermodynamic activity

activityactivitieschemical activity
Expressed with chemical activities a, instead of concentrations, the thermodynamic equilibrium constant for the water ionization reaction is:

Equilibrium constant

equilibrium constantsequilibriumdissociation constant
Expressed with chemical activities a, instead of concentrations, the thermodynamic equilibrium constant for the water ionization reaction is:

Molar concentration

Mnanomolarmicromolar
:where [H 3 O + ] is the molarity (≈molar concentration) of hydrogen or hydronium ion, and [OH − ] is the concentration of hydroxide ion.

Ionic strength

ionic balanceionic concentrations
When the equilibrium constant is written as a product of concentrations (as opposed to activities) it is necessary to make corrections to the value of K_{\rm w} depending on ionic strength and other factors (see below).