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The self-ionization of water (also autoionization of water, autoprotolysis of water, autodissociation of water, or simply dissociation 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 −.
In chemistry, molecular autoionization (or self-ionization) is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. [1]: 163 In most cases the oxidation number on all atoms in such a reaction remains unchanged. Such autoionization can be ...
For example, water undergoes autoprotolysis in the self-ionization of water reaction. 2 H 2 O ⇌ OH − + H 3 O + For example, ammonia in its purest form may undergo autoprotolysis: 2 NH 3 ⇌ NH − 2 + NH + 4. Another example is acetic acid: 2 CH 3 COOH ⇌ CH 3 COO − + CH 3 COOH + 2
K w is the constant for self-ionization of water. There are two non-linear simultaneous equations in two unknown quantities [A 3−] and [H +]. Many computer programs are available to do this calculation. The speciation diagram for citric acid was produced with the program HySS. [11] N.B.
Water is amphoteric, meaning that it can exhibit properties of an acid or a base, depending on the pH of the solution that it is in; it readily produces both H + and OH − ions. [c] Related to its amphoteric character, it undergoes self-ionization. The product of the activities, or approximately, the concentrations of H + and OH −
Self-ionization of water – Autoprotolysis or exchange of a proton between two water molecules; Water-in-water emulsion; Water purification – Process of removing impurities from water; Water treatment – Process that improves the quality of water; Water (data page) – Chemical data page for water; Hard water – Water that has a high ...
In pure water, there is an equal number of hydroxide and H + ions, so it is a neutral solution. At 25 °C (77 °F), pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see self-ionization of water). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. [7]
Because water is the solvent, and has an activity of one, the self-ionization constant of water is defined as K w = [ H + ] [ O H − ] {\displaystyle K_{\mathrm {w} }=\mathrm {[H^{+}][OH^{-}]} } It is perfectly legitimate to write [H + ] for the hydronium ion concentration, since the state of solvation of the proton is constant (in dilute ...