Ionization

Japanese: 電離 - でんり（英語表記）ionization

(1) This refers to the dissociation of an electrolyte into ions in a solvent (usually water). It is the same as ionization. For example, when sodium chloride is dissolved in water, almost all of it becomes separate hydrated sodium ions (with water molecules) and hydrated chloride ions. When acetic acid is diluted with water, some of the acetic acid molecules dissociate to produce hydrated hydrogen ions and hydrated acetate ions. The sodium chloride and acetic acid in this state are called electrolytes, and those with a high degree of ionization, such as sodium chloride, are called strong electrolytes, while those with a low degree of ionization (such as acetic acid) are called weak electrolytes.

This ionization is a type of chemical reaction, so the law of mass action can be applied. For example, for the acid HA,
HA＋H ₂ O―→H ₃ O ⁺ ＋A ^-
K = [H ₃ O ⁺ ][A ^- ]/[HA][H ₂ O]
The equilibrium constant K in this case is called the ionization constant.

(2) In the case of many gases, this refers to the case where atoms or molecules are collided with high-energy electrons or are exposed to light or radiation, losing electrons and becoming positive ions or negative ions.

(3) When a neutral molecule or atom acquires a positive or negative charge, it becomes an ion. For example, liquids such as water and ammonia are
_H2OH ⁺ + ^OH-
_2NH3NH4 ⁺ ⁺ _NH2-
and the resulting ions are in equilibrium with the original neutral molecule.

[Toda Genjiro and Nakahara Katsunori]

[Reference items] | Electrolyte | Ionization constant

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

(1)電解質が溶媒（多くは水）の中でイオンに解離することをいう。イオン化と同じ。たとえば、塩化ナトリウムを水に溶かすとほとんど全部が、ばらばらの水和した（水分子を伴った）ナトリウムイオンと水和塩化物イオンになる。酢酸を水で薄めると、酢酸分子が一部解離して水和した水素イオンと、水和した酢酸イオンを生ずる。このときの塩化ナトリウムや酢酸を電解質といい、そのなかで塩化ナトリウムなど電離の度合いの高いものを強電解質、低いもの（酢酸など）を弱電解質という。

　この電離も化学反応の一種であるから質量作用の法則が適用できる。たとえばHAという酸については、
　　HA＋H₂O―→H₃O⁺＋A^-
　　K＝[H₃O⁺][A^-]/[HA][H₂O]
このときの平衡定数Kを電離定数という。