Question

Where do `"OH"^(-)` and `"H"^(+)` ions come from?

Answer 1

From the dissociation or autoprotolysis of water.

Explanation:

Water is conceived to undergo an acid-base dissociation:

`H_2O rightleftharpoons H^+ + OH^-`

Alternatively,

`2H_2O rightleftharpoons H_3O^+ + OH^-`

Both reactions are somewhat conceptual, in that there is no species that corresponds to `H^+` or even `H_3O^+` (aka, the hydronium ion). As far as anyone knows, this acidium ion is 4-5 water molecules with an extra `H^+`, i.e. `H_9O_4^+`; a cluster of water molecules with an extra `H^+`. Whatever it is, we conceive that the species in solution is `H_3O^+`, and we write our reactions and model our equilibria accordingly.

We can certainly write the equilibrium reaction for the above equation, because it has been extensively measured:

`K_w = [OH^-][H_3O^+]` `=` `10^(-14)`, at `298K`. Would you expect `K_w` to increase or decrease at temperatures `>` `298K`?