Question

When an acid dissolves in water, what ion does water form?

Answer 1

Well, we conceive it to be the `"hydronium ion......."`

Explanation:

Water is conceived to undergo an autoprotolysis reaction as shown.....

`2H_2OrightleftharpoonsH_3O^+ + HO^-`, where ..........

`K_w=[H_3O^+][HO^-]=10^-14` under standard conditions.

When an acid, say `HX`, is dissolved in water, we conceive it to INCREASE the concentration of the characteristic cation, `H_3O^+`, and accordingly reduce concentrations of the characteristic anion, `HO^-`, so that the autoprotolysis equilibrium is maintained.

And a base likewise increases concentrations of the characteristic anion, `HO^-`.

We can quantify the acid-base equilibrium in water under standard conditions by the equation......

`2H_2OrightleftharpoonsH_3O^+ + HO^-`, where.....

`K_w=10^-14=[H_3O^+][HO^-]`

And taking `-log_10` of both sides.....

`-log_10K_w=-log_(10)10^-14=-log_10[H_3O^+]-log_10[HO^-]`

And thus `14=underbrace(-log_10[H_3O^+])_"pH"+underbrace(-log_10[HO^-])_"pOH"`

And thus our working equation.....`14=pH+pOH`.

See here for practical issues with respect to the uses of acids in water.