Question

What is an acid?

Answer 1

What is an acid?

Explanation:

Well, it is a species that increases the concentration of the characteristic cation of the solvent. And since the normal solvent is water, we would say that an acid is a species that increases concentration of the hydronium ion, `H_3O^+`, the `"acidium species...."`

In water we know that `"autoprotolysis"` occurs.....

`2H_2O(l) rightleftharpoons H_3O^+ + HO^-`

And under standard conditions we can quantify this equilibrium, i.e.

`K_w=[H_3O^+][HO^-]=10^(-14)`; and we can take `log_10` of both sides to give......

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

And on rearrangement......

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

And so (finally!), our working relationship.....

`14=pH+pOH`

And so at high acidity `pH` is low or negative; at neutrality `pH=pOH=7`, and under alkaline conditions, `pOH` is low or negative.

Anyway a lot to digest here....I would expect a 1st or 2nd year undergrad to be able to reproduce this......A level students should know that `14=pH+pOH`.