Question

On the pH scale, which numbers indicate a basic solution? An acidic solution? Why?

Answer 1

On the `pH` scale in aqueous solution, values of `pH>7` indicate a basic solution.........

Explanation:

And of course values of `pH<7` indicate an acidic solution. `pH`, literally `"pouvoir hydrogen"`, measures the concentration of `H_3O^+`, conceived to be the acidic entity in aqueous solution. Extensive measurement has confirmed the following equilibrium in water under standard conditions:

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

And this reaction can be quantified by taking the ion product:

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

If we take `-log_10` of both sides we get,

`-log_10[H_3O^+]-log_10[HO^-]=14`

But under our standard defintion of `pH` and `pOH`,

`pH+pOH=14`

And thus values of `pH>7` indicate a preponderance of hydroxide ions, and `pH<7` indicates a preponderance of hydronium ions.

What is `pH` at neutrality? Why?