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

1 Answer
Oct 2, 2016

Answer:

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?