Question

Which is not true regarding the pH scale?

Answer 1

Answer: When the concentration of hydroxide ions increases, the pH decreases.

Explanation: pH or pOH is the measure of acidity or alkalinity.  

pH is defined as negative logarithm of hydrogen ion concentration. pOH is defined as the negative logarithm of hydroxide ion concentration.

Acids have pH ranging from 1 to 6.9, neutral solutions have pH of 7 and bases have pH ranging from 7.1 to 14.

$pH=-\log [H^+]$

$pOH=-log [OH^-]$

$pH+pOH=14$

Thus as pOH and $OH^-$ are inversely related, a solution having high concentration of $OH^-$ will have low pOH and thus high pH.

Answer 2

The statement that is not true regarding the pH scale would be the final one. As the concentration of hydroxide ions in a corresponding solution increases, the pH would increase, as the solution becomes more basic and not as acidic. The pH values from 0-6 account or characterize a solution to be acidic, while from 8-14 would be basic. Increase in hydroxide will increase the pH from original value.