How do the numbers on the pH scale compare? Example - is a pH of 4 twice as strong as a pH of 2? Hint - the pH scale is not linear!

The numbers on pH scale can not be compared as the pH 4 is two times stronger than the pH 2. From the pH value, concentration of H_{3}O^{+} ions can be calculated, which is them compared the acidic strength of the solution. pH is negative log of hydronium ion.

pH=-log[H_{3}O^{+}]

Thus, from each pH value, the concentration of H_{3}O^{+} ions can be calculated and compared.