Question

which of the following is correct regarding the ph scale?
a substance with a ph of 3 is two times more acidic than a substance with a ph of 4
a substance with a ph of 3 is 10 times more acidic than a substance with a ph of 4
a substance with a ph of 3 is two times more alkaline than a substance with a ph of 4
a substance with a ph of 3 is 10 times more alkaline than a substance with a ph of 4

Explanation:

The pH scale is a logarithmic scale. A change of 1 unit on the pH scale represents a ten - fold change in acidity or alkalinity. A lower pH value indicates a more acidic substance. So, a substance with pH = 3 is more acidic than a substance with pH = 4. Since the difference in pH values is 1 (\(4 - 3=1\)), and because of the logarithmic nature (\(pH=-\log[H^{+}]\)), if \(pH_1 = 3\) and \(pH_2 = 4\), then \([H^{+}]_1=10^{- 3}\) and \([H^{+}]_2 = 10^{-4}\). The ratio \(\frac{[H^{+}]_1}{[H^{+}]_2}=\frac{10^{-3}}{10^{-4}} = 10\).

Answer:

A substance with a pH of 3 is 10 times more acidic than a substance with a pH of 4.