Can pH be less than zero?
a Yes
b No
The correct answer and explanation is:
The correct answer is: a. Yes.
pH can be less than zero, although it is rare. The pH scale typically ranges from 0 to 14, where 7 is neutral, values less than 7 are acidic, and values greater than 7 are basic. However, the pH scale is not strictly confined to these limits.
pH is a measure of the concentration of hydrogen ions (H⁺) in a solution. It is defined as the negative logarithm (base 10) of the molar concentration of hydrogen ions: pH=−log[H+]\text{pH} = -\log[\text{H}^+]
For typical substances, the hydrogen ion concentration in a solution is in the range of 10−710^{-7} to 10−110^{-1} mol/L, resulting in pH values from 0 to 14. However, pH can theoretically be less than 0 if the concentration of hydrogen ions exceeds 1 mol/L. For example, a very concentrated acid, such as a strong acid like hydrochloric acid (HCl) in extremely high concentrations, can result in a pH lower than 0.
When the hydrogen ion concentration becomes extremely high, such as 10 mol/L (which is beyond the usual concentration of common acids), the pH would be negative. In practical terms, this could occur in laboratory conditions with concentrated acid solutions, though it is not typical for everyday reactions or substances.
Thus, while pH values less than 0 are not commonly encountered, they are possible when dealing with highly concentrated acidic solutions. This phenomenon is more about the limits of the logarithmic scale used to define pH rather than a real-world limitation.