Why one pH unit represents a tenfold change
The pH scale is logarithmic rather than linear. In the usual dilute-solution approximation, a solution at pH 3 has ten times the hydrogen-ion concentration of one at pH 4. A two-unit difference corresponds to a hundredfold change, but pH alone does not describe every property of an acid solution.
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A scale built around powers of ten
pH expresses hydrogen-ion activity using a negative base-ten logarithm. In sufficiently dilute solutions, hydrogen-ion concentration provides a useful approximation to that activity.
The logarithmic scale compresses a wide range into manageable numbers. Moving down by one pH unit corresponds to a tenfold increase in hydrogen-ion activity.
- pH 4 to pH 3: tenfold increase.
- pH 4 to pH 2: hundredfold increase.
- Lower pH means greater hydrogen-ion activity.
pH is not a complete measure of how much acid a solution contains. Different solutions can have the same pH but respond differently when another substance is added, depending on their composition and buffering.
The useful lesson is the scale's spacing: neighbouring numbers do not represent equal additive steps in hydrogen-ion concentration.