Chemistry

Treating pH as Linear Instead of Logarithmic

Students treat a change from pH 3 to pH 6 like a simple doubling instead of a 1000-fold change in hydronium concentration.

Quick check

Why students make it

The pH scale looks like an ordinary number line, so it feels natural to compare pH values by adding, subtracting, or doubling. That intuition misses that pH is based on a base-10 logarithm.

The rule done right

pH is defined by pH=−log⁡10[H3O+]\mathrm{pH} = -\log_{10}[\mathrm{H_3O^+}]. A change of 1 pH unit means a 10×10\times change in [H3O+][\mathrm{H_3O^+}], and a change of nn pH units means a factor of 10n10^n.

Related resources

phlogarithmsacid-base