Acids, Bases and the pH Scale
From lemon to soap: the chemistry of every liquid in 14 units
Summary
The pH scale expresses how acidic or basic a liquid is with a number from 0 to 14. 7 is neutral, below 7 is acidic and above 7 is basic. The scale is logarithmic: a difference of 1 unit is a 10-fold difference in acidity.
In depth
Acidity is a measure of the concentration of hydrogen ions (H⁺) in a liquid. The pH value is its negative logarithm: pH = −log₁₀[H⁺]. That is why the difference between pH 3 (lemon) and pH 7 (pure water) is 10⁴, ten thousand times. Acids release H⁺ ions when they dissolve in water (HCl → H⁺ + Cl⁻); bases release OH⁻ ions (NaOH → Na⁺ + OH⁻). When H⁺ and OH⁻ combine they form water (H₂O), so mixing an acid with a base produces a neutralisation reaction. Universal indicator is a mixture of several organic dyes; it changes colour with pH and makes the reading visual.
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