Chemistry: Why pH Alone Can't Measure Acid Strength

The pH scale measures the hydrogen ion concentration of a solution, but it does not, on its own, tell you how strong an acid or base really is. This distinction between acid strength — the degree to which a substance dissociates — and pH value is central to chemistry, because two solutions can share an indicator colour while differing enormously in the concentration of H+ ions they contain. The scale runs from 0 to 14, with values below 7 indicating acids, values above 7 indicating bases, and each whole unit representing a tenfold change in hydrogen ion concentration. Universal indicator translates these values into colour, turning red in strongly acidic solutions and blue in weakly basic ones. Because the scale is logarithmic, a small numerical difference in pH corresponds to a large difference in [H+], so substances that look identical to an indicator may behave quite differently in a reaction.