pH becomes logarithmic, and strong/weak decouples from concentrated/dilute

pH is a number 0–14 read off universal indicator, roughly "how acidic". Strong acids have low pH. The distinction between "strong" and "concentrated" is often not drawn at all.

pH = −log₁₀[H⁺] and [H⁺] = 10⁻ᵖᴴ (3.1.4). pOH = −log₁₀[OH⁻], and Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 298 K so pH + pOH = 14 (3.1.5, 3.1.9). Kw is temperature-dependent, so the neutral pH is only 7 at 298 K. Strong/weak refers to the extent of ionisation (3.1.6), quantified by Ka, Kb, pKa, pKb (3.1.10), with Ka × Kb = Kw for a conjugate pair (3.1.11). Concentrated/dilute refers to moles per dm³ and is an entirely independent axis. pH can be negative or greater than 14.

pH becomes a calculation, not a reading; it becomes possible to have a concentrated weak acid with a higher pH than a dilute strong acid; and pKa becomes the working currency of acid strength (lower pKa = stronger acid, an inversion students repeatedly get backwards).

Logs are outside GCSE maths for many students, and universal indicator gives an integer.

RSC lists "understanding the difference between strong, weak, concentrated, dilute" as a top post-16 difficulty. Also: the belief that pH must lie in 0–14; the belief that all salt solutions are pH 7 (RSC explicitly flags this — CuSO₄(aq) is ~pH 4.6, sodium ethanoate ~pH 9); getting the pKa direction backwards; and simple calculator failure — not knowing which button gives 10ˣ.