7.2.6—The differences in behaviour between strong

A strong acid or base ionises essentially completely in water; a weak one establishes an equilibrium with a significant fraction un-ionised.

To compare strength, compare the ionisation equilibrium, not just the measured pH. A concentrated weak acid can have a lower pH than a very dilute strong acid while still being weaker.

0.001 mol dm⁻³ HCl is strong and nearly fully ionised, whereas 0.10 mol dm⁻³ ethanoic acid is weak and only partly ionised. Their pH values cannot alone define strength.

Strong does not mean concentrated, and weak does not mean harmless. Keep degree of ionisation and amount per volume separate.