Acids are redefined three times over

An acid is a substance that produces H⁺ ions in solution / turns litmus red / has pH below 7. Bases neutralise acids. Reactions: acid + metal, acid + base, acid + carbonate. Strong vs weak may be mentioned briefly.

Three nested definitions, each broader than the last.

1. Arrhenius (background): produces H⁺ / OH⁻ in water.
2. Brønsted–Lowry (Reactivity 3.1.1–3.1.3): an acid is a proton donor, a base a proton acceptor. Acid–base behaviour becomes relational — a species is an acid only with respect to a partner. Every proton transfer creates a conjugate acid–base pair (3.1.2), and amphiprotic species (H₂O, HCO₃⁻, HSO₄⁻, amino acids) can act as either (3.1.3).
3. Lewis (Reactivity 3.4.6–3.4.7, HL): an acid is an electron-pair acceptor, a base an electron-pair donor. This subsumes Brønsted–Lowry (H⁺ is an electron-pair acceptor) and unifies acid–base chemistry with organic mechanisms and transition-metal complexes: nucleophiles are Lewis bases, electrophiles are Lewis acids (3.4.7), and ligands binding to metal ions are Lewis-base/Lewis-acid interactions (3.4.8).

"Acid" changes from a property of a bottle to a role in a reaction. And the Lewis extension means that Reactivity 3.1 (acid–base) and Reactivity 3.4 (organic mechanisms, complex ions) are the same chapter of chemistry — which is exactly the conceptual unification the 2023 syllabus was restructured to make visible.

Arrhenius is sufficient for aqueous reactions in a school lab and requires no equilibrium.

Cannot identify conjugate pairs (a near-guaranteed Paper 1 question); cannot explain why NH₃ is a base despite containing no OH⁻; cannot explain why an ammonium salt solution is acidic (Reactivity 3.1.12); cannot see that "nucleophile" and "base" describe the same electron-pair donation and so learns organic mechanisms as unconnected rote sequences.