Proton Donor

Summary: An acid — a substance that donates H⁺ ions (protons) to another substance. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

In the Bronsted-Lowry theory of acids and bases, a proton donor is defined as a substance that donates a hydrogen ion (H⁺, which is simply a proton) to another substance, and this definition is synonymous with the term “acid” as used throughout IGCSE Chemistry. When hydrogen chloride gas dissolves in water, the HCl molecule donates a proton to a water molecule, forming the hydronium ion (H₃O⁺) and a chloride ion — in this reaction, HCl acts as the proton donor (acid) and H₂O acts as the proton acceptor (base). The Bronsted-Lowry model is more general than earlier theories because it explains acid-base behaviour even in non-aqueous solvents and accounts for substances like ammonia acting as bases despite containing no hydroxide ions. Every Bronsted-Lowry acid-base reaction involves the transfer of a proton from the acid to the base, creating a conjugate base (what remains of the acid after losing its proton) and a conjugate acid (the base after gaining a proton), which are collectively called a conjugate acid-base pair. This proton-donor definition is the most commonly examined acid definition at IGCSE level and underpins the understanding of neutralisation, pH, and salt formation across the syllabus.

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.