Learning focus
Develop precise chemical language, connect equations and practical observations, interpret periodic trends and apply salt-preparation rules to unfamiliar examples.
Proton definition
An acid is a proton donor and a base is a proton acceptor. A proton in this context is H+. The definition focuses on transfer during a reaction. Hydrochloric acid donates H+ to water or to a base, while OH- accepts H+ to make H2O.
Identifying the acid and base
Follow the proton. The species that loses H+ is the acid; the species that gains H+ is the base. In HCl + H2O -> H3O+ + Cl-, HCl donates H+ and is the acid, while water accepts H+ and acts as the base. The hydronium ion notation may appear in wider chemistry, although the syllabus commonly writes H+(aq).
Relationship to neutralisation
Many neutralisation reactions can be understood as proton transfer. A metal oxide contains oxide ions that accept protons, and a metal hydroxide contains hydroxide ions that accept protons. The resulting water is formed when the acidic proton becomes bonded to oxygen-containing species.
Limits of simple labels
A substance can act as an acid in one reaction and as a base in another if the reaction partners differ, although most syllabus examples have clear roles. The useful method is to analyse actual proton transfer rather than memorise a permanent label without context.
Practical or data skill
Use molecular cards to show proton transfer between HCl and OH-. Mark the proton before and after the reaction and identify donor and acceptor.
Examination tip
State both parts of the definition: acid donates H+; base accepts H+.
Review questions and suggested answers
Question 1
What is a proton donor?
Suggested answer
An acid.
Question 2
What does a base accept?
Suggested answer
A proton, H+.
Question 3
In H+ + OH- -> H2O, which species acts as the base?
Suggested answer
OH-, because it accepts H+.
