What distinguishes an acid from a base in an aqueous model?

Support map

Model diagram

Use paper, a calculator and supplied data only. Do not taste, smell, mix or heat chemicals; do not open household products or unknown containers. Neutral pH does not establish safety. Do not apply teaching neutralisation to skin, eyes, spills or unknown mixtures.

Situation and question

The notebook’s salt has dissolved, but the solution’s composition remains uncertain. Can clarity reveal whether it is acidic or basic? Lessons 50–58 distinguished molecules from ions. Now add acid–base interaction, using formulae and supplied information only, without handling samples.

New terms, fully explained

A Brønsted–Lowry acid donates a proton H⁺ and a base accepts one. In water, do not draw H⁺ as an isolated bare proton: use hydronium, H₃O⁺. The shorthand H⁺(aq) represents a hydrated proton. Acidity depends on the relative amounts per volume of H₃O⁺ and OH⁻, not colour or taste.

Predict first

Consider HCl + H₂O → H₃O⁺ + Cl⁻. Predict the proton donor and acceptor. Then change the example to NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. Can water play a different role? Before reading on, mark the change in the number of H atoms in each species.

Investigate or analyse given data

Copy both equations. Count H, O, Cl or N on each side, then add the charges. In the first equation, neutral reactants give +1 and −1 products. In the second, an equilibrium arrow allows reverse transfer; it does not mean equal amounts of every species.

Worked example

With HCl, water accepts H⁺ and acts as a base, leaving Cl⁻. With NH₃, water donates H⁺ and acts as an acid, producing NH₄⁺ and OH⁻. A conjugate pair differs by one proton: HCl/Cl⁻, H₃O⁺/H₂O or NH₄⁺/NH₃. The transfer does not destroy atoms.

A common mistake

Mistake: requiring every base formula to contain OH. NH₃ has no such group but accepts a proton. An alkali is a water-soluble base; the school aqueous description and the proton-transfer model provide different explanatory levels. A neutral individual species does not establish that the whole solution is neutral.

Transfer to a new case

Transfer case: HNO₃ + H₂O → H₃O⁺ + NO₃⁻. Identify the donor, acceptor and two conjugate pairs. Does the oxygen count change between H₂O and H₃O⁺? Explain why total product charge is zero although the solution contains charged species.

Answer and recall

Check: HNO₃ donates and H₂O accepts; the pairs are HNO₃/NO₃⁻ and H₃O⁺/H₂O. Water and hydronium each contain one O; one proton is added. The +1 and −1 charges balance. Reconstruct both original equations and explain water’s different roles.

Exercise

In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, who donates H⁺? Name NH₃’s conjugate pair.

Answers

When to revisit

Close the page and, after 10 minutes, reconstruct the map, define the terms in your own words and solve the new case. Repeat without hints after a day and a week. If an error persists, return to the worked example and write down the missing step.

Sources and limits

• OpenStax: acid–base models

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