Forget litmus paper and sour tastes. At this level an acid is defined by one thing only: what it does with a proton. Get that single idea straight and the entire topic unlocks.
📚 What you need to know
A Brønsted–Lowry acid is a proton donor.
A Brønsted–Lowry base is a proton acceptor, and it needs a lone pair to do it.
A proton is simply H+ — a hydrogen atom that has lost its only electron.
Acid and base always work as a pair. Nothing can donate a proton unless something else accepts it.
In water, the proton attaches to a water molecule to give the hydronium ion, H3O+.
The theory is not limited to aqueous solutions — proton transfer happens in the gas phase too.
The formation of ions in this way is called dissociation or ionisation.
What a proton actually is
A hydrogen atom is one proton and one electron, with no neutrons in its commonest isotope. Take the electron away and what is left is a bare nucleus — a single proton. That is why H+ and “a proton” mean exactly the same thing in this topic.
This matters more than it sounds. When an acid donates H+, the hydrogen leaves without the pair of electrons that used to bond it in place. Those electrons stay behind, which is precisely why the acid ends up negatively charged.
Follow the electrons and the charges explain themselves. A loses a proton but keeps two electrons, so it becomes negative; B gains a proton but shares its own pair, so it becomes positive.
This is why the definition of a base is worth more than “something that accepts a proton”. A proton has no electrons of its own, so it cannot form a bond by itself. The base must supply both electrons for the new bond — which is a coordinate bond, exactly the kind you met in bonding.
Acids and bases in water
Dissolve hydrogen chloride gas in water and the water does the accepting.
Hydrogen chloride in water
HCl(g) + H2O(l) → H3O+(aq) + Cl–(aq)
HCl gives the proton away, so it is the acid. Water accepts it using a lone pair on the oxygen, so here water is the base. The product H3O+ is called the hydronium (or oxonium) ion.
A lone proton in water would be a naked nucleus with an enormous charge density. It never exists on its own; it is always attached to something with a lone pair.
You will see acid dissociation written both ways: HCl → H+ + Cl– and HCl + H2O → H3O+ + Cl–. Both are accepted. The only rule is that if you use H3O+, water must appear on the left so the equation balances.
It does not need water at all
The older definition of an acid tied everything to aqueous solution. Brønsted–Lowry does not, and that is its real advantage. Hold an open bottle of concentrated hydrochloric acid next to one of concentrated ammonia and a white smoke of ammonium chloride forms in the air between them.
Proton transfer in the gas phase
HCl(g) + NH3(g) → NH4Cl(s)
No solvent, no solution, no beaker — but a proton still moves from HCl to the lone pair on the nitrogen. The definition holds.
Brønsted–Lowry acid
Brønsted–Lowry base
Does what
donates H+
accepts H+
Must have
a hydrogen atom it can release
a lone pair of electrons
Charge change
becomes one unit more negative
becomes one unit more positive
Examples
HCl, HNO3, CH3COOH, H3O+, HSO4–
NH3, OH–, H2O, CO32–, Cl–
Notice that H2O appears in both columns, and so does more than one ion. Whether a species behaves as an acid or a base depends on what you put it next to — which is the subject of two pages from here.
WORKED EXAMPLE
Identify the Brønsted–Lowry acid and base in each reaction, and justify each choice. (a) HNO2(aq) + H2O(l) ⇌ NO2–(aq) + H3O+(aq) (b) CH3NH2(aq) + H2O(l) ⇌ CH3NH3+(aq) + OH–(aq)
(a) track the hydrogenHNO₂ loses an H and becomes NO₂⁻. H₂O gains one and becomes H₃O⁺.acid: HNO₂ base: H₂O(b) track it againCH₃NH₂ gains an H to give CH₃NH₃⁺, using the lone pair on its nitrogen. H₂O loses one to give OH⁻.acid: H₂O base: CH₃NH₂Water is the base in (a) and the acid in (b). Never assume its role — work it out from the equation in front of you.
WORKED EXAMPLE
Explain why the chloride ion, Cl–, can act as a Brønsted–Lowry base, but the ammonium ion, NH4+, cannot.
ChlorideCl⁻ has four lone pairs, so it has electrons available to bond to an incoming proton.it can accept H⁺, forming HClAmmoniumIn NH₄⁺ the nitrogen has already used its lone pair to bond a fourth hydrogen. There is nothing left to offer.no lone pair, so it cannot act as a baseNH₄⁺ can only go the other way: it has four hydrogens to give away, so it is an acid.
💡 Exam tip
Learn the two definitions word for word: proton donor and proton acceptor. Nothing else scores.
Say proton, not “hydrogen”. A hydrogen atom would take its electron with it.
When identifying roles, follow the hydrogen count from left to right. Whichever species loses one is the acid.
If asked why a species can act as a base, mention the lone pair explicitly.
Keep H+ or H3O+ consistent within a single equation, and balance it.
⚠️ Common mix-up
Defining an acid as “something that produces H+ in water”. That is the older, narrower definition, and it misses gas-phase reactions entirely.
Saying a base donates OH–. Some bases do, but the definition is about accepting protons.
Assuming water is always the base. It is whichever the other reactant forces it to be.
Forgetting the lone pair when explaining how a base accepts a proton.
Writing H3O+ without adding H2O to the left-hand side, leaving the equation unbalanced.
Up next: Conjugate Acid–Base Pairs — once you can see the proton moving, you can see that every acid leaves a base behind it, and every base creates an acid. They come in twos.
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