You already know that acids taste sour and turn litmus red. That tells you what acids do. Brønsted and Lowry told us what acids are, and their answer is beautifully small: an acid is something that hands over one tiny particle. Get that one particle straight and the whole topic opens up.
📘 What you need to know
A hydrogen atom is one proton and one electron. Pull the electron off and only the proton is left — so H+ and “proton” are the same thing.
A Brønsted–Lowry acid gives a proton away. It is a proton donor.
A Brønsted–Lowry base takes a proton in. It is a proton acceptor.
A base must have a lone pair. The proton has no electrons of its own, so the base has to supply both electrons for the new bond.
Acids and bases always work in pairs. Nothing can donate a proton unless something else is there to accept it.
This works outside water too — even between two gases with no solvent at all.
In water a bare proton never floats about alone. It sticks to a water molecule to give H3O+. Writing H+(aq) or H3O+(aq) both earn the mark.
What is actually moving
Start with the simplest atom there is. Hydrogen has one proton in its nucleus and one electron going round it. Nothing else. So when hydrogen loses its electron, the whole atom is reduced to a single naked proton.
That is why chemists use the two words as if they were the same word, because here they really are. When a question says “proton transfer”, picture an H+ hopping from one molecule to another. Nothing bigger than that is happening.
Why this matters: a proton is unbelievably small and carries a full positive charge packed into almost no space. It cannot sit on its own in solution — it will grab the nearest thing with spare electrons. That “grabbing” is the whole of acid–base chemistry.
The two definitions
Here they are, and they are short enough to learn word for word.
Learn these exactly
acid = proton (H+) donor base = proton (H+) acceptor
Notice what is not in those definitions. No mention of water. No mention of pH. No mention of OH−. That is deliberate — it is what makes the Brønsted–Lowry idea so much wider than the “acids make H+ in water” definition you met earlier.
Examiners are fussy about the word “donor”. Writing “an acid loses a proton” is weaker, because losing sounds accidental. Donating means it gives the proton to something. Always name what it gives it to.
Why a base needs a lone pair
This is the part most students skip, and it is the part that makes everything else make sense.
A proton arrives with zero electrons. A covalent bond needs two. So if the proton is going to bond to the base, the base has to hand over both electrons itself. The only electrons a molecule has spare are its lone pairs. No lone pair, no bond, no base.
That is why ammonia (lone pair on N), water (two lone pairs on O) and the hydroxide ion (three lone pairs on O) are all bases, but methane, CH4, is not. Methane has no lone pairs at all, so it has nothing to offer an incoming proton.
The charges are worth checking: A started neutral and ends up 1− because it kept the electron pair. B started neutral and ends up 1+ because it gained a bare proton.
It does not have to happen in water
Almost every example you meet will be in solution, so it is easy to assume water is part of the rule. It is not. Take the lid off a bottle of concentrated hydrochloric acid next to a bottle of ammonia solution and a white cloud forms in the air between them. Two gases, no solvent, and a proton still moves.
The gas-phase example is the one to quote if a question asks you to show that the Brønsted–Lowry theory is not limited to aqueous solution.
Acid or base is a job, not a name
Here is the idea that trips people up. “Acid” is not a label a molecule wears for life. It is a role it plays in one particular reaction. Change the partner and the role can flip.
Water is the classic example. Put water with HCl and water is the base, because HCl is desperate to give its proton away. Put the same water with ammonia and water becomes the acid, because ammonia is even hungrier for a proton than water is.
Same molecule, opposite jobs
HCl + H2O → Cl− + H3O+ (water is the base)
NH3 + H2O ↔ NH4+ + OH− (water is the acid)
So never answer “water is a base”. Answer “water acts as a base in this reaction, because it accepts a proton from HCl”. Substances that can play both roles have their own name, and that is the next-but-one page.
🧩 Spotting the acid and the base in any equation
Count the hydrogens on each reactant and on each product.
Find the species that lost a hydrogen going left to right. That reactant is the acid.
Find the species that gained a hydrogen. That reactant is the base.
Check the charges moved by 1 in the right direction. The acid gets one unit more negative, the base gets one unit more positive. If they did not, you have picked the wrong pair.
Worked examples
WORKED EXAMPLE
Identify the Brønsted–Lowry acid and base in: HNO3(aq) + H2O(l) → NO3−(aq) + H3O+(aq)
Step 1: count hydrogens on the left and the rightHNO3 has 1 H → NO3− has 0 H, so it lost one.H2O has 2 H → H3O+ has 3 H, so it gained one.Step 2: match that to the definitions
Lost a proton = donated it = acid. Gained a proton = accepted it = base.
Step 3: check the charges shifted by one each way0 → 1− for the acid, and 0 → 1+ for the base. Balanced.HNO3 is the acid, H2O is the basethe charge check takes three seconds and catches nearly every slip
WORKED EXAMPLE
In HCO3−(aq) + H2O(l) ↔ H2CO3(aq) + OH−(aq), state the role of the water and explain your answer.
Step 1: track the waterH2O (2 H) becomes OH− (1 H), so the water has lost a hydrogen.Step 2: where did that hydrogen go?HCO3− (1 H) becomes H2CO3 (2 H). It picked the proton up.Step 3: name the role
Water donated a proton, so here it is behaving as the acid.
H2O is acting as the Brønsted–Lowry acidsay “acting as”, not “is” — the same water is a base in the previous example
WORKED EXAMPLE
Explain why NH3 can act as a base but CH4 cannot.
Step 1: what does a base have to do?
It has to accept a proton, and the proton brings no electrons with it.
Step 2: so where does the bonding pair come from?
From the base. It must supply both electrons, which means it needs a lone pair.
Step 3: compare the two moleculesNH3: N has 3 bonding pairs and 1 lone pair.CH4: C has 4 bonding pairs and 0 lone pairs.NH3 has a lone pair to donate; CH4 has none, so it cannot bond to H+“lone pair” is the mark word here — do not just say ammonia is more reactive
💡 Exam tip
Use the exact wording: proton donor and proton acceptor. “Gives away H+” usually scores, but the standard phrase never fails.
If a question asks you to explain, name the species the proton went to or came from. A role with no partner is only half an answer.
When you need to prove a base is a base, mention the lone pair. That single phrase is often worth its own mark.
H+(aq) and H3O+(aq) are both accepted, but if you use H3O+ you must include H2O on the other side of the equation.
Keep the phrase “acts as” in your vocabulary. It shows the examiner you understand the role can change.
Learn the gas-phase HCl and NH3 example. It is the standard evidence that the theory is not limited to solutions.
⚠ Common mix-up
Saying a base must contain OH−. That is the older Arrhenius idea. Ammonia has no OH− anywhere and is still a base.
Thinking the proton travels alone through the solution. It is far too reactive. It always lands on a water molecule as H3O+.
Confusing “proton” with the whole hydrogen atom. The atom keeps its electron; the ion does not. Only H+ moves.
Labelling water permanently. Water is a base with HCl and an acid with ammonia. Read the equation before you decide.
Mixing up strength with role. A weak acid is still an acid. Whether it donates easily is a different question, covered later in the topic.
Forgetting the charge changes. If your chosen acid does not become one unit more negative, you have mislabelled something.
Up next: Conjugate Acid–Base Pairs. You have already met them in the diagrams above — now we give them their proper name and learn to spot the two pairs in any equilibrium.
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