Water has appeared as the acid on one page and the base on the next. That is not sloppy writing — some species genuinely go either way, and which way depends entirely on what you put them next to.
📚 What you need to know
An amphiprotic species can both donate and accept a proton.
To manage it, a species needs a hydrogen atom it can release and a lone pair to accept with.
Water is the standard example: a base with HCl, an acid with NH3.
Others: HCO3–, HSO4–, H2PO4–, HPO42– and amino acids.
Amphoteric is the wider term: acts as an acid and as a base, by any mechanism.
All amphiprotic species are amphoteric; not all amphoteric species are amphiprotic.
Al2O3 is the classic amphoteric-but-not-amphiprotic case.
Water, both ways
Put water next to a stronger acid and it has no choice but to accept the proton. Put it next to a base and it becomes the best proton donor in the flask. The molecule has not changed — the comparison has.
Both neighbours of water on this ladder are its conjugates: H3O+ is its conjugate acid, OH– is its conjugate base.
Water behaving as a base
HCl(g) + H2O(l) → H3O+(aq) + Cl–(aq)
Water behaving as an acid
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)
What a species needs to qualify
Two requirements, and both must be met.
A hydrogen atom that can leave as H+, otherwise it can never act as an acid.
A lone pair, otherwise it has nothing to bond an incoming proton with.
Run those two tests and the awkward cases sort themselves out. CO32– has lone pairs in abundance but no hydrogen at all, so it can only ever be a base. NH4+ has four hydrogens to give away but the nitrogen has already committed its lone pair to the fourth N–H bond, so it can only ever be an acid.
A useful shortcut: most amphiprotic ions are the middle rungs of a ladder. Carbonic acid can lose two protons in turn — H2CO3, then HCO3–, then CO32–. The species at the top has nowhere to go but down, the one at the bottom nowhere but up, and whatever sits in between can do both.
Species
Acting as an acid, it becomes
Acting as a base, it becomes
H2O
OH–
H3O+
HCO3–
CO32–
H2CO3
HSO4–
SO42–
H2SO4
H2PO4–
HPO42–
H3PO4
HPO42–
PO43–
H2PO4–
Amino acids are amphiprotic too, and it is not a coincidence. Each one carries a –COOH group that can donate a proton and an –NH2 group with a lone pair that can accept one. In solution they often do both at once, giving an ion with a positive and a negative end.
Amphiprotic or amphoteric?
These two words are used loosely in everyday chemistry, and the IB is fussy about them. Amphoteric means a substance behaves as an acid in some reactions and a base in others. Amphiprotic is narrower: it specifies how — by actually donating and accepting protons.
Draw this once and the distinction sticks. The small circle sits entirely inside the big one, and nothing sits outside the big one.
Aluminium oxide reacts happily with acids, which shows basic character, and with alkalis, which shows acidic character. It is textbook amphoteric. But look at the formula: there is no hydrogen anywhere in Al2O3, so it cannot donate a proton and cannot be amphiprotic.
Al2O3 behaving as a base
Al2O3(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2O(l)
Al2O3 behaving as an acid
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
WORKED EXAMPLE
Show, using two equations with water, that the hydrogencarbonate ion HCO3– is amphiprotic.
As an acid — it donates a proton to waterHCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq)As a base — it takes a proton from waterHCO₃⁻(aq) + H₂O(l) ⇌ H₂CO₃(aq) + OH⁻(aq)it does both, so it is amphiproticNotice water swaps role between the two equations as well. In the first it is the base; in the second it is the acid.
WORKED EXAMPLE
State, with a reason, whether each species is amphiprotic: (a) CO32– (b) HSO4– (c) NH4+ (d) Cl–
(a) carbonatenoPlenty of lone pairs, but no hydrogen to donate. It can only ever be a base.(b) hydrogensulfateyesIt can lose its H to give SO₄²⁻, or accept one to give H₂SO₄.(c) ammoniumnoFour hydrogens to give away, but the nitrogen lone pair is already used in the fourth bond, so it cannot accept.(d) chloridenoLone pairs but no hydrogen — the same failure as carbonate.
WORKED EXAMPLE
Explain why aluminium oxide is described as amphoteric but not amphiprotic.
Why it is amphotericIt neutralises hydrochloric acid, behaving as a base, and dissolves in sodium hydroxide, behaving as an acid.it shows both acidic and basic characterWhy it is not amphiproticAl₂O₃ contains no hydrogen, so there is no proton it can donate.no proton to give awayAmphoteric describes the behaviour; amphiprotic specifies the mechanism. This oxide passes the first test and fails the second.
💡 Exam tip
To prove a species is amphiprotic, write two equations, one where it donates and one where it accepts. A sentence alone rarely gets full marks.
Use water as the partner in both equations — it is amphiprotic itself, so it can always play the other role.
Run the two tests: is there a hydrogen to lose, and is there a lone pair?
Keep the two words apart: amphiprotic is about protons, amphoteric is about behaviour.
Learn Al2O3 as the standard exception. It comes up repeatedly.
⚠️ Common mix-up
Using the two words interchangeably. Every amphiprotic species is amphoteric, but the reverse fails.
Calling CO32– amphiprotic because it looks similar to HCO3–. Check for the hydrogen.
Forgetting the lone pair requirement and assuming any species with a hydrogen qualifies.
Writing only one equation when the question asks you to show a species is amphiprotic.
Giving water the same role in both equations. If your species donates, water must accept, and vice versa.
Up next: The pH Scale — you can now say who gives protons and who takes them. The next question is how many are floating about, and that needs a number.
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