Amphoteric and Amphiprotic Substances

Most substances pick a side. Hydrochloric acid is always an acid; sodium hydroxide is always a base. But a handful of substances refuse to commit. Put them with an acid and they act as a base. Put them with a base and they act as an acid. Chemists call these substances amphoteric, from the Greek amphoteros, meaning “both of two”.

Amphoteric behaviour isn’t a curiosity. It’s the reason water can have a pH, the reason your blood stays at a steady pH, and a key part of how aluminium is extracted from its ore.

Amphoteric vs amphiprotic

The two words overlap but aren’t identical.

• Amphoteric: can react as either an acid or a base, under any definition (including Lewis).
• Amphiprotic: can either donate or accept a proton (the Brønsted–Lowry version).

Every amphiprotic substance is amphoteric, but not every amphoteric substance is amphiprotic. Aluminium oxide, for example, is amphoteric, but it contains no hydrogen, so it can’t donate a proton. Water and the hydrogencarbonate ion are both amphoteric and amphiprotic.

Water: the most important amphiprotic substance

With hydrogen chloride, water accepts a proton, acting as a base:

HCl + H₂O → H₃O⁺ + Cl⁻

With ammonia, water donates a proton, acting as an acid:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

And with itself, it does both at once:

H₂O + H₂O ⇌ H₃O⁺ + OH⁻

This self-ionisation happens to only a tiny fraction of molecules: at 25 °C, about two water molecules in every billion are ionised at any moment. But it matters enormously. It sets [H₃O⁺] and [OH⁻] in pure water at 1.0 × 10⁻⁷ mol/dm³ each, which is why neutral water has a pH of 7 at 25 °C. Every pH calculation you ever do relies on this amphiprotic behaviour. See the pH scale explained.

Amphiprotic ions

Many ions that still carry an acidic hydrogen are amphiprotic.

Ion	As an acid (donates H⁺)	As a base (accepts H⁺)
HCO₃⁻	→ CO₃²⁻	→ H₂CO₃
HSO₄⁻	→ SO₄²⁻	→ H₂SO₄ (very unfavourable)
H₂PO₄⁻	→ HPO₄²⁻	→ H₃PO₄
HPO₄²⁻	→ PO₄³⁻	→ H₂PO₄⁻
HS⁻	→ S²⁻	→ H₂S

Whether one of these ions acts mainly as an acid or a base in water depends on how its two tendencies compare. Hydrogencarbonate is a slightly better base than acid, which is why baking soda solution is mildly alkaline, with a pH around 8.3. Hydrogensulfate is a much better acid than base, so sodium hydrogensulfate solution is quite acidic and is sold as a pH reducer for swimming pools.

Why this matters in your body

The HCO₃⁻/H₂CO₃ pair keeps your blood at pH 7.35–7.45. If acid enters the blood, hydrogencarbonate accepts the protons. If base enters, carbonic acid donates protons to neutralise it. Phosphate ions (H₂PO₄⁻/HPO₄²⁻) do a similar job inside cells. Both systems depend on amphiprotic ions. Read more in buffers explained.

Amphoteric metal oxides and hydroxides

On the periodic table, metal oxides are generally basic and non-metal oxides acidic. The metals near the dividing line produce oxides and hydroxides that can go either way.

Aluminium oxide and hydroxide

With an acid, aluminium oxide acts as a base:

Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O

With a strong alkali, it acts as an acid, dissolving to form an aluminate ion:

Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]

Aluminium hydroxide behaves the same way. If you add sodium hydroxide solution drop by drop to a solution of an aluminium salt, a white precipitate of Al(OH)₃ forms first. Keep adding sodium hydroxide and the precipitate dissolves again as [Al(OH)₄]⁻ forms. This “precipitate, then dissolve in excess” behaviour is a standard test that distinguishes aluminium ions from magnesium or calcium ions, whose white precipitates don’t dissolve.

This property is used industrially. In the Bayer process, bauxite ore is treated with hot sodium hydroxide solution. Aluminium oxide dissolves as aluminate; iron oxide and other impurities don’t, so they can be filtered off. Pure aluminium hydroxide is then precipitated and heated to give pure aluminium oxide for electrolysis.

Zinc oxide and hydroxide

Zinc behaves similarly:

ZnO + 2HCl → ZnCl₂ + H₂O ZnO + 2NaOH + H₂O → Na₂[Zn(OH)₄]

Zinc hydroxide is also a white precipitate that dissolves in excess sodium hydroxide.

Other amphoteric oxides

• Lead(II) oxide (PbO) and tin(II) oxide (SnO) dissolve in acids and in concentrated alkalis.
• Beryllium oxide (BeO) is amphoteric, unlike the basic oxides of the rest of Group 2. It’s one reason beryllium is said to resemble aluminium.
• Chromium(III) oxide (Cr₂O₃) is amphoteric.
• Copper(II) hydroxide dissolves slightly in very concentrated alkali, though it’s mainly basic.

There’s a pattern. Amphoteric oxides tend to come from metals with a high charge density: small ions with a fairly high charge. These ions pull strongly on oxygen, so the metal–oxygen bond has significant covalent character, halfway between an ionic (basic) and a covalent (acidic) oxide.

Across Period 3: the full spectrum

Period 3 shows the transition beautifully:

Oxide	Nature
Na₂O	Strongly basic
MgO	Basic
Al₂O₃	Amphoteric
SiO₂	Weakly acidic (reacts with hot, concentrated alkali)
P₄O₁₀	Acidic
SO₃	Strongly acidic
Cl₂O₇	Strongly acidic

Aluminium sits right at the turning point. You can explore these elements on the interactive periodic table to see how their other properties change across the row.

Amino acids: amphoteric molecules of life

Amino acids have an acidic carboxylic acid group (–COOH) and a basic amine group (–NH₂) in the same molecule. In water near neutral pH, the acid group gives its proton to the amine group, forming a zwitterion: a molecule with both a positive and a negative charge, but no overall charge.

H₃N⁺–CH₂–COO⁻ (glycine as a zwitterion)

• In acid, the –COO⁻ group accepts a proton → the molecule becomes positively charged.
• In base, the –NH₃⁺ group loses a proton → the molecule becomes negatively charged.

This is why proteins can buffer biological fluids, and why each amino acid has an isoelectric point, the pH at which it carries no net charge.

Amphoteric is not the same as neutral

It’s tempting to think an amphoteric substance must be “halfway”, and therefore neutral. That isn’t what the word means. A neutral substance, like sodium chloride, simply doesn’t react much as either an acid or a base. An amphoteric substance reacts readily as both, choosing its role according to its partner. Aluminium oxide is insoluble and gives no pH change in pure water, yet it reacts vigorously with both hot hydrochloric acid and hot sodium hydroxide. Being amphoteric describes versatility, not weakness.

How to identify amphoteric behaviour

1. Does it react with both an acid and a base? Test with dilute HCl and with NaOH solution.
2. Does its hydroxide precipitate and then redissolve in excess alkali? Classic sign of Al³⁺, Zn²⁺ and Pb²⁺.
3. Does it carry an H that can be lost and a lone pair that can gain an H⁺? Then it’s amphiprotic.

Key takeaways

• Amphoteric substances can act as acids or bases; amphiprotic ones specifically donate or accept protons.
• Water is amphiprotic, and its self-ionisation defines neutral pH.
• Ions like HCO₃⁻ and H₂PO₄⁻ are amphiprotic and are the basis of biological buffers.
• Oxides and hydroxides of aluminium, zinc, lead, tin, beryllium and chromium(III) are amphoteric; they dissolve in excess alkali.
• Amino acids are amphoteric, forming zwitterions.
• To compare acid–base definitions, see acid–base definitions.