Definition of Amphoteric

What does Amphoteric mean?

An amphoteric compound is one that can react with both acids and bases.

Amphoteric behavior applies to acids and bases defined in the Bronsted-Lowry sense and in the wider Lewis sense.

The word amphoteric comes from Greek, where Ampho means both or both kinds - i.e. amphoteric compounds can react with both acids and bases.

Examples of Amphoteric Behavior

Example 1a: Water reacts with acid:

H2O + HCl ⇌ H3O+ + Cl-

Example 1b: Water reacts with base:

H2O + NH3 ⇌ NH4+ + OH-

Example 2a: Beryllium hydroxide reacts with acid:

Be(OH)2(s) + H2S04(aq) → BeS04(s) + 2H2O(l)

Example 2b: Beryllium hydroxide reacts with base:

Be(OH)2(s) + 2OH-(aq) → [Be(OH)4]2-(aq)

Example 3a: Aluminum oxide reacts with acid:

Al2O3(s) + 6H30+(aq) + 3H2O(l) → 2[Al(OH2)6]3+(aq)

Example 3b: Aluminum oxide reacts with base:

Al2O3(s) + 2OH-(aq) + 3H2O(l) → 2[Al(OH)4]-(aq)
Amphoteric Substances
The following are examples of amphoteric substances:
  • Water, amino acids, hydrogen carbonate ions, and hydrogen sulfate ions
  • Metal and metalloid oxides and hydroxides including: aluminum, antimony, arsenic, antimony, bismuth, beryllium, chromium, cobalt, copper, gallium, germanium, gold, iron, lead, silver, tellurium, tin, zinc

Note that the tendency is for elements on the left of the periodic table to form purely basic oxides and hydroxides, while elements on the right form purely acidic oxides. The elements that form amphoteric oxides tend to be those placed more centrally in the periodic table.

Sometimes substances that we might normally consider an acid or base can show amphoteric behavior. For example, H2SO4 is an acid when studied in water, but acts as a base in superacids, reacting with the superacid.

See also amphiprotic.

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