Base

Summary: A substance that accepts hydrogen ions (H⁺). Metal oxides and hydroxides are common bases. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

A base is a substance that accepts hydrogen ions — a proton acceptor.

Oxides and Hydroxides of Metals

Bases are typically oxides or hydroxides of metals:

A base that dissolves in water is called an alkali — it releases OH⁻ ions. Not all bases are alkalis (e.g., CuO is a base but insoluble — cannot form an alkaline solution).

Characteristic Properties of Bases

Reaction with Acids

Bases react with acids to produce a salt + water (neutralisation):

Base + Acid → Salt + Water

• CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
• MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
• Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)

This is the characteristic reaction of all bases.

Reaction with Ammonium Salts

Bases react with ammonium salts to produce a salt + water + ammonia gas:

Base + Ammonium Salt → Salt + Water + Ammonia

• Ca(OH)₂(s) + 2NH₄Cl(s) →(warm)→ CaCl₂ + 2H₂O + 2NH₃(g)
• NaOH(aq) + NH₄Cl(aq) → NaCl(aq) + H₂O(l) + NH₃(g)

Test for NH₃: Damp red litmus paper turns blue. This reaction is used as a test for ammonium ions.

Bases as Proton Acceptors

In the Brønsted-Lowry theory, a base is a proton (H⁺) acceptor.

When ammonia dissolves in water:

• NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
• NH₃ accepts a proton from H₂O → NH₄⁺ (base)
• H₂O donates a proton → OH⁻ (acid)

Key properties of bases:

• Turn red litmus blue
• Have pH greater than 7 (if soluble)
• React with acids to form salt + water (neutralisation)
• Metal oxides and metal hydroxides are bases
• Soluble bases are called alkalis (e.g., NaOH, KOH, Ca(OH)₂)

Common bases: Sodium hydroxide (NaOH), calcium oxide (CaO), copper(II) oxide (CuO), ammonia (NH₃), magnesium oxide (MgO)

See also: Acids and Bases, Alkali, Neutralization, Acid, Oxides

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.