Acid

Summary: A substance that donates hydrogen ions (H⁺) when dissolved in water. A proton donor. Tags: igcse chemistry definition acids-bases Created: 2026-07-14 Last Updated: 2026-07-14

An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. In the Brønsted-Lowry definition used at IGCSE, an acid is a proton donor.

Characteristic Properties of Acids

Acids have characteristic reactions with metals, bases, and carbonates.

Reaction with Metals

Acids react with metals above hydrogen in the reactivity series to produce a salt + hydrogen gas:

Metal + Acid → Salt + Hydrogen

• Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
• Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
• Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Test for H₂: Lighted splint → squeaky pop.

Metals below hydrogen (Cu, Ag, Au) do NOT react with dilute acids.

Reaction with Bases

Acids react with bases (metal oxides and hydroxides) to produce a salt + water:

Acid + Base → Salt + Water (Neutralisation)

• CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
• MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Reaction with Carbonates

Acids react with carbonates to produce a salt + water + carbon dioxide:

Acid + Carbonate → Salt + Water + Carbon Dioxide

• CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
• Na₂CO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

Test for CO₂: Bubble through limewater → turns milky/cloudy.

Acids and Indicators

Effect on Litmus

Acids turn blue litmus red. Litmus paper is the simplest test for acidity.

Effect on Thymolphthalein

Acids have no effect on thymolphthalein — it remains colourless. (Thymolphthalein only changes colour in alkaline conditions, pH 9.3–10.5.)

Effect on Methyl Orange

Acids turn methyl orange red. (Methyl orange is yellow in alkali; red in acid.)

H⁺ Ions in Aqueous Solution

Aqueous solutions of acids contain H⁺ ions (hydrogen ions). These are what make a solution acidic. The higher the concentration of H⁺ ions:

• The lower the pH
• The stronger the acidity

Acids dissociate (ionise) in water to release H⁺:

• HCl(aq) → H⁺(aq) + Cl⁻(aq)
• H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)

Acids as Proton Donors

In the Brønsted-Lowry theory used at IGCSE, an acid is a proton (H⁺) donor. When HCl dissolves in water:

• HCl donates H⁺ to H₂O → H₃O⁺ + Cl⁻
• HCl is the proton donor (acid)
• H₂O is the proton acceptor (base)

Strong vs weak acids:

• Strong acids (HCl, H₂SO₄, HNO₃) — completely dissociate in water
• Weak acids (CH₃COOH, H₂CO₃) — partially dissociate in water, an equilibrium is established

Hydrochloric Acid as a Strong Acid

HCl is a strong acid — it completely dissociates in aqueous solution:

HCl(aq) → H⁺(aq) + Cl⁻(aq) (single arrow → shows complete dissociation)

At the same concentration, strong acids have a lower pH (more H⁺ ions), faster reaction rates, and higher electrical conductivity than weak acids.

Ethanoic Acid as a Weak Acid

CH₃COOH is a weak acid — it partially dissociates in aqueous solution:

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq) (equilibrium arrow ⇌ shows partial dissociation)

Most of the ethanoic acid remains as undissociated CH₃COOH molecules.

See also: Acids and Bases, Indicators and pH, Neutralization, Strong Acid, Weak Acid, Base, Carbonates

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