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An acid is a substance that produces hydrogen ions, H⁺(aq), when dissolved in water.

The higher the concentration of hydrogen ions in the solution, the lower the pH. (Higher tier)

An alkali is a substance that produces hydroxide ions, OH^-(aq), when dissolved in water.

For example, hydrochloric acid ionises completely into hydrogen and chloride ions:

HCl(aq) → H⁺(aq) + Cl^-(aq)

Strong acids include:

• hydrochloric acid
• sulfuric acid
• nitric acid

For example, sodium hydroxide ionises completely into sodium and hydroxide ions:

NaOH(aq) → Na⁺(aq) + OH^-(aq)

Strong alkalis include:

• sodium hydroxide
• potassium hydroxide

For example, ethanoic acid is a weak acid.

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

The ‘reversible arrow’ (⇌) indicates a reversible reaction.

For example, ammonia is a weak alkali:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺ (aq) + OH^-(aq)

The ‘reversible arrow’ (⇌) indicates a reversible reaction.

Dilute and concentrated solutions (higher tier)

• A concentrated acid contains a large number of acid particles dissolved per unit volume.
• A dilute acid contains a small number of acid particles dissolved per unit volume.