Acids and alkalis

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.

curriculum-key-fact
(Higher tier) Strong acids completely ionise in water. They break up completely to produce a high concentration of hydrogen ions in the solution.

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
curriculum-key-fact
(Higher tier) Strong alkalis completely ionise in water. They break up completely to produce a high concentration of hydroxide ions in the solution.

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

NaOH(aq) → Na+(aq) + OH-(aq)

Strong alkalis include:

  • sodium hydroxide
  • potassium hydroxide
curriculum-key-fact
(Higher tier) Weak acids only partially ionise in water. Only a small fraction of their molecules break into hydrogen ions when added to water.

For example, ethanoic acid is a weak acid.

CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)

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

curriculum-key-fact
(Higher tier) Weak alkalis only partially ionise in water. Only a small fraction of their molecules break into hydroxide ions when added to water.

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.