Acids and alkalis - AQA SynergyConcentrations and strengths of acids - Higher

Dilute and concentrated solutions

A solution forms when a solutedissolves in a solvent. The concentration of a solution is a measure of how 'crowded' the solute particles are. The more concentrated the solution, the more particles it contains in a given volume.

When solutions are described as dilute or concentrated:

• a dilute solution contains a relatively small amount of dissolved solute
• a concentrated solution contains a relatively large amount of dissolved solute

Take care to use the word 'dilute' correctly. It can be used as an adjective to describe the concentration of a solution (as here), or as a verb to describe the process of adding more water to a solution to decrease its concentration.

Strong and weak acids

Acids in solution are a source of hydrogen ions, H⁺. The hydrogen ions are produced when the acid dissociates or breaks down to form ions.

Strong acids

Strong acids completely dissociate into ions in solution. For example, hydrochloric acid is a strong acid. It ionises completely to form hydrogen ions and chloride ions:

HCl(aq) → H⁺(aq) + Cl^-(aq)

Nitric acid and sulfuric acid are also strong acids.

Weak acids

Weak acids only partially dissociate in solution. For example, ethanoic acid is a weak acid. It is only partially ionised to form hydrogen ions and ethanoate ions:

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

The ⇌ symbol is used in the equation to show that the reaction is a reversible reaction and does not go to completion.

pH and hydrogen ion concentration

The pH of a solution is a measure of its concentration of hydrogen ions:

• the higher the concentration of H⁺ ions in an acidic solution, the lower the pH
• the lower the concentration of H⁺ ions in an acidic solution, the higher the pH

This means that, for a given concentration in aqueous solution, the stronger an acid, the lower the pH.

The more concentrated the solution of an acid, the lower its pH will be.

pH of alkaline solutions

The higher the concentration of OH^- ions in an alkaline solution, the higher the pH.