A solution forms when a solute dissolves 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:
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 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 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.
The pH of a solution is a measure of its concentration of hydrogen ions:
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.
The higher the concentration of OH- ions in an alkaline solution, the higher the pH.
A solution of 1 g/dm3 hydrochloric acid has a pH of 1.6. Predict its pH when it is diluted to 0.1 g/dm3.
The hydrogen ion concentration decreases by a factor of 10, so the pH increases by 1 from 1.6 to 2.6.
A solution of 2 g/dm3 hydrochloric acid has a pH of 1.3. Predict its pH when it is diluted to 0.02 g/dm3.
The hydrogen ion concentration decreases by a factor of 100, so the pH increases by 2 from 1.3 to 3.3.