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.
If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1 unit.
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.