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 reduce its concentration.
Strong acids completely dissociate into ions in solution. For example, hydrochloric acid is a strong acid. It completely dissociates 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 into ions in solution. For example, ethanoic acid is a weak acid. It only partially dissociates 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. The higher the concentration of H+ ions in an acidic solution, the lower the pH.
A pH of 1 represents a hydrogen ion concentration of 0.1 mol/dm3.
If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.
If the hydrogen ion concentration increases by a factor of 100 (102), the pH decreases by 2.
The higher the concentration of OH- ions in an alkaline solution, the higher the pH.
A solution of 0.8 mol/dm3 hydrochloric acid has a pH of 0.1. Predict its pH when it is diluted to 0.08 mol/dm3.
The hydrogen ion concentration decreases by a factor of 10, so the pH increases by 1 from 0.1 to 1.1.
A solution of 0.5 mol/dm3 hydrochloric acid has a pH of 0.3. Predict its pH when it is diluted to 0.005 mol/dm3.
The hydrogen ion concentration decreases by a factor of 100 (or 102), so the pH increases by 2 from 0.3 to 2.3.